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How to Harden Off Seedlings

Acclimate your seedlings before you transplant.

Transplanting seedlings into your garden this year? Plan at least a week in advance to "harden off" indoor-grown seedlings before setting them into the still-cold earth. We need to acclimate indoor-grown transplants because the cushy confines of a warm home or greenhouse are really nothing like the great outdoors: Inside, seedlings are treated to steady temps, consistent light, and attentive watering; outside, seedlings face bracingly cold soil, scorching sun, wind, rain, and other tests of their endurance. Although plants are generally pretty forgiving, give them time to adjust to their new environment and they'll do much better.

A happy tomato seedling grown for our annual seedling sale in April. 

Take "Baby Steps."

Hardening off is an incremental process. Slowly introduce seedlings to outdoor conditions in "baby steps":

  • First, pick an overcast day when outdoor temps are 45°F or warmer. 
  • Begin by setting transplants out for 1-2 hours in a sheltered location protected from wind and direct sun; then, return your transplants indoors until the following day. 
  • For the next few days, gradually increase the time spent outdoors. Don't put plants out on especially windy days or when temps are below 45°F.
  • As plants adjust, move them to a spot with direct morning sun. The intense afternoon sun can burn tender leaves.
  • Gradually increase exposure to afternoon sun by moving them or leaving them to linger for longer in the same spot. 
  • Harden off plants over a period of 1-2 weeks, until seedlings can tolerate a full day of outdoor exposure.  If day and nighttime temps both hold at 50°F or warmer, your precious seedlings will be ready to plant! Choose an overcast day or wait until late afternoon to set them out.

Melons and Cucumbers are especially sensitive to cold temps, so be sure not to leave them out overnight if the forecast predicts temperatures lower than 50°F. Many gardeners keep row cover on hand to protect young plants from unexpected dips in temperature. Interested in planting seedlings grown by us? If you're local, you're in luck! Each year, we host a popular seedling sale for local pick up in Accord, NY. We'll announce the sale soon by email and on our Instagram page. Sign up for our newsletter to stay in the loop!