Whether you grow your own seedlings (Congratulations!) or buy your garden plants, they will need some preparation for the transition out to the “real world”.  There are big differences between indoors (house or greenhouse) and the tough world outside and plants need a little help to adapt to those hard conditions outside.  This process is called “hardening off” and it is easy if you follow these basic steps. 

Hardening off is a stressful transition for plants because they have to adapt to multiple environmental factors at once.  In addition to getting direct sunlight outside vs through a window or from artificial lights, they also are dealing with new temperature swings (day to night), wind stress (flexing & dehydration), and varying moisture levels.  The best approach is to make sure plants have some time to get used to the different conditions. Think of a pale Vermonter hitting a tropical beach in January for too long in the sun!

Depending on how you raised the plants indoors and the species, most plants do best with a gradual transition over 10-14 days.  Start by putting them outside on warmer, cloudy days (or in a shady spot) for 2, 4, 6, then 8 hours per day.  After 4-5 days of this they can be placed in direct sunlight for about 4 hours per day for a few days and then full sun all day and overnight for a couple days before transplanting.  Keep them moist and protected from strong winds. 

A good preparation for hardening is to “exercise” upright plants (tomatoes, peppers, zinnias, etc.) with an oscillating fan blowing on them for 4-6 hours each day on low (use a timer).  Alternately, if you gently brush over them with your hand or a rod 10 times back and forth to imitate a breeze several times a day they will develop thicker, stronger stems and not grow so tall.  Commercial greenhouses always have fans running to exercise plants and to control white flies and harmful soil fungus (“damp-off”).  If you would like to see how hardening is done there is a good, short video on hardening off plants at  https://www.almanac.com/video/how-harden-plants

All this is under optimal circumstances, assuming you have the time.  This is just a target to shoot for, few people will actually do the whole transition perfectly so do what you can.  It’s important, I think, to understand how the young plant physiology has to change and try to avoid sudden changes, especially into the direct sun.  Finally, there is little advantage to getting plants out at the earliest possible date, so a delayed transplant is probably better to minimize transplant shock.  Please refer to my previous article on “When To Plant” for some more guidelines.

What experiences have you had with hardening off plants?  Please share them in the Comments below.