Choosing fabric weight for spring vs. fall
Last lesson was about knowing when a row cover is the better tool than the cold frame. Today let's talk about which cover, because "row cover" isn't one product. It's a whole shelf at the garden store, and the weight you grab matters more than people think.
Row cover fabric is rated by weight, usually in grams per square meter. You'll see it labeled a few different ways depending on the brand, but the practical breakdown is this:
- Lightweight (about 0.5 oz/sq yd): Barely there. Mostly for bug protection and a couple degrees of frost buffer. Lets a lot of light through.
- Midweight (about 1.0 - 1.5 oz/sq yd): The workhorse. Handles a light frost, maybe down to the mid-20s depending on how it's set up.
- Heavyweight (about 2 oz/sq yd or more): Serious frost protection, down into the low 20s or high teens for some products. Blocks more light too.
Here's what actually happens if you don't think about this: you buy one roll of whatever's on the shelf, use it in April and October both, and wonder why it either cooks your seedlings in spring or lets everything die in a hard fall freeze. The fabric doesn't know what month it is. You have to match it to the job.
Spring: go lighter than you think
In spring you're usually protecting young, tender transplants from an occasional late frost, not fighting a real cold snap. Lightweight or midweight does the job. You want light getting through, because spring seedlings are trying to grow, not just survive.
Spring is also when the sun angle is climbing fast and afternoons warm up quick even with cold nights. A heavyweight fabric traps too much heat under there on a sunny 55-degree April day. Same problem as the cold frame lid, honestly — you cook the thing you're trying to protect. I use midweight for most of my April cover and only reach for heavyweight if the forecast has an actual hard freeze in it, like below 28.
Remember my opinion on frost dates: they're a suggestion, not a promise. The valley will burn you with a warm week in April followed by one nasty night. Keep a heavier cover on hand even after you think you're past it, because you probably aren't.
Fall: go heavier, and go heavier sooner than feels natural
Fall's a different problem. You're not protecting something young and fragile, usually — you're stretching mature lettuce, spinach, root crops, whatever's already established, and you're fighting real cold that's only going to get worse, not better, as the weeks go on.
I run midweight through most of October and switch to heavyweight once nighttime lows start flirting with the mid-20s. That was the fall I kept lettuce and spinach going into the first week of November, the one I mentioned a few lessons back. Heavyweight cover, double layered on the coldest nights, and I didn't say much about it to anybody. Just quietly liked knowing I'd stretched it further than most people bother to.
One thing about fall cover specifically: light matters less. The days are already shorter and the plants aren't pushing hard growth anymore, they're mostly just holding on. So you can afford the light-blocking tradeoff that comes with heavier fabric. In spring you can't, not as much.
The part nobody warns you about
Whatever weight you're using, you're probably watering under it at some point, even if it's just occasional. And this is where I'll bring up something that took me most of a summer to figure out.
My drip emitters kept clogging on the far end of a bed, second season I gardened here, and I could not figure out why that end kept struggling while the near end did fine. Turned out it was the hard water. Utah County water is loaded with mineral content and it builds up inside emitters over a season, slow and quiet, until flow just stops on the far end of a line. I finally soaked the emitters in vinegar and got them working again. Half the valley has this problem under their row covers and cold frames and doesn't know it's the water, not the equipment.
So if you're running irrigation under fabric all season, check your emitters now and then. Pull one at the far end and look at it. If it's crusty white, that's your hard water, not a bad part.
A quick rule of thumb
If you only buy one weight to start, get midweight. It covers spring frost protection and most of fall without hitting either extreme. Add heavyweight later once you know how hard your fall actually gets and whether you're the type to push lettuce into November like I do.
Fabric's cheap compared to losing a bed of transplants or a good stand of fall greens. Buy more than you think you need. You'll use it.
Before next time: dig up whatever row cover you already own, or look up weights on what's available locally, and figure out which category it falls into. We'll talk about hoops and anchoring next, and it helps to know what you're actually working with.