Just yesterday we were tending to some immature onion plants out in the garden:
Outdoor Plants: Insulation is Key
For green thumbs like us utilizing mulch-gardening methods, keeping plants toasty warm on chilly spring evenings is as simple as pulling some hay around and over your seedlings. We can't stress this method of gardening enough—especially if you live in a place with hard-to-love soil. For us, planting is so simple you don't need tools. Nor do you need any fertilizer, undue amounts of irrigation, or weeding. Read more about mulch gardening here.
If you don't utilize mulch-gardening methods, you'd be wise to keep your plants well-mulched at least throughout early spring. To do this, simply buy a square of hay (or more if you have a big garden) and put a couple handfuls around each of your plants. Don't have access to hay or straw? Other great insulators are bark, sawdust, newspaper, and compost.
Cold Frames are another great way to protect your outdoor plants early in the season. But don't bother spending upwards of $200 for one—just scavenge some old windows and scrap wood to make a cold frame all on your own. For more information on this, click here.
Don't have any of the above, and anticipate a frost? Track down an old blanket, tarp, or drop cloth and throw that over your plants. In a pinch, this simple act can save all your precious plants. Just be sure to take the cover off during the day to allow your plants access to light and air.
Greenhouse Plants: Turn Up the Heat
While your greenhouse may soak up the sun during the day, on a really cold night your plants will still be susceptible to early Spring chills.
Some gardeners beat this issue by heating their greenhouse with compost. That's right—they just make a hot compost pile right in the center of all the action. This is certainly a practical way to go—but we avoid it because of the potential for it to attract additional mice (mice have a particular fondness for young squash plants and seeds).
You can also find some discarded, 50-gallon drums or horse-tub waterers, paint them black, fill them with water, and put those in the greenhouse along the south wall. They will absorb the heat during the day, and release the heat all night long. The same will work if you paint some plastic juice bottles black, fill them with water, and put them on the same shelves as your plants.
When you buy your plant flats, buy them with clear, plastic lids. These lids are great for insulating your seedlings when they're still very young.