Spring and fall gardening is loads of fun when you're trying to outsmart the weather. That statement can either be wildly sarcastic or spot-on, depending on how well you've prepared for unexpected cold snaps.
If you've been gardening for any amount of time, you've learned not to rely on local weather forecasts to tell you when to run out in your skivvies and protect your plants from a surprise frost. A garden on one end of town can be devastated by a sudden temperature dip while another just a half-mile away sailed through it with no ill-effects. Even within your suburban-sized garden, you'll have different microclimates, affected by sun exposure, dips in the soil, raised beds, and windbreaks.
Aside from the whole climate change thing, we mortals have some simple solutions to Mother Nature's shoulder-season shenanigans.
What causes frost?
Ready for some math? Frost occurs when surface temperatures dip below freezing (32°F) while the air temperatures remain above. When moisture in the air clings to plants and soil, ice crystals form.
In a light frost, the soil temperatures don't stay suppressed for long, allowing the plants to recover. But when they dip more than a few degrees below freezing for several hours or more, we're dealing with hard frost territory — the kind that turns our beloved annuals and seedlings to mush as cells suffer damage from cellular expansion and contraction caused by the freeze/thaw cycle.
Frost damage prevention methods: The good, the iffy, and the ridiculous
Not to be Captain Obvious here, but one battles frost using one or both of the following approaches: Heat retention through insulation or active heating of the plants and soil. The not-so-obvious runner-up is beating the weather to the punch.
Smudge pots and fans
Have you ever seen that ridiculously saccharine movie A Walk in the Clouds? If you have, then you'll never look at a vineyard without picturing Keanu Reeves and Jaqueline Bisset wearing wing-like paddles as they fluttered down the rows fanning fire-warmed air over precious grapevines. Winegrowers and orchardists still use "smudging" today, though they've mostly replaced oil and wood fuels with more eco-friendly propane or natural gas, and mechanical fans to distribute the heat.
If you're a pyro, go ahead and try this at home. Be sure to check your insurance policy, though, and scale things down as appropriate. Or just leave out the fire and stick with the fans. Stagnant air speeds up the freezing process, so keep it moving.
Cloche is the French word for bell, and in the early 1600s, French gardeners — who perfected high-density urban market gardening — used bell-shaped clear, blown-glass domes over their plants to protect them from frost. These eventually evolved into the oxymoronic square cloches, or what we'd recognize as cold frames.
Back in the day, The French heated their cloches and cold frames with rotting horse manure gathered from the streets of Paris. You can try the same with your early-stage compost or scavenged livestock poop as long as the animals weren't fed fodder sprayed with aminopyralid, clopyralid, or picloram herbicides. These will kill most garden plants.
Nowadays, we can make our cloches from stuff we'd usually send straight to the recycling plant. Put the reuse in "use, reuse, and recycle" with these DIY hacks, starting with the milk jug cloche:
- Remove and discard the bottom of a one-gallon milk jug.
- Place the jug over your plant, avoiding contact between the foliage and the plastic.
- Adjust ventilation as necessary by fiddling with the lid.
For smaller plants, use one-liter bottles. You can experiment with any kind of clear or light-permeable rigid plastic covering; I've used the lids from cakes and pre-roasted grocery-store chickens. As with old-school glass cloches, though, you'll have to tip them up to let fresh air in, or better yet, cut a few vent slits in the top.
High tunnels (also called "hoop houses") are arched buildings covered with rigid or sheet clear plastic. They're usually made from PVC, tubular aluminum or steel ribs, or cattle panels wired together and braced at the bottom. These are effective, low-cost alternatives to greenhouses, though militant HOA officers might get snitty about them.
Low tunnels are simply scaled-down versions of the above. The skin traps heat through solar radiation, and even when you throw them together after dark, the air trapped within serves as an insulating buffer.
Some green thumbs prefer to cover the low tunnels with garden cloth — thin, non-absorbent material made from woven polyester or polypropylene — that helps insulate plants from frost while aiding with ventilation. They'll trot it out again when bug pressure's high, as it can protect tender veggies from insect predation.
Floating row covers
Most market and home gardeners define floating row covers as woven garden cloth gently draped over their row crops. While garden cloth comes in rolls, you can cut them to size to drape over individual plants. Sure beats running out at 2:00 am to toss garbage bags and your partner's undershirts all over the yard and looks slightly better, too.
By "organic," we mean easily bio-degradable. Pile up some fluffy chopped straw, dried leaves, garden wool, or even shredded newsprint to insulate seedlings, small plants, and surrounding soil. Anytime you mulch, though, keep moist material at least an inch away from plant stems to avoid rotting.
Plastic sheet mulch
Clear plastic, whether used as row covers or sheet mulch at soil level, becomes doubly effective when moisture condenses on its surface. Clear plastic works best when air gaps between it and the soil so moisture droplets can refract and intensify solar radiation.
This is why clear sheet mulch is twice as effective at warming the soil as is black, opaque sheeting. Under optimal conditions, it can raise daytime soil temperatures by as much as 14°F.
The clear stuff will trigger early germination, but this includes weed seeds. That's not necessarily a bad thing, as you can trick weeds into sprouting in early spring or late winter, then hoe 'em up or uncover them for the next freeze or frost. The idea is to get them to expend themselves well before it's time to plant your spring garden.
Dense structures absorb heat during the day, slowly releasing it as ambient temperatures cool. One way to keep your plants from frosting up is to dig a few more empty milk jugs out of your neighbor's trash, fill them with water, and set them among your plants. Even better? Paint the jugs black, or cover them with black trash bags to make them more heat-absorbent. Or — duh — just fill them with hot water before you set them out.
Plants withstand cold weather best when they're well-hydrated, so a thorough watering at soil level, while the temperatures are well above freezing, is an excellent way to prep your garden for a frost. This is especially true of newly-transplanted trees, shrubs, and bedding plants with underdeveloped roots. Irrigate them early in the day to give the plant's cells and the soil itself plenty of time to soak up insulating moisture. And since wet soil serves as better thermal mass than dry soil, it will retain that heat longer.
If you live where the ground doesn't completely freeze up, maintain a consistent irrigation schedule. Cold, dry winter air dehydrates soil and plants faster than you'd think.
The "Fight Freeze With Freeze" approach
Here's a hard-core Hail Mary against frost damage: Use mist emitters throughout the night to glaze your plants with a protective ice coating. While this seems to be the world's best example of counterproductivity, if it's done right (huge if), the quickly-forming ice will seal in and insulate foliage and flower buds.
This method isn't recommended for the faint-of-heart. It's a last-ditch effort used by citrus growers in Florida or California, but if it's done incorrectly, it can destroy the plant, tree, or shrub. Even when it's done right, your neighbors will think you're an idiot.
Container garden frost protection
Speaking of low-hanging fruit, here's an easy solution for protecting your potted plants: Put them in an unheated outbuilding. We'd say "bring them inside," but your home's room temperature might throw your little darlings into shock. You could slowly acclimate many plants to warm indoor conditions, but pre-frost panic isn't a good time to make the introduction. Most garages or garden sheds will be a few degrees warmer than unprotected spaces. Or, move the pots against a south-facing wall to let thermal mass work its magic.
Grow the right varieties for your region and season
Some plants don't just survive but thrive when they've gone through late spring or early frost. Choose frost-tolerant varieties recommended by local gardeners, and purchase fresh seeds harvested from naturally-resistant genetics. Once you've figured out how your favorite veggies, vines, and ornamentals manage in the various microclimates in your garden — with or without the above preventative measures — you can push the limits with more delicate plants.
So don't get caught with your pants down the next time frost falls in your garden. As much as tighty-whities will keep your young tomatoes snuggly (we're talking about actual tomatoes here), there are better ways to save — or extend — your growing season!