Four Ways to Protect Plants from Frost
It’s May! The weather is beautiful! Much better than last year, when May tried to pretend it was June.
Except it’s going to get down to 39 degrees tonight here in central North Carolina, and frost is predicted for Saturday. I jumped the gun and already planted my tomatoes, peppers and basil. What to do??
I’ve been nursing these tender little seedlings since February, starting them myself mostly from seed I saved, watering them with care, hauling them out on fine days so they can get some extra sun. I can’t bear to see them harmed. Even in a normal year, I would not be eager to run to the store for starts to replace plants killed by frost. At several dollars a plant, I really can’t afford it. The hardware store and the greenhouse do not carry the varieties I have grown so attached to. And I’m not doing any unnecessary trips to public places, given the virus situation.
Luckily, there are some easy inexpensive (or free!) DIY solutions for a late frost, if you know ahead of time that one is immanent.
First, it’s important to know which of your plants are in danger. I’m not concerned about the starts and seeds I planted in March. Cabbage, collards, carrots, beets, peas, turnips, radishes, arugula and mustard spinach are all tolerant of temperatures down to 32 degrees, which is why they are in the early planting group in the first place. In contrast, members of the family Solanaceae (peppers, tomatoes, eggplant, potatoes) do not tolerate frost. Cucurbits like cucumbers, melons, squash and pumpkins are similarly tender.
If you live in the US, your local Cooperative Extension Service can give you a calendar of suggested planting dates for every vegetable you can think of (for local readers, here’s the one for our area). The predicted low is 33, so I’m concerned for the tomatoes, potatoes, peppers, basil and the sad young figs I started from cuttings and have been nursing for a year.
1. Cold Frames
For the figs, the solution is obvious. Last fall I heeled them in (this just means buried) in their pots inside a cold frame. I’ve got three cold frames now going on three years old and still functional, each made from a single sheet of ¾” plywood and a $5 storm window from the local Habitat for Humanity ReStore (total cost: about $90). Apparently no one uses storm windows anymore, because there is always an ample supply available used from the ReStore or on Craigslist. With frost expected, I just popped the storm window back on the frame. It will protect the figs and also the seedling onions planted in front of them.
I also put the window on the cold frame where I planted lettuce, spinach and cilantro starts. These crops won’t die at 33 degrees but they may get a little leaf damage, and I’m looking forward to harvesting some of that stuff next week. Protecting them will keep their growth rapid and reduce their stress, extending the time until they bolt (make seed). I need to be careful to keep the window cracked during the day, though, because even at 50 degrees daytime temperature, on a sunny day it can definitely get way too hot inside the frame for lettuce, which will also cause it to bolt.
A cold frame is a pretty amazing structure. Simple and robust, it can last for years and extend the harvest of cold-tolerant crops well into the winter without supplemental heat, even in climates as far north as Maine. Leave the snow on to add an extra layer of protection for very cold nights. Brush it off to get some sunlight in for earlier spring growth.
If you have a raised bed and the plants aren't much above the sides of it yet, you're already half way to a cold frame. Just cover it temporarily with something overnight. A window is ideal but a sheet of plywood would also work if you pull it off in the morning. Even stapling a trash bag over it will give some protection.
2. Row Cover
For the poor little tomatoes, I sacrificed the second layer of row cover that has been over my cabbages and other brassicas, which are quite tolerant of the temperatures we’ve been experiencing, even down to 33 degrees. The double row cover has done its first job of speeding their growth as young seedlings when the weather was even cooler, and now some have already been harvested while some are approaching maturity. I left one layer to do the row cover’s second job, which is preventing cabbage white moth from laying eggs. Without row cover, I lost all my cabbage and most of the rutabaga to cabbage white last year.
Yesterday when I opened up the brassica row to do some weeding and thinning, I discovered a handful of volunteer tomatoes that had sprouted under there and tolerated all the swings in temperature over the last month. They’re a similar size to my pampered indoor starts, so I know for sure a double layer of row cover is sufficient for tomatoes to thrive, if they are well-enough established and not suffering from recent transplant. I left them nestled there among the cabbages, and will use them to replace any of my transplants that meet a band end in the next couple of weeks.
If you’re thinking forward to fall and want to have some handy row cover of your own, this is the one that kept my beets, chard and arugula alive all through the winter. (That is an affiliate link, and there may be others in this post. My commission supports this blog first, and then the Cool Effect). The nice thing about row cover is it breathes well, keeping the plants from overheating or molding, and it is permeable to rain or water from a can, so it’s not hard to keep things watered.
If like me you’re desperately trying to remedy a planting error, you don’t have time to wait for shipping. For brief periods a sheet can do the same job, but a sheet is heavier, and you don’t want it weighing down your plants especially if wet. If all you’ve got on hand is a sheet, you may want to use it to construct the next frost solution.
3. Low Tunnel
Basically, this is MacGyver’s tiny cheap greenhouse protecting my recently transplanted basil and peppers. I’m concerned the basil is too fragile to tolerate even a light row cover, and the peppers are rather tall. I used a bunch of lengths of pretty substantial wire to make hoops, pressing the cut ends several inches into the ground (any wire will work, but these are the scraps from constructing my awesome tomato cages).
Then I covered those hoops with what looks to be 6 mil plastic. I had this roll kicking around the barn, and actually cannot even remember buying it. It might be salvage. It was the right width when only partly unfolded, so it’s several layers. The ends need to be pretty well secured with stakes or weights (in this case bricks from the chimney of the old farm house we tore down). If you don’t have plastic and aren’t willing to go to Lowes or similar right now, a sheet will work.
I scrounged up some string, tied it to the bottom of the first hoop, and passed it over the top of the plastic and then around the next hoop to secure the plastic. It’s best to bend a loop into the hoop to hold the string down especially if you’re using a sheet, but several layers of plastic is probably stiff enough to do without the loop temporarily.
The most labor intensive portion of the whole project was untangling the string. After we used it to mark the placement of the posts on our new pole barn, I carefully stored my string in a wad inside a tire out in the field. Because a true homesteader always keeps their string in a wad. Apparently.
Several winters ago I kept my whole fall garden under low tunnels like this, and while it pretty much worked, I wasn’t pleased at the wind resistance of the structure over time, or with the ease of access. Row cover is better on both counts, but I’m out of that and the low tunnel will do for this week. Like the cold frames but unlike the row cover, it’ll get too hot in there if we get sunny days, so I’ll have to watch it and lift up the sides of the plastic for ventilation.
The potatoes took some damage at 40 degrees the other night, and they are too far apart and too awkwardly arranged for a low tunnel. I copied my mom, who covered her few tomatoes with plastic 1-quart yogurt containers. It took every spare bucket and every empty pot I could find lying about the farm, plus a couple of mom’s yogurt containers for the little ones. It’s not pretty, it’s wind-vulnerable and this is a labor-intensive solution, because each pot has to be pulled off during daylight hours and replaced at night. But it’ll work for a day or two, and that’s all I need.
Each of these methods is good for about five degrees of protection, which is enough for my particular needs this weekend. The tomatoes won’t be happy, but they’ll live and recover. To get more protection, the methods can be layered. A cold frame under a tunnel might be worth eight or ten degrees of protection, and row cover under a cold frame under a tunnel could give you 12 to 15. That’s enough to get carrots and beets through the winter in most of the US!
My absolute favorite source for these and other season extension ideas is Elliot Coleman’s Four Season Harvest. The text is inspiring and the diagrams are excellent. As I learn to feed us more and more from our land, I’m leaning more on his advice about choosing varieties for the particular conditions in which I want to grow. What a delicious and satisfying way to eat.
If you’re in for a frost this week, I hope your delicate tomatoes are safe on the windowsill. And if they’re not, let us know in the comments how you protected them.