Buy This in August or Your Fall Garden Ends on One October Night
- Fall Garden
- Frost
It is somewhere in the nineties as I write this and the last thing on my mind is a cold night. That is exactly the problem. The fabric that saves a fall garden has to be bought while the tomatoes are still splitting, because the night you need it is not a night you get any warning about.
It usually goes like this. You do everything right in August. Lettuce and spinach in by the middle of the month, a second row of radishes, kale transplants going in strong. September is glorious and the bed looks better than anything you grew all summer, because fall greens genuinely are better. Then somewhere in the first half of October a clear, still, dry night drops the yard to 30 for four hours before dawn, and by eight in the morning half of it is translucent and slumped. The garden center sold out of frost blanket a week earlier. The bedsheet you threw over the bed at eleven at night was touching the leaves, so the leaves it touched froze anyway.
None of that is a gardening failure. It is a shopping failure that happened in August.
What the fabric is actually doing
Row cover carries no heat of its own. What it does is trap the heat your soil absorbed all day and is radiating back out after sunset, and stop that heat from rising straight into a clear sky.
Which is why the whole thing hinges on clear and still. On a cloudy, breezy night at 34 degrees, nothing in your garden is in trouble. On a clear, dead-calm night at 34, the air a few inches above your lettuce can be several degrees colder than whatever the thermometer on your porch says, because everything on the ground is dumping its heat upward into open space with nothing to bounce it back. That is the night that kills. It is also the night row cover is built for, because a sheet of spun fabric is a ceiling.
Two things follow from that, and both are where people go wrong. The cover has to go on before sunset, or in the early evening at the latest, while there is still heat in the ground to catch. Throwing it on at eleven o’clock is throwing a lid over air that is already cold. And the edges have to be sealed to the ground with bricks, boards, soil, sandbags, whatever you have. A cover flapping loose at one end is a chimney.
Which weight to buy
The number on the bag is ounces per square yard, and it is the only spec that really matters. Heavier fabric holds more heat and lets through less light, so you are trading growth for protection.
| Weight | Frost protection | Light through | What it is for |
|---|---|---|---|
| ~0.5 oz | a couple of degrees | most of it | Insects, wind, a marginal night |
| ~0.9 oz | around four degrees | roughly two thirds | The all-purpose fall choice |
| ~1.5 oz and up | six to eight degrees | about half | Hard freezes, short-term use |
For a fall vegetable bed in most of the country, the mid weight is the one to own. It will carry greens through the first several frosts without you having to take it off on cloudy days, and it is light enough that hoops can be flimsy. The heavy stuff is worth having a smaller piece of for the genuinely bad nights, but leave it on for a solid week of gray weather and your spinach will stretch and go pale.
Buy wider than the bed. The fabric has to drape well past the edges to be anchored, so a four-foot bed wants at least six feet of cloth.
The hoops matter more than the fabric
Wherever the fabric rests directly on a leaf, that leaf is in contact with the outside temperature and freezes on schedule. You will see it the next morning as a stripe of damage running across the bed exactly where the cloth sagged.
Nine-gauge wire hoops, half-inch PVC bent over rebar stubs, old fencing wire cut into lengths. Every three or four feet. What you want is an air gap and a shape that sheds a downpour instead of collecting a puddle that sits on the plants and freezes solid.
A cold frame does the same job with more commitment and better results, and if you have ever thought about building one, now is the month, while you can still get the lid square without wearing gloves.
What it does and does not save
Frost protection buys weeks at the front of the cold season, not months, and it buys them for the crops that were already inclined to survive. Kale, spinach, mache, leeks, parsnips, collards, most Asian greens: these want the cold and get sweeter after a frost, and a cover mostly keeps their leaves undamaged so they stay worth eating. Lettuce, chard, cilantro, beets and carrot tops all take a light frost under cover and keep going for a good while.
It will not save peppers, basil, beans or tomatoes. Those are done at the first real frost regardless of what you drape over them, and the effort is better spent picking everything green that night.
One more thing to have straight before you commit to a covered bed is your own first frost date, because the whole plan is built around it. Garden Pro Planner keeps a planting calendar tuned to your local frost dates, so you can see how many weeks a crop has left before the average first frost rather than guessing from how the weather feels.
Order the fabric this week. Put it somewhere you will find it in the dark, with the bricks, because the night you need it you will be doing all of this at dusk with a flashlight in your teeth, and the difference between a fall garden that ends in October and one you are still cutting from at Thanksgiving is about twenty minutes of work you did not have to improvise.
