Spring Cover Crops for Vegetable Rotations

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

One principle of managing soil for improved health is to always have a crop growing on the soil. This will maintain or add organic matter, provide benefits from the action of growing roots, and recycle nutrients.

Where fall cover crops were not planted due to late harvest, spring cover crops can be planted in early April to provide soil health benefits where vegetables and field crops are not scheduled until late May or the month of June.

The most common cover crop options for late March or early April planting include spring oats, mustards and annual ryegrass. Plant oats at 90-120 lbs per acre, mustards at 10-20 lbs per acre, and annual ryegrass at 20-30 lbs per acre.

Field peas are another option; however, we are somewhat south of the best zone for spring planting. One type of field pea is the winter pea which is often fall planted in our area but can be spring planted. It has smaller seed so the seeding rate is 30-60 lbs per acre. Canadian or spring field peas are larger seeded and used as a spring cover crop planted alone at 120-140 lb/A.

Mixtures can also be used. Field peas are well adapted to mixing with spring oats or with annual ryegrass. Reduce seeding rates of each component when using in mixtures. Recommended seeding rates are 70 lbs of oats per acre and 40 lbs/A of Austrian winter peas or 80 lbs/A of Canadian or spring field peas.

Many mustard family crops have biofumigation potential. When allowed to grow to early flower stage and then incorporated into the soil, they release compounds that act as natural fumigants, reducing soil borne disease organisms. Some biofumigant mustard varieties and blends include ‘Pacific Gold’, ‘Idagold’, ‘Caliente’, ‘Trifecta’, and ‘Kodiak’. Other mustard family crops serve as non-hosts, trap crops, or deterrents for pests. In research at the University of Delaware biofumigation using early spring planted biofumigant crops such as ‘Image’ radish, ‘Dwarf Essex’ rapeseed, or ‘Nemat’ arugula showed potential for managing root knot nematode populations. When used as a biofumigant, mustard family cover crops should be grown to achieve maximum biomass by adding 60-100 lbs of nitrogen per acre. Nitrogen is also required to produce high biomass with spring oats and annual ryegrass at similar rates. When planting mixtures with peas, nitrogen rates should be reduced.

An often-forgotten spring seeded legume crop that can also be used is red clover. Red clover can be frost seeded into small grains, seeded alone, or mixed with spring oats or annual ryegrass. Seeding rates for pure stands would be 10-16 lbs/A, for mixtures 6-10 lbs/A.

Spring planted radishes and mustards as cover crops.

Are You Growing Cover Crops for Maximum Benefits?

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

For most of Delaware, the optimal window for cover crop planting is in the month of September and we are nearing the end of the planting window where soil health benefits can be maximized. As we move into October, cover crop selection becomes limited due to reduced daylength and lower temperatures thus limiting potential soil health benefits.

Vegetable growers understand the benefit of growing cover crops to maintain soil heath. Most vegetable production systems are tillage intensive and organic matter is oxidized by soil microorganisms at a high rate. Cover crops are an important means to add organic matter back into vegetable production systems.

Cover crop acreage has been growing in the region, largely due to nutrient management efforts and an emphasis on growing cover crops for soil health benefits. Successful programs have been implemented by the USDA-NRCS and Conservation Districts to increase cover crop plantings for soil improvement.

Nutrient management goals and soil health goals are not necessarily the same. You can think about this with the question are you growing cover or crops?

In nutrient-management based cover crop programs, the goals are to have crops that can take up residual nitrogen and provide cover to reduce erosion losses. Non-legumes predominate, with most of the acres planted in small grains such as rye with some recent use of radishes. No fertilizer or limited fertilizer can be used with these cover crops. In this case, the answer to the question above is that a cover is being grown. While there will be soil health benefits, they are not maximized.

In contrast, when soil improvement is the primary goal, the cover crops are grown as crops. You are growing plants to maximize the benefits they provide. To increase organic matter and improve soil health the main goal is to produce maximum biomass above ground and below ground. A secondary goal would be to provide different types of organic matter with cover crop mixtures to support a diverse soil microbial environment.

In other situations, the goals will be different. With leguminous cover crops a goal may be to maximize the amount of nitrogen fixed. With soil compaction reducing crops such as radishes, the goal is to maximize the amount of “biodrilling” – the amount of tap roots being produced. With biofumigant crops, the goal is to maximize the production of fumigant-like chemicals the crops produce. With mulch-based systems, the goal is to maximize above ground biomass.

What these soil improvement and specific use goals have in common is the need to treat the cover crop as a crop to optimize plant growth. This would include seeding at the proper rate to achieve optimal stands, planting at the right time, using seeding methods to get maximum seed germination and plant survival, having sufficient fertility to support good plant growth, providing water during dry periods, managing pests (insects, diseases, weeds), and inoculating legumes. If cover crop mixtures are being used, the ratios of seeds being planted must be considered to have the best balance of plants in the final stand.

The best cover crop stands are obtained with a drill or seeder that places the seed at the proper depth, at the proper seeding rate, with good soil to seed contact. Fertilization and liming programs should be used to support season-long growth – fertilizers and other soil amendments will be necessary in most cases. Nitrogen will need to be added for non-legumes.

When the crop is terminated is also key. The cover crops should be allowed to grow to the stage that maximizes the benefits they offer before killing the crops. Allowing a winter cover to grow for an extra week in the spring can make a large difference in the amount of biomass produced.

Again, consider the question are you growing a cover or a crop? The answer is important to achieve your cover crop goals.

Above-ground biomass for a mulch-based vegetable production system after spring burn-down. Note the differences between the cover crop strips.

Winter Killed Cover Crops for Vegetable Cropping Systems Revisited

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Cover crops that put on significant growth in the fall and then die during the winter can be very useful tools for vegetable cropping systems. These winter killed cover crops add organic matter, recycle nutrients, improve soil health, and allow for earlier spring vegetable planting.

Winter killed cover crops that are late-summer and early-fall planted include spring oats, several mustard species, and forage and oilseed radish. Earlier planted summer annuals (millets; sorghums, sudangrasses, and hybrids; annual legumes such as cowpeas or forage soybeans; buckwheat and many others) can also be used as winter killed species. Timing of planting will vary according to the species being used and winter killed species selection will depend on when fields will be available for seeding. Summer annuals should be planted in August for use in a winter killed system to obtain sufficient growth.

Spring oats and mustard species can be planted from late August through September. For best effect, forage and oilseed radishes should be planted before the middle of September. Spring oats, radishes and mustards are not suited for October or later planting because they will not produce adequate fall growth.

The winter killed non-legumes mentioned above will benefit from the addition of 30-60 lbs of nitrogen.

The following are several options for using winter killed species with vegetables:

1) Compaction mitigation for spring planted vegetables. Where there are compacted fields, the use of forage radish has worked very well as a winter killed cover crop by “biodrilling”. The extremely large taproot penetrates deep into the soil, and after winterkilling, will leave a large hole where future crop roots can grow. Oilseed radish also provides considerable “biodrilling”. Winter killed radishes works well with spring planted crops such as spinach, peas, early sweet corn, and early snap beans. One issue with radishes is that on mild winters they may not fully winter kill.

Fall growth on mustards and radishes that will then winter kill. A potential winter kill mix would include a radish, a mustard, and spring oats.

2) Early planted vegetables. A wide range of early planted vegetables may benefit from winter killed cover crops. For example, peas no-till planted or planted using limited vertical tillage after a winter killed cover crop of forage radish, oilseed radish, or winter killed mustard have performed better than those planted after conventional tillage. Early sweet corn also has potential in these systems as do a wide range of spring vegetables including spinach, potatoes, and cabbage. Winter killed radishes and mustards also have the advantage of outcompeting winter annual weeds leaving relatively weed free fields and recycling nutrients from the soil so that they are available in the spring for early crops (decomposition has already occurred).

3) Mixed systems with windbreaks for plasticulture. By planting planned plasticulture bed areas with winter killed cover crops and areas in-between with cereal rye you can gain the benefits of these soil improving cover crops and eliminate the need make tillage strips early in the spring. The winter killed areas can be tilled just prior to laying plastic.

4) Bio-strip till. By drilling one row of forage or oilseed radish and other adjacent rows with rye or other small grains, you can create a biodrilled strip that winter kills and that can be no-till planted into the spring without the need for strip-till implements. This presents dozens of options for strip tilling (seed or transplanted) spring vegetables.

Notes from the Annual Meeting of the American Society for Horticultural Sciences (ASHS)

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Each year, the ASHS has an annual meeting bringing together scientists working with specialty crops (vegetables, fruits, ornamentals). This year the meeting is in Washington DC. The following are some notes from sessions I have attended over the last 2 days that have relevance to our Delmarva growers.

  • Sweet corn planted into selected biodegradable black plastic mulches were shown to provide equal weed control, production, and earliness to standard black polyethylene mulch and eliminate mulch disposal costs.
  • Pepper production under biodegradable plastic mulch was equivalent to standard black plastic mulch again eliminating the need for mulch disposal.
  • Low rate compost application in potato (1 ton/a) reduced nitrogen needs and improved quality and yield in potato production.
  • Reduced curing temperatures and time of curing as well as delayed vine termination (mowing just before digging) reduced internal defects in ‘Covington’ sweet potato
  • Using white or reflective mulch did not improve broccoli production compared to black plastic mulch (we have a similar study currently in Delaware)
  • Progress is being made in breeding beets for lower levels of geosmin, the compound that gives beets the earthy taste.
  • Grafting tomatoes onto certain vigorous rootstocks can improve yield in high tunnel production, even in the absence of soil-borne disease.
  • From Matt Kleinhenz at Ohio State University “Commercial microbe-containing crop biostimulants are advertised to maintain or enhance crop growth. More than two-hundred such products ranging in composition (e.g., bacterial, fungal, both; cfu/ml) are currently available. To date, outcomes from standard statistical approaches common in product evaluations, variety trials, and cultural management comparisons show that significant increases in yield or quality have been rare, regardless of inoculation parameters or experimental conditions.”
  • A multistate project is underway to see if there are long term benefits to the “soil balancing” philosophy of soil management — specifically, balancing percentages and ratios of calcium, magnesium, and potassium through applications of lime, gypsum, and other materials to improve soil physics (tilth) and biology and, thereby, crop yield and quality and weed control. Past, shorter-term studies have shown no benefits to soil balancing but some growers and crop advisors disagree. This multi-state research aims at answering claims that University research on soil balancing has not been long term and thus is biased.
  • Recently, a finely ground (<0.5 micron) liquid limestone-based product (Top Flow 130; Omya, Oftringen, Switzerland) was developed for agriculture use to be injected through drip irrigation tubing. Research by Tim Coolong in Georgia showed that Top Flow 130 could be used to adjust pH in a plasticulture system, but that the effects would occur within a zone of 4 inches on each side of the drip irrigation tubing. This may be useful for situations where pH has dropped below 5.2 in plasticulture beds.
  • UV blocking plastic in high tunnel covers were shown to reduce Japanese beetle activity greatly in high tunnel raspberry production.

Sunscald on Vegetable Leaves

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

I know it may seem odd to see an article about sunscald or sunburn on leaves with the week of rain we just had, but leaves came in over the last week as the rain started and the damage had been done days before this. It is also possible that there will be a greater chance for sunscald in the coming days as growers try to get their transplants out. An area on the leaf turning papery white or tan is usually the first indication of scald on plants (Fig. 1).

Figure 1. Scald on crucifer leaf (top) and bean leaf (bottom)

Many of these plants were set in the field after coming straight out of the greenhouse or off the trailer. Before the rains we had a few days of very hot temperatures and intense sunlight. In figure one you can see that only certain parts of the leaf are scalded (these are the areas that had direct sunlight on them for an extended period) and the tissue next to the scalded area is still bright green. In the transplant production house plants are exposed to filtered light so the leaves are good at absorbing as much light as possible. The problem with taking plants straight from this type of environment to the field is that the plants at times are not ready for the extra UV light they are going to receive. The leaf tissue rapidly becomes desiccated with the extra light/heat exposure, causing light tan to white discoloration on the leaves and stems of sensitive plants. At times even established plantings can experience this as well, especially during an unexpected heat wave, which we had (believe it or not) as a heat wave is defined as 3 or more consecutive days of temperatures at or over 90°F. Once leaves are damaged, all that can be done is to support the plant until it manages to grow new leaves. Hardening off the transplants would have prevented the sunscald on the new transplants, but with all the cool wet weather we had growers were forced to get their plants out when they could. Make sure to appropriately water and feed plants that have sunscald while they are recovering.

Flooding, Waterlogged Soils, and Effects on Vegetable Crops with Special Consideration for Plasticulture Vegetables

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

We have had widespread flooding in vegetable crops in May due to heavy and  extended rains. Soils in some field areas have remained waterlogged for several days. Over a 10-day period from May 12, 2018 at our Georgetown, Delaware Research station, 7.5 inches of rainfall fell. There were 4 days with rainfall over 1 inch and one day receiving 3 inches of rain. Many surrounding areas had over 10 inches of rain during this period.

Climate scientists predict that extreme weather events will become more common on Delmarva over the next several decades. This will present additional challenges for vegetable growers related to flooding, wet weather diseases, nutrient losses, ability to do timely harvests, field compaction, other wet soil issues, and resulting crop losses.

In 2018, initial plantings of watermelons and other fresh market vegetables have been made; peas are nearing harvest, and significant acres of pickles, snap beans, and sweet corn are in the field. Many processing vegetable fields have already had significant crop losses (sweet corn, snap beans, peas) due to flooding.

In flooded soils, the oxygen concentration drops to near zero within 24 hours because water replaces most of the air in the soil pore space. Oxygen diffuses much more slowly in water filled pores than in open pores. Roots need oxygen to respire and have normal cell activity. When any remaining oxygen is used up by the roots in flooded or waterlogged soils, they will cease to function normally. Therefore, mineral nutrient uptake and water uptake are reduced or stopped in flooded conditions (plants will often wilt in flooded conditions because roots have shut down). There is also a buildup of ethylene in flooded soils, the plant hormone that in excess amounts can cause leaf drop and premature senescence.

In general, if flooding or waterlogging lasts for less than 48 hours, most vegetable crops can recover. Longer periods will lead to high amounts of root death and lower chances of recovery.

While there has been limited research on flooding effects on vegetables, the following are some physiological effects that have been documented:

  • Oxygen starvation to vegetable roots will cause roots to cease to function resulting in plant collapse with limited recovery potential
  • Oxygen starvation in root crops such as potatoes will lead to cell death in tubers and storage roots. This will appear as dark or discolored areas in the tubers or roots. In carrots and other crops where the tap root is harvested, the tap root will often die leading to the formation of unmarketable fibrous roots.
  • Ethylene buildup in saturated soil conditions can cause leaf drop, flower drop, fruit drop, or early plant decline in many vegetable crops.
  • Leaching and denitrification losses of nitrogen and limited nitrogen uptake in flooded soils will lead to nitrogen deficiencies across most vegetable crops.
  • In bean crops, flooding or waterlogging has shown to decrease flower production and increase flower and young fruit abscission or abortion.
  • Lack of root function and movement of water and calcium in the plant can lead to calcium related disorders in plants. There is a potential for higher incidence of blossom end rot in tomatoes, peppers, watermelons, and other susceptible crops when fruits are forming and soils are saturated.

Low lying areas of fields are most affected by excess rainfall. However, cropping practices can also increase water standing. In vegetables, field compaction will reduce water infiltration leading to increased crop losses in wet weather.

Plasticulture Concerns in Wet Weather
In plasticulture, water can accumulate and persist between rows of plastic mulch because of the impervious surface of the mulch. Because much of the rainfall runs off the plastic, water pooling can be serious problem in plastic mulched fields, especially where row middles have become compacted. Vining crops that fruit into the row middles can have vines and fruits sitting in water and this produces ideal conditions for diseases of wet conditions to develop. A prime example is Phytophthora capsici (a water mold) that needs saturated soils or standing water to infect plants (fruits).

When water overflows the bed tops of plastic mulched crops, whole beds become saturated as water enters the planting holes. This often leads to plant losses as beds take a very long time to dry once saturated in this way and oxygen is very limited in the root zone.

To avoid water accumulation between plastic mulched beds, tilling with a deep shank or a subsoiler in row middles can help improve drainage. Cut drainage channels at row ends to reduce blockage (dams) that can back up water. Where practical, section plasticulture fields and install cross drains to remove extra water to improve drainage and reduce water damage potential. Growers may also choose not to plant lower areas in the field prone to water damage where plastic is laid.

In some crops such as peppers and strawberries, high raised beds will improve drainage significantly and can reduce losses to water standing between plastic rows. Another option in watermelons (and other strongly vining crops) grown on plastic is to reduce plastic bed width and increase distance between rows to limit impervious surfaces.

In some crops in our region (plasticulture strawberries for example), cover crops such as ryegrass are being grown between beds to reduce erosion. Research on row middle management will be a priority for the future.

Compaction between mulched beds can lead to increased ponding.

When water goes over top of beds they become saturated for long periods leading to plant losses. In this case the water just missed going over the bed (note the trash line).

Identifying Poorly Drained Areas for Phytophthora capsici Management
Growers with crops susceptible to Phytophthora capsici (P. cap) are encouraged to evaluate fields with susceptible crops (all vine crops, tomatoes, peppers, lima beans) for drainage issues where this disease can proliferate. The primary keys to P. cap management are limiting standing water, the potential for saturated soils, and water movement across the crop.

Recovering from Flooding or Waterlogging
One option to aid in vegetable crop recovery after floods or waterlogging is to aerate the soil by cultivating (in crops that can be cultivated) as soon as you can get back into the field. This allows for oxygen to enter the soil more rapidly. To address nitrogen leaching and denitrification losses, sidedress with 40-50 lbs of N where possible depending on the crop and crop stage.

In vegetable fields that remain wet, consider foliar applications of nutrients. Since nitrogen is the key nutrient to supply, spraying with urea ammonium nitrate (28 % N solution) alone can be helpful. These can be sprayed by aerial or ground application. Use 5 to 20 gallons of water per acre. The higher gallons per acre generally provide better coverage. As with all foliar applications, keep total salt concentrations to less than 3% solutions to avoid foliage burn.

Future Considerations
To address excess water challenges in the future, vegetable growers will need to invest in and plan for drainage in every field. Solutions including land levelling, surface drainage, tiles (tile wells, patterned tiling), and pumping may all need to be considered. See the article by James Adkins in this issue on drainage basics.

Row middles with ponding due to a field depression.

Revisiting Compost for Vegetable Production

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Each year I field questions from vegetable growers on the use of compost for their production systems. The availability of commercial compost has fluctuated over the years based on the companies operating in the region. With the entry of Perdue AgriRecycle into the compost field another quality compost is now available for use by growers on Delmarva.

In the composting process, organic stock material sources such as yard wastes, manure and litter, wood waste, food scraps and garbage, paper, hatchery waste, or other waste materials are combined in a proper mix to create a carbon to nitrogen ratio that will promote the growth of microorganisms that then decompose the materials, producing a dark, humus-rich end-product. In addition, in the composting process, the compost piles will heat up to between 130-170° F, killing pathogens of concern in the materials. A properly produced compost can be used for vegetable production without concerns for transferring plant pathogens or human pathogens.

Compost will contain plant nutrients, the level of which depends largely upon the stock materials used. Nitrogen content may be significant; however, much of the nitrogen will be in organic form and will be slowly available over several years. Most of the potassium will be readily available while phosphorus availability is more variable.

While compost does contain plant nutrients, the more important benefit that it provides is stable organic matter. Because it has already been decomposed, the organic component contains humus-like materials that will decompose very slowly when added to the soil. This means that compost will immediately raise the organic matter of the soil. This in turn will increase the cation exchange capacity (CEC) of the soil, improve soil moisture holding capacity, and improve soil physical characteristics (reduced compaction, improved aeration, decreased crusting).

Research has also shown that certain composts can reduce the incidence of soil borne diseases and pests. This is most likely because the organic addition promotes more diversity in soil microorganisms that can compete with pathogens and the improved physical properties of the soil (such as reduced compaction) that limits the impact of certain pathogens. Newly finished compost also contains beneficial microorganisms that directly affect plant pathogens by antibiosis or hyperparasitism. Some composts have also been shown to induce resistance to pathogens in crop plants.

When using compost, growers should first receive an analysis of the material. From this analysis you should look at the following:

Compost Maturity and Stability – Only use mature compost that has finished the composting process and that is stable. Immature compost will continue to decompose, and can cause soil imbalances in some cases.

Nutrient Content – As previously stated, compost has a base nutrient content. You need to account for available nutrients in the nutrient management plan for the crop the compost will be used on. Much of the nitrogen will be in organic form and only a portion will be available for the growing season.

Electrical Conductivity (EC or salts levels) – Composts that use manure or poultry litter as part of the stock materials can accumulate salts (particularly potassium) at elevated levels. The elevated salt content must be accounted for when determining application rates so that salt injury does not occur with crops.

Calcium Carbonate Equivalent (lime value) – Lime is generally not added in the composting process; however, high pH materials such as hatchery waste sometimes are composted. This means that certain composts may have more liming value.

Moisture Content and Physical Condition – Compost will be partly water. With higher moisture composts, you will be paying for more water and less of the humus material and nutrients. In addition, higher moisture composts do not spread as well. Compost should be adequately screened so that the product spreads well.

In research at the University of Delaware with several compost materials, a rate of 5-7 tons per acre showed yield benefits on sandy soils in the first year with several vegetable crops. However, specific effects on a grower’s farm will depend on soil type, existing organic matter, existing soil health, and compost source; therefore, rates should be adjusted accordingly.

The decision to use compost is also an economic one. Compost can cost anywhere from $15.00 to $50.00 per ton depending upon the source and distance for transport. Growers need to consider the soil improving and nutrient value of the compost and evaluate that against other soil improvement programs such as cover cropping and green manure crops.

Frost and Freeze Considerations in Vegetables Revisited

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

As we move into October, frost becomes a factor in harvest and recovery of vegetables. Later in the fall, freezes can become a concern. The first frost on inland sites generally occurs by the third week in October in the middle of Delmarva. However, this can vary quite a bit. For example, the first temperature below freezing in the Laurel, DE area occurred on Oct 26 in 2016 (31.3), Oct 18 in 2015 (31.6), Oct 20 in 2014 (31.9), Oct 24 (29.6) in 2013, and Oct 13 (28.9°F) in 2012. The first hard freeze (below 28°F) in the Laurel area occurred on Nov 12, Nov 15, Nov 8, Oct 26, and Nov 5 from 2016 to 2012 respectively. Coastal areas will see a delay in frost. For example, Kitts Hummock, near the Delaware Bay, had first frosts on Nov 12, Oct 19, Nov 8, Nov 9, and Nov 6 over the last 5 years.

Light to moderate frosts will not affect cool season vegetables such as cole crops, lettuce, and spinach. Some cool season crops, such as Brussels sprouts, broccoli, kale, and collards will handle freezing conditions. In contrast, cauliflower, once frozen, will deteriorate quickly. Warm season vegetables vary considerably in their ability to tolerate a light frost. For example, pepper is more cold tolerant in the fall than tomato which is severely damaged by frost. Pumpkins and winter squash will have leaf and vine kill with light frost but fruits will remain marketable. Heavier frosts and freezes will damage the fruit. Sweet potatoes must be dug quickly after a frost kills vines and will suffer root damage if soil temperature drops below 40°F. We often have significant acreage of beans still out in the fall. Snap beans and lima beans will have leaf damage but still can be harvested with a light frost. It is when temperatures drop below 28°F and pods freeze that harvest recovery is affected. When lima beans are frosted, you may have several weeks to get into the field and harvest. However, if there is pod freezing, the harvest window drops to a few days, depending on the day temperatures, before seeds start to “sour”.

For unprotected frost sensitive vegetables, it is important to follow weather forecasts closely for risk of frost or freeze. Clear sky conditions after a cold front moves through will be the highest risk for frost or freeze. When risk is high, growers should harvest all marketable produce ahead of the frost or freeze in warm season crops. For example, harvest all tomatoes (ripe, breakers, and mature greens) prior to a frost.

Floating row covers offer the best protection of sensitive vegetables against frost and freeze injury, depending on the thickness of the row cover, expect 2-6°F degrees of protection. Moist soil also can store some heat, lessening frost, and sprinklers can be used for fall frost protection (see past articles on spring frost protection).

Peppers will tolerate frosts in the fall, tomatoes will not

Lima bean harvest is minimally affected after a light frost. However, after a freeze, lima beans must be harvested within 48 hours.

Brussels sprouts are frost and freeze tolerant to 20°F.

Hail Damage Showing Up in Scattered Vegetable Fields

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

This has been a strange spring weather pattern we have been having. It has been cloudy and wet for the last 10 days or so and on top of all that we have had isolated down pours with hail. In some fields growers were not aware of the hail that had passed through and wonder now what could have caused the type of damage they are seeing in their crop. The damage on tomatoes and onions that I saw was from pea-size hail (Figures 1 and 2). The damage to tomatoes was always one-sided or even a quarter of a side of the tomato that was not covered by foliage. There was some tomato foliar damage, but not much. The noticeable and important damage was to the developing fruit. Developing vegetable fruit is often more sensitive to damage from hail than the stems and even leaves which are more durable and can take some small hail damage. Onion leaves were more beat up, but the bulbs all looked good in the fields I visited. In cucurbit fields the damage again was usually on only one side of the fruit with much lighter damage to stems and leaves (Fig. 3). If you have not had a chance to check out your vegetables or fruit, be sure to do so in the next few days to get an idea of how much, if any, damage occurred.

If you do have hail damage be sure your fungicide spray coverage is thorough and that plants are at least sufficient if not better in nutrient levels. This may be a good time to try some biostimulants that are purported to help plants overcome stressful conditions. Be sure to treat only some or half of your damaged plants to see if the biostimulant worked or not. Damaged fruit should be removed from plants as it will be unmarketable, but will continue to drain the plant’s resources until it is removed.

Figure 1. Hail damage to tomato on only one side of fruit by pea-size hail

Figure 2. Hail damage to onions

Figure 3. Hail damage to zucchini on only one side of fruit and light damage to stems