Note: These work for both spring and fall but you’ll have to modify some of the controls for larger or smaller plants as I note below
Here’s what I used to do in the nursery:
Watering The Plants
Last thing at night on cold nights, I’d go out and mist all the plants – put as fine a spray on top of them as possible but thoroughly soak them all down. By the time I’d finished, everything (including me) was thoroughly soaked through and wet everywhere.
First thing in the morning (and I do mean first thing) before the sun had hit the plants, I’d repeat the soaking. Sometimes there was frost or ice on the plants -that’s OK. That frost had given off heat in forming (remember to get colder, a material must lose heat) and protected the tender leaves.
When I put more water on that wasn’t freezing, it too added “heat” and melted off the frost leaving green leaves behind.
This technique works on many plants *if* they’ve been hardened off and the frost isn’t a heavy one.
Nothing is going to stop 5 degrees of frost but if it’s only 1 degree, then this technique works.
For *really* cold nights, it is far better to cover plants with a wet sheet or other insulating material (not plastic)
(I note this was a problem with the nursery when we had over 25,000 plants outside)
Covering The Plants
It is possible to cover with an old sheet and then soak the sheet with water.
Frost fabric also helps while the plants are short (tomato seedlings) but can be difficult if you’re trying to cover taller plants (mature tomatoes)
Cardboard boxes work – just put them over top of the plant and weight them down.
If you’re trying to cover a plant that’s very tender and breaks easily (impatiens or annuals) then sometimes it helps to drive stakes into the ground to help hold the cloth up over top of the plant. Think of a mini-tent so the weight of the fabric/water won’t crush the plants.
Plastic is next to useless – don’t even bother with the effort.
Here’s a source of frost fabric at decent prices
You can see other practical organic vegetable gardening tips here.