Emmalea Ernest, Extension Fruit & Vegetable Specialist; emmalea@udel.edu
Cool season vegetables like cole crops and lettuce have been transplanted into the field and the first warm season crops will go into the field soon. As you prepare to transplant, don’t skip the step of acclimating transplants to outdoor conditions before planting them in the field, commonly known as “hardening off”.
Every year, spring brings erratic weather with huge swings in temperatures, light levels and humidity. Transplants that have been growing in a controlled greenhouse environment are not ready to thrive in these conditions. Transplants should be placed in a semi-protected outdoor space for a week before being planted in the field. Suitable locations include a porch or pavilion, or an area with a building blocking prevailing winds and providing some shade. Limiting water and fertilizer during hardening will also help transplants acclimate to field conditions.
The hardening process will:
- Allow plants to build up the waxy cuticle layer on their leaves to keep them from drying out under dry or windy conditions.
- Acclimate leaves to higher light conditions to prevent sunburn.
- Slow growth and control transplant height.
These broccoli transplants were being hardened off. Plants in the center were a variety that didn’t root in as quickly as the others and wilted when exposed to wind.
Transplants that are hardening should be moved into a greenhouse or shed if freezing temperatures, thunderstorms or high winds are expected. Placing transplants on a wagon for hardening will facilitate this. As conditions become more favorable for establishment later in the growing season, hardening is not as crucial, but can still be important, especially if light conditions in the transplant production area are different than the field (i.e. shade cloth is being used on the greenhouse).