You could take the worst portion of one to use as a patches. Cut out a squareish shape an inch larger all the way around than the tear or hole.. Stitch around the edge, 1/4' inside the patch.and 1/4' outside the tear. That's it.
If the covers are mostly in good shape, it would be more cost effective to buy a yard of lightweight sew in interfacing to use for the patches. Check out the link below.
I always liked using row cover, but it rarely lasted more than a year. Toss in a windy season and it didn't stay together at all. I ended up saving the shredded chunks of it, stuffing old pillow cases and using those as insulation blocks in places in the greenhouse where the wind was getting in, like around the door, at the ends, where the plastic came to the ground but was moving around due to age, storms, the shifting of the soil there.
What I use now are sheer white curtains, which are quite inexpensive, come in 8-foot lengths, are meant to be in the sun, have two channels, top and bottom, to put rope or string through to secure them, and clothes pins also hold them on nicely. I've noticed that the home goods stores put the expensive sheer curtains at eye level, and the cheaper ones at the bottom of the display near the floor. So far they aren't cheaper in quality, they are just as nice, but are sometimes 1/2 of the price.
Don't fall for the My-Place-Is-Special, It-Won't-Happen-Here Syndrome.
Which weight cover are you using? the thicker covers last longer, and if it is for frost protection you can use frost netting instead which is much stronger and will last around 10 years, even being pulled about by a tractor to put it on and take it off.
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