Preparing Your Seedlings to Thrive Outdoors

  • Difficulty Rating: Beginner
Preparing Your Seedlings to Thrive Outdoors

It never fails. I’ve been guilty of the same crime: the untimely, overly optimistic and false belief that spring has sprung early.

All it takes is that one warm March weekend to lure us into the false belief that winter must be over. Our cabin fever has made us delusional yet again. The result, as always, is the premature planting of millions, maybe even billions of seedlings in gardens across America, thinking we’ve dodged the bullets of Old Man Winter. But alas, we didn’t. We rarely do and yet how often do we repeat this same exercise in futility, year after year? Usually within a few short days, sub-freezing temperatures are back and all our work and money spent on summer seedlings are down the drain.

Don’t despair. We’ve all done it. But sooner or later you should realize, it really is best to exercise restraint. Hold off on planting those classic summer vegetable and annual seedlings until the last risk of frost for your area has passed. Then and only then can you plant with confidence in knowing that your efforts will not be in vain. Lest we forgot, there are cool season crops, like spinach, lettuce, kale, collards and turnips, Brussels sprouts, peas and broccoli. They like the cold. In fact, in many cases, they even taste better after being exposed to a couple of frosty nights. Warm season crops on the other hand, dissolve into mush if exposed to the same conditions. You know the classics of summer: tomatoes, beans, squash, cucumbers, and peppers. But in our haste for that first vine-ripe tomato of the season, we often start too early.

Yet there are a few tricks to get your seedlings off to a faster start. First, know that all-important date of your last frost. The safest rule of thumb is to wait until after this date before planting warm season seedlings in the ground. An easy way to find out this date is to contact your local county extension service. You can use this website to locate the closest office in your area: The extension agent or Master Gardener on duty should be able to provide this information.

If you start your seedlings indoors, it’s always a good idea to work backwards from the last frost date when sowing your seeds. The seed package usually gives the number of days or weeks it takes from germination to planting outside. Use this as your guide and plan accordingly. However, when it is time to transplant seedlings outdoors, it’s a smart idea to “harden them off” first. The term means to toughen up your plants a bit, to gradually condition them over a week to 10 days to outdoor temperatures. Considering they’ve spent their entire life so far in a 70’ish degree climate, it can come as quite a shock to suddenly adapt overnight (literally) to near freezing temperatures.

In order to address this common condition, gradually expose the seedlings to more time outdoors each day. On the first day, place them outdoors for an hour, avoiding direct sunlight. On day two, add an hour. Continue this process, adding an hour each day and giving them more time in the sun. By the end of the week, assuming temperatures remain above freezing, you can leave them out into the evening, or even overnight. After a week to 10 days, and frost-free weather, plant outside with confidence that your plants can stand up to the sun and cool (but not freezing) nights.
If you purchase your seedlings from a nursery or garden center, unless you know for a fact that they have been left outside for several days and nights before you bring them home, this hardening off process should be applied to these as well. But you might be able to shorten the process by several days assuming they’ve had a head start on spending time outside. Otherwise they are just as susceptible to cold temperatures as the ones you might have started from seed at home.

If you want to try and get a jump on the growing season, and you just can’t wait until after the last frost date, there are still a few things you can do to hasten the planting season. One of the most well-know tools for this is called the “Wall O’Water”. Basically it consists of a perimeter wall of water-filled plastic tubes that encircle each seedling, one per plant. The water-filled tubes absorb heat during the day, and at night, release that heat around the plant, raising the ambient temperature a few degrees, and providing some protection from the elements. It works pretty well and is a favorite tool of most gardeners trying to get a jump on the season.

Another technique you can deploy is to use either store-bought or homemade cloches. A cloche is just a fancy name for a clear cover that traps the warmth of the sunlight and rising heat from the soil, enough to raise the ambient temperature around your tender seedlings a few degrees and prevent potential frost from killing your plants. Clear plastic milk jugs work great for this. Cut the bottom off and place the remaining part over your plant. Cut out part of the handle and place a stick down through the opening so that it is inserted into the soil, enough to keep your jug in place, even in high winds. One important cautionary point here; cloches work so well that if you don’t remove them in the morning or at least provide a way for the heat to escape out the top, you can cook your plants in a matter of hours. Again, milk jugs work well because you can simply remove the plastic lid each morning as a way for heat to escape.

Finally, you can use cold frames. These are mini-greenhouses, usually about 24-36 inches or so tall, set right into the garden beds. They usually have solid wood sides with a clear top lid made of glass or plastic. You often see them made with old windows, hinged onto the frame. They are fantastic for trapping and holding the heat of the day and keeping your plants warm enough to thrive through cold nights. And as before, be sure to vent the opening each day. Otherwise you will likely cook your plants, even on the coldest of days.