Master Gardeners of Northern Virginia

Over 40 Years of Growing Gardens and Gardeners

Phlox Subulata (moss pink) in landscape. Photo © Elaine Mills

May is the Time to Harden Off Seedlings

by Extension Master Gardener Nancy Dowling

🥕🍅 Welcome to a new column on vegetable gardening from veteran EMG Nancy Dowling,
a former coordinator at the MGNV Organic Vegetable Garden. 👩🏼‍🌾

In this series on growing vegetables from seed, we have covered how to grow seeds indoors (March) and how to safely transplant those seedlings into pots once they’ve established their true leaves (April). This month, we address hardening off, the process of exposing these transplants to outdoor conditions. Hardening off enables the plant to handle the environmental conditions—strong sunlight, cool nights, and less-frequent watering—they will face when they are planted outside.

This process normally takes one to two weeks and therefore should be started one to two weeks before you expect to move plants to your garden after the last frost date has passed. The pictures below show plants prior to hardening off and after one week outside. 

Young tomato plants growing in plastic cups on a shelf under a light.
Transplants ready for hardening out. Photo © Nancy Dowling
Several young tomato plants growing in small pots on a metal table outdoors.
Growth after one week outside. Photo © Nancy Dowling

Several factors should be considered:

  1. the temperatures expected outside during this period
  2. shade and protection from wind
  3. the gradual increase in exposure to the outside and to sunlight over this period
  4. continued watering throughout this process.

The ideal temperature for these young, vulnerable plants is between 50–70 degrees Fahrenheit. If the temperature fluctuates wildly during this period, as it has this year, then only sporadic exposure outside may be possible each day. This means taking them out in optimal temperatures and bringing them back in when temperatures are too high or too low. Cold frames are an excellent tool for helping alleviate the need to bring them in if temperatures are not drastically above or below 50–70 degrees. A makeshift cold frame might be rigged with garden cloth and/or plastic, as seen in the pictures below, or by using an outdoor table as a roof with cloth or plastic draped to the floor or ground and secured against wind and cold. Never let the plastic create too hot an environment or cut off air for too long. 

A balcony garden covered with protective fabric, showcasing rows of green plants and small containers.
Cloth covering for cool nights. Photo © Nancy Dowling
A covered outdoor patio area with a clear plastic tarp draped over a wooden surface, accompanied by metal furniture components.
Plastic covering for cold days/ nights. Photo © Nancy Dowling
Young tomato plants growing in black plastic and white cups on a wooden surface.
Outside and still watering from tray below. Photo © Nancy Dowling

The idea in hardening off plants is to introduce them to harsher conditions, like sun and some wind, over time. Strong sun exposure too early can cause the leaves to burn and slow their growth and development.

It is important to continue to provide water for these plants when they are outside, just as we did during the seeding and transplanting periods, so use outdoors whatever bottom tray you used for those previous stages and water from below. Over time, you can use less water, but do not let the tray dry out completely. Wilting is a sign the plants are too dry.

Over the one-to-two-week period, ideally you will see the stems thicken, the leaves fill out, and the plant increase in size. See photo below. Once the plants have adapted to shade, sun, and wind, and the temperature is cooperating, they can stay out overnight.