Extending Your Strawberry Season with Day Neutral Varieties

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

Plasticulture strawberry planting season is quickly approaching. Growers seeking to extend their strawberry seasons should consider planting a portion of their area to day-neutral varieties. Day-neutral strawberries start fruiting 12-14 weeks after planting and have the potential to give late fall as well as early April through July production. Currently, the three varieties that have shown the most potential for extended production on Delmarva are Seascape, San Andreas, and Albion.

Albion, in particular, has shown great flexibility for season extension. It is very flexible on when it is planted in the late summer or early fall. August plantings will yield some late fall production, particularly in high tunnels. While much less productive in the main Chandler season in the spring, it has some unique properties that make it valuable to growers. First, it will give some early production, ahead of Chandler. Second, even though production is lower, it produces evenly over an extended period of time from April through July. In general, it will give 5-6 weeks more production than Chandler. It is a large, firm berry that, while not as sweet early in the season, has good quality in May and June.

Early August plantings of San Andreas will yield more fall production than Albion and San Andreas has comparable yields to Chandler in the spring with continued production through June. Both Albion and San Andreas have good quality and are firm berries that will stand up to regional shipping.

Seascape has been around for a long time and was the first of the larger sized day-neutral berries to show commercial potential in our area; however, Seascape has a softer berry and does not ship well so is best adapted to U-pick and local sales. Some grower in the region have had luck growing Seascape with multiple spring plantings spaced about three weeks apart from March through June giving summer and fall sales. Both Albion and San Andreas can also be planted in the spring for extended summer sales. Production in the heat of July and August will decline or stop unless there is a cool summer.

Because these day-neutral varieties keep blooming throughout the season, it is critical to maintain fertility, particularly with nitrogen, potassium, and calcium through fertigation. Albion, in particular, has high nitrogen needs to produce well. Disease management is also critical because these varieties bloom for an extended season. Gray mold fungicide sprays must be applied regularly throughout the extended seasons.

When to Plant Plasticulture Strawberries

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

Chandler has been our main plasticulture strawberry and has shown consistently high yields. For most of Delaware, the recommendation has been to plant Chandler the second week in September. However, Chandler is more sensitive to fall and winter temperatures than other varieties and in warmer conditions Chandler will put on too much growth, leading to small berries the following spring; therefore, knowing when to plant is difficult. If you could accurately predict fall and winter temperatures, you could adjust planting dates, but, of course, this is not possible.

One strategy has been to make multiple plantings of Chandler one week apart starting the second week in September. This will insure that a part of the crop will come out of winter with the proper number of crowns (not too many, not too little). Unfortunately, this means that part of the crop will be low yield and part will have small berries.

Another strategy is to switch to varieties that are less susceptible to putting on too much growth. This is where the variety Camarosa may have a fit; it is less temperature sensitive than Chandler in the fall and is not prone to putting on excessive growth. Camarosa has not performed as well on Delmarva compared to North Carolina.

Sweet Charlie, the early berry that also can put on a second late crop, is normally planted 7-10 days ahead of Chandler. It is not an option to replace Chandler. For other varieties being tried, we still do not have enough research in our region to know if they can be replacements for Chandler. Flavorfest has performed well but does not produce over as long of a season as Chandler.

Another strawberry that should be considered by growers is Albion, a day-neutral variety. It too is not sensitive to when it is planted in the fall. While much less productive in the main Chandler season, it has some unique properties that make it valuable to growers. First, it will give some early production, ahead of Chandler. Second, even though production is lower, it produces evenly over an extended period from April through early July. In general, it will give 5-6 weeks more production than Chandler. It is a large, firm berry, that, while not as sweet early in the season, has good quality in May and June. Research at Cornell and Penn State has shown that Albion needs much higher levels of nitrogen than the other common varieties and when fertilized properly will give higher yields over an extended period.

Sunburn in Fruiting Vegetables and Fruit Crops

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

Recent weather has produced conditions where there is high potential for sunburn in fruits and fruiting vegetables. Growers may need to consider ways to protect against sunburn. Sunburn is most prevalent on days with high temperatures, clear skies and high light radiation. We commonly see sunburn in watermelons, tomatoes, peppers, eggplants, cucumbers, apples, strawberries, and brambles (raspberries and blackberries).

There are three types of sunburn which may have effects on the fruits. The first, sunburn necrosis, is where skin, peel, or fruit tissue dies on the sun exposed side of the fruit. Cell membrane integrity is lost in this type of sunburn and cells start leaking their contents. The critical fruit tissue temperature for sunburn necrosis varies with type of fruit. Research has shown that the fruit skin temperature threshold for sunburn necrosis is 100 to 104°F for cucumbers; 105 to 108°F for peppers, and 125 to 127°F for apples. Fruits with sunburn necrosis are not marketable. Injury may be white to brown in color.

The second type of sunburn injury is sunburn browning. This sunburn does not cause tissue death but does cause loss of pigmentation resulting in a yellow, bronze, or brown spot on the sun exposed side of the fruit. Cells remain alive, cell membranes retain their integrity, cells do not leak, but pigments such as chlorophyll, carotenes, and xanthophylls are denatured or destroyed. This type of sunburn browning occurs at a temperature about 5°F lower than sunburn necrosis. Light is required for sunburn browning. Fruits may be marketable but will be a lower grade.

The third type of sunburn is photooxidative sunburn. This is where shaded fruit are suddenly exposed to sunlight as might occur with late pruning, after storms where leaf cover is suddenly lost, or when vines are turned in drive rows. In this type of sunburn, the fruits will become photobleached by the excess light because the fruit is not acclimatized to high light levels, and fruit tissue will die. This bleaching will occur at much lower fruit temperatures than the other types of sunburn. Damaged tissue is often white in color.

Storms that cause canopies in vine crops to be more open will expose fruits to a high risk of both sunburn necrosis and photooxidative sunburn.

Genetics also play a role in sunburn and some varieties are more susceptible to sunburn. Varieties with darker colored fruit, those with more open canopies, and those with more open fruit clusters have higher risk of sunburn.

Control of sunburn in fruits starts with developing good leaf cover in the canopy to shade the fruit. Fruits most susceptible to sunburn will be those that are most exposed, especially those that are not shaded in the afternoon. Anything that reduces canopy cover will increase sunburn, such as foliar diseases, wilting due to inadequate irrigation, and excessive or late pruning. Physiological leaf roll, common in some crops such as tomato, can also increase sunburn.

In crops with large percentages of exposed fruits at risk of sunburn, fruits can be protected by artificial shading using shade cloth (10-30% shade). However, this is not practical for large acreages.

For sunburn protection at a field scale, use of film spray-on materials can reduce or eliminate sunburn. These materials are kaolin clay based, calcium carbonate (lime) based, or talc based and leave a white particle film on the fruit (such as Surround, Screen Duo, Purshade and many others). There are also film products that protect fruits from sunburn but do not leave a white residue, such as Raynox. Apply these materials at the manufacturer’s rates for sunburn protection. They may have to be reapplied after heavy rains or multiple overhead irrigation events.

While particle films have gained use in tree fruits, their usefulness in vegetables is still unclear. Research in a number of states has shown reduced fruit disorders such as sunburn in peppers and white tissue in tomatoes when applied over those crops. Watermelon growers have used clay and lime based products for many years to reduce sunburn in that crop in southern states.

There are some drawbacks to the use of particle films. If used for sunburn protection on fruits, there is added cost to wash or brush the material off at harvest. Where overhead irrigation is used, or during rainy weather, the material can be partially washed off of plants, reducing effectiveness and requiring additional applications. Produce buyers can also have standards relating to the use or particle films and may not accept products with visible residues.

Sulfur and Vegetable Crops

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

With the recent heavy, leaching rains, we are seeing signs of sulfur deficiency in some vegetable crops. Sulfur is considered one of the secondary macronutrients that vegetable crops require for growth. Sulfur is a component of four amino acids and is therefore critical for protein formation. It is also a component of certain glycosides that give pungency to mustard family crops (greens, cole crops) and Allium crops (onions, garlic).

In the last 25 years, as industrial air pollution has been reduced (especially pollution from coal fired power plants) we have had less sulfur deposition from rainfall. Sulfur deficiencies are more common and sulfur additions in fertilizers or manures is being required for many crops to produce high yields.

Most of the sulfur in the upper part of the soil is held in organic matter. Upon mineralization, sulfur is found in the soil as the sulfate ion (SO42-) which has two negative charges. The sulfate ion is subject to leaching, especially in sandy textured soils (loamy sands, sandy loams). It does accumulate in the subsoil but may not be available for shallow rooted vegetables.

Sulfur can be added by using sulfate containing fertilizers such as ammonium sulfate, potassium sulfate, and K-mag (sulfate of potassium and magnesium). It is also a component of gypsum (calcium sulfate). In liquid solutions, ammonium thiosulfate is often used as the sulfur source. Sulfur is also found in manures and composts. For example, broiler litter has about 12-15 lbs of sulfur per ton.

In vegetable crops, sulfur removal is generally in the 10-20 lb/A range. Mustard family crops (cole crops such as cabbage and broccoli, mustard and turnip greens, radishes) remove between 30 and 40 lbs/A of sulfur. Research in our region has shown response to added sulfur for sweet corn and for watermelons. In Florida research it was shown that adding 25 pounds of sulfur per acre boosted yields by 1.7 tons per acre in tomatoes. Similar results were found with strawberries.

Our general recommendations are to apply 20-30 lbs of sulfur per acre on sandy soils for most vegetable crops. Remember to take credit for any sulfur being added with fertilizer sources such as ammonium sulfate (24% sulfur).

One vegetable where we want to limit sulfur is with sweet onions. Because sulfur increases onion pungency, and sweet onions are sold based on their low pungency, we limit sulfur applications to this crop.

Botrytis Fruit Rot (Gray Mold) and Crown Rot in Strawberries

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

Strawberry growers in the region should initiate programs to control Botrytis fruit and crown rots in strawberries.

Gray mold of strawberry fruit is caused by the fungus Botrytis cinerea. This pathogen can also cause crown rots that can weaken or kill plants. The crown rot phase of the disease often develops under floating row covers that are used to promote growth or protect against cold events.

The reservoir for this fungus is mycelium in dead strawberry leaves. This mycelium becomes active in the spring and starts to produce spores on the old leaf tissue, which then spread to blooms. Most infections start at the bloom stage but symptoms usually do not develop until close to harvest (the fungus does not become active until the fruit enlarges). Ripening fruits can also be infected. Conditions conducive for infection are temperatures between 70 and 80 °F and wet conditions (rain, dew, fog, irrigation). The most critical period for applying fungicides to control gray mold is during bloom.

Fungicide Recommendations From our Commercial Production Recommendation Guide:


Start spraying at 5-10% bloom, because most fruit infections occur through the flower. Repeat every 7-10 days. Spray less frequently during prolonged dry periods, but spray every 5-7 days during very wet periods. For season-long control it is usually sufficient to spray once a week for 4 weeks. Tank-mix and rotate fungicides from different FRAC codes to reduce the chance of fungicide resistance development.

Application #1, apply ONE of the following: Thiram 480DP 4.4 lb/A, Captan 80WDG 3.7 lb/A, or Switch 62.5WG 11.0 to 14.0 oz/A

Application #2, apply ONE of the following if resistance is NOT SUSPECTED on your farm: Fontelis 1.67SC (except Jewel, L’Amour and Clancy varieties) 16.0 to 24.0 oz/A, or Elevate 50WDG1 1.1 to 1.5 lb/A

If testing, observation, or frequent prior use of the above materials indicates high resistance risk, apply ONE of the following instead: M3 Thiram 480DP 4.4 lb/A, Captan 80WDG 3.7 lb/A, or Captevate 68WDG 3.5 to 5.25 lb/A

Application #3: Same as Application #1

Application #4. For subsequent applications, ROTATE BETWEEN the following fungicides with different modes of action: Fontelis 1.67SC (except Jewel, L’Amour and Clancy varieties) 16.0 to 24.0 oz/A, Switch 62.5WG 11.0 to 14.0 oz/A, or Elevate 50WDG1 1.1 to 1.5 lb /A.

Note: If Pristine is included in the schedule for Anthracnose control, in most cases that may provide satisfactory Gray Mold control and separate application of specific products may not be necessary.

If Botrytis is a problem it can often be traced back to poor sanitation (removing old leaves) from plantings, mistiming of bloom sprays, or a combination of the two. However, we are seeing resistance of Botrytis to several fungicides. If fungicide resistance is suspected, resistance testing may be warranted. There is a laboratory at Clemson University that is doing Botrytis resistance testing.

Strawberries should be cleaned of all dead leaves after row cover removal to limit Botrytis infections. Note freeze damage to flowers (black centers).

Plasticulture Strawberry Management 2018

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

If growers have not done so already, plasticulture strawberries should be cleaned to remove dead leaves and other dead plant material. Winter injury has left many dead leaves that will serve as a major source of Botrytis spores during bloom (the critical stage for infection). Dead material can also lead to crown rots in strawberry plants.

Plasticulture strawberries should have nitrogen applications prior to bloom. Base recommendations are 25 lbs/a of nitrogen at greenup and another 25 lbs/a of nitrogen 2-3 weeks later. If fertigating weekly, addition of 3-5 lbs of nitrogen per acre per week may be warranted. Nitrogen is critical prior to and during early bloom. Altering between potassium nitrate and calcium nitrate as the nitrogen source will often improve fruit quality.

Growers are also encouraged to take petiole and leaf tissue samples for laboratory analysis. To collect and submit strawberry tissue samples, follow these guidelines:
● select the most recently mature, healthy, trifoliate leaves from uniform field areas and the same variety;
● detach the petioles from the leaves as you collect them and save each separately;
● include leaves and petioles from 20 to 25 plants; and
● then submit leaves and petioles together as one sample.

We have a lab on Delmarva that can run these tissue samples. Leaf tissue nutrient levels should be maintained as follows: N (%) 3–4, P (%) 0.2–0.4, K (%) 1.1–2.5, Ca (%) 0.5–1.5, Mg (%) 0.25–0.45. When in full bloom, petiole tissue nitrate content should be between 4000-6000 ppm and then will decrease thereafter.

Petiole nitrate levels for most plasticulture strawberry varieties in ppm:
● greenup, pre-bloom 600-1500 ppm;
● bloom 4000-6000 ppm,
● main fruiting period 3000-5000 ppm,
● later fruiting decrease each week from 3000 to 1000 ppm.

Note that some varieties, such as Flavorfest, require much lower nitrogen levels.

Further note that day neutral varieties such as Albion that fruit into July should maintain higher levels of petiole tissue nitrate later in the season than June bearing types.

Plasticulture Strawberry Planting and Fall Growth Considerations Revisited

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

The next 10 days are the optimum period for planting strawberries in the plasticulture system. The variety Chandler, which has the most acreage in our region, is very sensitive to planting date. For highest yields, Chandler should be planted by September 20. Sweet Charlie and Camarosa should be planted 7-10 days earlier than Chandler for best yields. Most other June bearing type strawberry varieties should be planted by September 20 for best spring yields. Day neutral varieties such as Albion and San Andreas are less sensitive to planting date but should be planted by the third week in September for the best early spring yields.

Strawberry establishment in the plastic bed takes 3-4 weeks. During establishment, the goal is to have plants root as quickly as possible in the soil and start to send out new growth. This requires attention at planting. Most Delmarva growers are using plugs. Plant so that the plug is at the level of the soil or is just covered with a small amount (1/8”) of soil but avoid getting soil into the crown of the plant. Deep planting will result in reduced stands and weak plants due to rotting in the crown area. Shallow planting (where part of the plug is out of the ground) will result in plugs desiccating and reduced stands. Soil should be firm around the plug and water provided at planting. It is advantageous to overhead irrigate several times, even with water provided by drip lines, to reduce plant shock. It is also hard to wet beds completely with the drip system in sandy soils thus affecting establishment.

Rooting also requires adequate bed soil temperature. Raise high beds, the higher the better to allow for good drainage. Lay plastic making sure there is a firm crowned bed. The goal is to have the plastic tight against the soil to allow for good heat transfer. Loose plastic will have poor heat transfer and can reduce fall growth. Beds with depressions that allow water to accumulate can lead to disease problems in strawberries.

The goal coming out of the establishment period is to have 3 or more fully green leaves on the plant. After establishment, plants will send out new growth and develop branch crowns during October and November. The goal by late fall is to have 2-3 branch crowns form from the mother plant. Crown growth occurs when temperatures are above 50°F. Flower buds are also initiated during this time. Often, growers receive plugs or plants later than September 20. For later plantings, low tunnels offer an opportunity to maintain temperatures above 50°F for a longer period achieving this goal. Early row covers may also be used to achieve this goal – research has shown that early row covers may not increase crown number but can increase flower bud initiation in the fall. While planting too late can reduce spring yields, planting too early risks too many crowns being developed, especially in Chandler, leading to smaller unmarketable berries (Sweet Charlie and Camarosa are less prone to this problem as is Albion). That is why we don’t plant in late August on Delmarva

Plant size in the fall is also critical for high yields the following spring. Plants should be about 8 inches in diameter going into winter. Sugars produced in leaves are translocated into the crowns of the plant where they are converted into starch for winter storage. This starch is then used in the spring at greenup. Inadequate starch storage will also lead to lower yields in the spring. Plants should also go into winter with enough leaves to help insulate the crown.

Reduced Fruit Size in Strawberries

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

Several growers have commented that fruit size in strawberries is smaller than normal in 2017. This poses the question “what affects strawberry fruit size?”

In plasticulture strawberries, one critical factor with varieties such as Chandler is the number of branch crowns that develop in the fall. Early planting or extended warm weather in the fall may cause plants to produce excess crowns leading to too many buds, flowers and fruits per plant in the spring and, consequently, small berries. This is also a common problem with carry-over plasticulture strawberries where crown thinning was not done or was inadequate.

Another cause of smaller sized strawberries is related to pollination. Strawberries are aggregate fruits. That is, they have multiple ovules per receptacle where the fruit is formed. The strawberry receptacle may have up to 500 ovules per berry. You will see these as “seeds” on the outside of the strawberry fruit which are called achenes. To have the largest berry possible, you need as many of these ovules to be successfully pollinated as possible. With pollination the receptacle tissue around the achenes will develop to form the strawberry fruit.

Strawberries have both male and female flower parts on the same flower and can self-pollinate. Wind and rain can move pollen within the flower. However this usually does not allow for full pollination of all the ovules. Bees such as honey bees or bumblebees are usually necessary to allow for complete pollination. Some flowers actually produce bigger berries with cross pollination with pollen from other flowers. Incomplete pollination will often result in smaller or misshapen berries.

Strawberry flowers are not heavy nectar producers. However, bees do visit the flowers and studies have shown that where native bees are limited, adding hives of honey bees or bumble bees increased productivity. It is recommended that each flower receive 16-25 bee visits. This is particularly true of the king berries, which form from the first flower to open on a fruiting truss.

This additional pollination by insects is limited when row covers are placed over fields for extended periods during flowering, by poor weather for honey bee flights (rainy, windy, cold), or by other actions affecting pollinator performance.

Low Level of Two Spotted Spider Mites in Strawberries

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

Because North Carolina was reporting two spotted spider mite infestations in their strawberry fields I decided to visit some of our fields over the last week. This is 2-3 weeks earlier than I normally visit fields, but with this mild winter and the reports out of North Carolina I decided to start earlier. Most strawberry fields were on plastic but some were matted row production and a few in high tunnels. I was surprised to find very low levels of mites in the fields, with a few hot spots of mites in some high tunnels. There was only one species of mite found: the two spotted spider mite, Tetranychus urticae. Overwintering female two spotted spider mites are an orangish-red (Fig. 1) and most of the mites that can be seen with a naked eye will appear reddish in color. Spider mites overwinter as adults in the soil or leaf litter, although they may remain somewhat active in high tunnels through the winter. I found mite eggs in several high tunnel strawberries, but not in any outdoor strawberries. The light yellowish eggs are pearl-like in appearance and are attached to the undersides of leaves or stems (Fig. 2). These overwintering populations of mites can be difficult to control as they are “entrenched” in the strawberries. Feeding damage by mites that occurs before fruiting can cause the most loss in yield, but after the first strawberry harvest plants can tolerate much greater rates of infestation. Growers should check their strawberries for mites now, especially if they are in a high tunnel. If mites are found now you need more than 5 mites per leaflet (1/3 of a leaf) to justify the expense of a miticide application.

Figure 1. Overwintered two spotted spider mite female with orangish-red coloration.

Figure 2. Many two spotted spider mite eggs on back of a leaf

The most difficult thing to accomplish for good control is getting adequate spray coverage. Many of the spray applications do a good job of covering the top of the leaves, but do a poor job of reaching the underside of the trifoliates. The underside area of the leaf that usually sees very little chemical deposition is in the ‘palm’ of the leaf (Fig 3). These are the areas where mites can still be found even after a few sprays and need to be carefully checked a few days after an application. Good coverage is essential. One grower used a leaf blower-like back pack sprayer and applied two sprays of 1% (by volume) horticultural oil 7-10 days apart. He got excellent spray coverage on the underside of his leaves and consequently excellent control of the mites that were present. By using two applications about one week apart it is possible to control not only the adults and nymphs, but the eggs too. Oil is a good management tactic to use at this time of year as the plants are small and any possible burn from using the oil is a very low risk. An added benefit of the oil is that is rather inexpensive. I would like to see more growers use something like oil now and save the other chemicals for later in the season when plants are much bigger and there is a flare up of mites or other pests. Using oils now will also greatly reduce any development of mite resistance to other chemicals over the course of the season. If miticides are needed Kanemite, Acramite and Portal are all excellent miticides.

Figure 3. Underside areas of strawberry leaf where mites can hide from spray applications

False Spring Concerns – Again

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

Some peaches, nectarines, plums, and apricots have started to bloom — four weeks ahead of normal. Other fruits such as strawberries may be blooming ahead of schedule in plasticulture systems. These fruit crops are at great risk of losses due to freeze events. Other fruits such as pears, cherries, and blueberries may also flower early and be at risk.

The expected temperatures near 20°F at the end of this week will damage many of these flowers. The long range outlook for March and April shows a return to more seasonable weather which will slow bloom. However, crops already in bloom will be exposed to the potential of freezing temperatures throughout the rest of March and April.

Peach in various stages of flowering including open bloom, the most susceptible state to freeze damage on March 2, 2017.

Open nectarine flower on March 2, 2017.

Plum with high percentage of open flowers on March 2, 2017

Pluot in full flower on March 2, 2017.

Normally, the average date of the last frost in Delaware is somewhere between April 20-25. We still have seven weeks of worry ahead for our fruiting crops.

For all these fruit crops the most susceptible stage of injury is when flowers have just opened. Open blooms are damaged at 32-34° F. Blooms prior to opening have higher cold tolerance as do small fruit. For most fruits, critical temperature for losses after fruits have formed is 28-30° F.

Frost and freeze protection methods vary with fruits and the type of freeze expected. Advective freezes occur with freezing temperatures and high winds. This is the most difficult to protect against. For strawberries, two layers of floating row covers may be the most effective strategy for advective freezes. Double covers have been shown to be more effective than single heavy covers in this case. Irrigation along with double covers can provide even more protection if done properly.

Radiation (or radiant) freezes occur on cold, still nights. In this case cold air is near the ground and warmer air is above. Wind machines and helicopters have been successfully used to stir the air and raise the temperatures in orchards in this case. Row covers in strawberries will protect against radiation freezes too. Other options are over the top sprinklers, ground sprinklers, and heaters.

Over the top sprinkling is commonly used for frost protection but it has to be done properly. How this works is that as clear ice forms on plants heat is released. For frost production, overhead sprinkler systems are designed to deliver 0.1 to 0.2 acre-inches of water per hour. This method is used for radiant freeze or frost protection when wind speeds are low and temperatures 24°F or above. The key is to keep ice formation occurring through the night and continue through melt in the morning. Remember that initially, until ice starts forming, there will be evaporative cooling of the plant. The latent heat of fusion (water freezing) will release heat (approximately 144 BTUs/lb of water), whereas evaporative cooling will absorb heat from the plant (absorbing approximately 1,044 BTUs/lb of water) and lower plant temperatures. Therefore, irrigation must start well above critical temperatures. Also, the volume of water needed needs to be matched with the expected temperature drop and wind speed. In addition, uniformity of water application is critical. This is difficult to do in high wind situations. In orchards, under-tree sprinklers can also be used to release heat (hot air then rises, protecting blooms).

Heaters that are placed throughout an orchard will add heat. Large numbers of small heaters are preferred (40 per acre). This is accomplished with fuel oil fired heaters, gas/propane heaters, or burn barrels using wood or other fuel (check with regulatory agencies before using open burning in barrels). Heaters are much more efficient and less are required if they are used in conjunction with wind machines.

Wind machines or frost protection fans above an orchard or vineyard mix the warmer air above the inversion layer with the colder air at ground level to protect against radiation frosts. These large fans can be permanently installed and will cover as much as 10 acres. Another type of fan is placed at ground level and pushes the cold air upward, again achieving mixing. Portable fans are also available.

The following is a good factsheet on frost protection on berry crops: http://www.fruit.cornell.edu/berry/production/pdfs/frost%20protection%20tips%20techniques.pdf