Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Strawberry season started early this year and is now winding down. The heat over the holiday weekend has essentially shut down additional flowering and fruiting in our spring bearing varieties so expect limited production into June. The critical high temperature for strawberries varies but any temperatures in the high 80s or 90s will reduce or stop flowering altogether.
There has been significant interest in the use of repeat blooming strawberries for extended production. These “day neutral” varieties can provide good spring production with continued production into the summer and fall, depending on the planting date. Day neutral strawberries are different from spring bearing types because they are not triggered to flower by daylength and can flower and fruit repeatedly as long as temperatures are suitable. Day neutrals vary in their ability to flower during the summer, and have been classified as either weak or strong. Strong day neutrals produce flowers and runner sparsely during the summer, flowers form on runners, and plants tend to be small with a moderate number of crowns. Intermediate and weak day neutrals, have more of the spring bearing characteristics, such as a stronger tendency to runner in summer. We recommend strong day neutrals for summer and fall production in our area.
In the past, the day neutral varieties Tribute and Tristar were used in this region; however, berry size is small. Certain day neutral varieties bred for use in other regions (California and Northern Europe) have been successfully used in this region and have larger berry size and higher yields, Seascape, Evie 2 and Evie 3 for example. Unfortunately these varieties have soft berries. Albion has shown good promise as a day neutral with large firm berries, small plants, and long term, even production and low disease pressure. It is lower yielding than others but works well in our region. Other California bred day neutral varieties such as Monterey and Portola have shown promise for our area. The USDA breeding program in Beltsville, MD is currently selecting day neutral varieties that would be better adapted to our region.
Day neutrals can be planted in the spring for summer, fall and carryover spring production; planted in summer for fall and carry over spring production; and planted in fall for spring, summer and fall production the next year.
For summer production with day neutral varieties, the use of aluminized reflective plastic mulch with drip irrigation is recommended. Additional provision for heat abatement will be necessary. This may include low volume misters for evaporative cooling during hot daytime temperatures or the use of white or reflective shade cloth. Drip irrigation should be run during the day to further limit bed heating.
Remove runners from all plants throughout the season. Runnering decreases markedly after fruiting begins, so while this task is somewhat intensive early in the season, it becomes insignificant later.
Flowers should be removed for 6 weeks following planting to allow the plants to achieve sufficient size for fruiting. Failure to remove flowers will result in small plants and low yields. Extending the period of flower removal beyond 6 weeks will result in larger plants, berries and second-year yield, but less production in the first year. Varying the flower removal period will not affect the timing of production peaks.
Day neutrals benefit from a continuous supply of nitrogen and potassium. Additional phosphorus is not necessary provided an adequate supply has been incorporated before planting. Apply 5 to 6 lbs/A of nitrogen through the drip irrigation system every week. Calcium nitrate is the preferred source of nitrogen early in the season, UAN solution can be substituted when temperatures warm. Supplement preplant potassium with 10 lb/A of K2O at monthly intervals, or 2 lb/A at weekly intervals through the drip irrigation system during the growing season. Day neutrals tend to be heavy consumers of boron because of their large commitment to reproduction. Monitor leaves occasionally to ensure that boron levels do not fall below 30 ppm. An application of 2 lb/A Solubor may be required in midsummer if boron levels are too low.
Gray mold is the biggest disease problem of day neutral strawberries. Because berries are continuously present, mold inoculum tends to increase during the season. Remove moldy berries from the planting, and protect flowers every 10 days to 2 weeks with an application of fungicide, especially after rainy periods.
To extend production in the fall floating row covers, clear row covers, or a combination can be used to conserve heat. If fall production is to be targeted, mid-summer plantings on black plastic mulch would be recommended but overhead irrigation is essential for establishment.
Day neutral plantings can be carried over to a second year. Plants should be cut back and crown thinning may be necessary in some varieties.
Some information in this article was taken from this factsheet: http://www.omafra.gov.on.ca/english/crops/facts/89-099.htm