Setting out, bringing in
For a seedling, going from a warm living room to the outdoors is like arriving at Dulles Airport from Honduras, in spring, without even a sweater — especially for tender seedlings like those of tomatoes. Even hardy ones like broccoli are uncomfortable below 45 degrees when first set out, because life on the inside has not prepared them for it.
So it helps to set seedling flats outside for an hour or so per day at first, gradually increasing the time over a week or two. Put them in a spot where it’s easy to bring them in and out. Better yet, use a cold frame with an automatic temperature-activated arm that will open and close the top as needed — a great timesaver! If you have lots of seedlings, the indoor-outdoor game is like housebreaking a puppy.
Acclimating to sun
An even greater risk is the strong spring sun. Unless your plants have been in a greenhouse, they will not be ready for outdoor light, which is much more intense than that of a sunny window or a plant light. It would be like flying back to that beach in Honduras without sunscreen, and it might permanently weaken or even destroy your precious crops.
If you have a tree that’s leafing out, set your flats in its dappled shade. Or drape that cold frame with a few layers of translucent row cover, and remove one each day. I use an outdoor metal table with an open lattice pattern that lets in partial light. I can throw a blanket over it on scary cold nights.
Preparing for wind and rain
Other big shocks for those young plants are the stress of wind, which can dry out their tissues, and the moisturizing but potentially bruising effect of pelting rain. Outdoors, in reaction to these forces, they’d develop thicker cell walls and thicker, stronger stems. It’s the same principle (called thigmomorphogenesis) that makes young trees, allowed to sway in the breeze without rigid staking, form sturdy trunks.
You can help seedlings prepare for wind even before they go outside. Use a small fan, placed at a distance, or just sweep your hand gently along their tops whenever you walk by. If they are in a greenhouse or some other spot where you can use a hose, watering from above with a wand is a good rain simulation.
If you buy your seedlings from a garden center, all the same cautions apply. Have they been indoors there, or under shade cloth or lattice? If so, you may need to harden them off just as you would if they were home-grown.
If, on the other hand, all these little plantlets have been sharing the house with you for weeks on end, you may be delighted to give them, and you, some breathing room.
Damrosch is a freelance writer and the author of “The Garden Primer.”