Grow Strawberries with Kratky
Lesson 14 of 14
Strawberries are a fruiting crop, so they build on Lesson 13: a bigger container, topping up and pollinating by hand. They add one rule of their own: keep the crown out of the solution.
Choose day-neutral plants
June-bearing strawberries fruit once a year and need shorter days before they flower. Day-neutral strawberries keep flowering and fruiting through the season, which suits a grow light on a timer. In a University of Delaware indoor trial under 14-hour days, June-bearing types flowered poorly, while the day-neutral varieties 'Cabrillo', 'Royal Royce' and 'Monterey' gave the best production. 'Albion' is another widely sold day-neutral variety (Cornell University).
Start from plants, not seed
Strawberries are slow and uneven from seed. Start with bare-root plants from a nursery or with rooted runners, the small daughter plants that grow on stems from a mother plant. Cornell researchers root runners straight in water, which is close to what happens in a Kratky container. If your plants come in soil, rinse it gently off the roots before planting.
Keep the crown dry
The crown is the short, thick base where the leaves and roots meet. When Cornell researchers rooted runners in water, they held each plant so that only the very bottom touched the water, because constant wetness can lead to crown rot and poor rooting.
- Set the plant in the net cup with the crown at or just above the rim and only the roots below it.
- Fill clay pebbles around the roots to hold the plant upright, not over the crown.
- Fill the solution until it just touches the bottom of the net cup, as in Lesson 8. Never higher.
Container and solution
- Size: a strawberry plant is much smaller than a tomato, but it fruits for months. Give each plant more solution than a lettuce jar holds, or grow several plants in one covered tote.
- Nutrients: use a hydroponic nutrient, and as with other fruiting plants, many growers switch to a formula with more potassium once flowers appear (Lesson 13).
- pH: aim for 5.5 to 6.0, the same range as Lesson 7.
- Topping up: follow Lesson 10. Add solution only to the lower roots and keep the air gap, so the crown stays dry.
Light and temperature
The Delaware trial grew day-neutral plants under LED lights for 14 hours a day, at 72 °F by day and 65 °F at night. When the growing space went above 86 °F, the plants almost stopped producing new flowers. Keep strawberries out of hot spots, and keep the solution cool as in Lesson 9.
Pollinate by hand
Indoors there are no bees and no wind. Flowers that are poorly pollinated often give small or misshapen berries. A SARE-funded project is now testing whether hand pollination improves yields and reduces malformed fruit in Kratky strawberries.
- Once flowers open, brush the center of each flower gently with a small, soft paintbrush.
- Move from flower to flower so pollen spreads between them.
- Repeat every day or two while the plant is flowering.
Runners and upkeep
- Runners: snip them off as they appear, so the plant puts its energy into fruit. The Delaware growers removed runners every week. Keep one or two if you want new plants.
- Old leaves: remove brown or dead leaves at the crown so air can move around it.
- Pests and disease: powdery mildew and spider mites were the main problems in the Delaware trial. Check the undersides of the leaves during your weekly check (Lesson 10).
Harvest
Pick berries when they are red all over. Strawberries do not keep ripening after picking, so a half-white berry stays half-white. Pinch or snip the stem just above the berry rather than pulling it.
Your task for this lesson
Get two or three day-neutral plants and set up one per container. Write down the date of the first flower and the first ripe berry, so you know how long your setup takes.
Sources
- Helmer, J., Timmons, M.B. and Mattson, N. Cornell University CEA. CEA Strawberry Runner Propagation for Deep Water Culture.
- Johnson, G. and Ervin, E. University of Delaware, SARE project ONE22-417. Providing a Research Base for Indoor Lighted Production of Strawberries in a Repurposed Poultry House.
- Heiden, N. Levo International, SARE project FNE25-117. Investigation of the Importance of Hand-Pollination and Root Inoculation for Strawberry Production in Simplified Hydroponics.