You Need to Know Shade Values and Installation Tricks for Greenhouse Shade Cloth
This fabric comes in a variety of uv-resistant materials that resist degradation and you should be able (with some careful handling and installation) to get several years out of each sheet before it begins to degrade. There are some materials on the market that also claim to have sunlight-repellant characteristics but I haven't seen any data from researchers on the practicality of this on the home garden scale.
Greenhouse shade cloth is available in different densities for different areas of the continent. So if you live in the north with a USDA zone rating, you're probably looking at shade factors between 30-50% reduction in light. In the retail houses at my nursery, we used 50% and it worked well for us. Lower percentage of shade would let you grow more roses for example, but on the home greenhouse, I'd go with the 50% in the north.
Southern gardeners might be well advised to take upwards of 70% shade protection because of the higher sun-levels. You'll need this extra protection to keep that greenhouse at any temperature less than a sauna.
I always purchased a full roll for the commercial greenhouse and had grommets installed around the edges at the factory. On the home or hobby greenhouse, you might not be able to get grommets installed so there's an alternate system below. Grommets reduce the tearing and increase the lifespan of the greenhouse shade cloth so if you can get them at the size you require for your greenhouse (unlikely on the home scale) go with it.
If you can't get grommets, spread the fabric over the greenhouse roof and use 1x2 pieces on the edges. Now, many home greenhouse owners simply tack the greenhouse shade fabric to these 1x2 boards. This is wrong.
Do not tack the fabric to the board. Wrap the fabric around the board (actually, you put the edge of the fabric on the board and roll the board up to get it all wrapped up. Then you tack the board to the greenhouse. The wrapping (several turns at a minimum) holds the shade fabric in place and there's no pulling at any one spot to create tears.
Treat the entire outer edge of the fabric this way. Roll every outer edge around a 1x2 board and tack or screw the board to the greenhouse.
Put it on on a reasonably warm day, make it as tight as you can. It will stretch a bit in the heat and pull down a bit at night. If you put it on cold, it will be tight but will become quite floppy in mid-summer under the heat of summer sunshine.
Remove all greenhouse shade fabric in the fall.
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