Gordon Johnson, Extension Vegetable & Fruit Specialist; email@example.com
So far, in 2023, the weather has been ideal for strawberry production. Strawberries produce best when daytime temperatures are in the 70’s and nighttime temperatures are in the 50’s. The cooler nighttime temperature promotes continued flower initiation, and the warm daytime temperature promotes fruit ripening.
In our region, commercial strawberries are grouped into two classes based on their flowering habit; those that flower and fruit mostly once in the spring and those that continue to flower and fruit as long as temperatures are moderate. Strawberries thrive in cooler weather and production is affected when temperatures are hot.
Short day or June-bearing has been used to describe plants which start to bloom in late winter and produce one fruit crop per spring. Day-neutral, long day, or repeat blooming have all been used to describe plants which flower multiple times and produce multiple crops over the course of the spring, cooler parts of the summer and fall.
Most of our commercial strawberries planted on Delmarva are June-bearing types. They start initiating flower buds after planting in the fall and begin flowering in late winter and spring as temperatures warm up. Flowering will continue until critical high temperatures are reached. This critical temperature at which flowering slows or ceases varies with the cultivar, but ranges are 80 to 86 °F daytime and 75 to 78 °F nighttime. At these temperatures runners form instead of flower-bearing branch crowns. So far, we have not reached these temperatures in 2023.
Day-neutral varieties (such as Albion, Seascape, San Andreas, and others) are being planted more frequently now on Delmarva. These varieties will continue to flower profusely under these mild temperatures but will stop flowering once temperatures hit the 90’s. They will then start flowering naturally again later in the summer when temperatures drop down below the critical values noted above. Day-neutral varieties are also induced to produce flowers as days get longer.
Runner formation in day-neutral varieties is controlled by genetics so that certain varieties produce more runners than others. In addition, stress will promote runner formation (nutrient stress, water stress, high temperatures). To keep day-neutral varieties producing in the summer, remove all runners, fertilize weekly with nitrogen, and keep them well watered. Irrigating in the morning will help to keep beds cooler longer through the day and white plastic mulch will also keep temperatures lower.
Figure 1. Runner formation in day-neutral strawberries as seen in these Albion plants is largely variety dependent but can be induced by stress such as high temperatures.
Figure 2. Current temperatures have been ideal for flowering and fruiting in strawberries.
Figure 3. June-bearing strawberries starting to produce runners which is promoted by high temperatures and long days.