What they should say about knitted shadecloth is that won't unravel VERY FAR. In general it is much more prone to damage than woven shadecloth. Knitted cloth often comes without tape or grommets. This is only suitable for areas that are completely bordered with a method of complete linear attachment such as wood with a batten or commercial metal channel with wire lock. You cannot have a loose edge flapping around. If it is taped, it is probably ok, but you don't often see it taped. Knitted cloth stretches a great deal.
Woven on the other hand does not stretch, does not unravel when damaged and will usually come taped, although it can be purchased without tape if you have battens or wire lock channels. In general, it makes a better 'tarp' than knitted that can be attached with grommets. I have used both and prefer woven.
In your area 30 to 50% would be acceptable for maples, certainly no more dense than 50%. I live in an area that is much hotter and drier than VA and 50% works just fine most deciduous trees. If you have natural shade in the afternoon, I would use 30% rather than 50%. The latter would be more suited for an open, all day sunny location.
Brent
EvergreenGardenworks.com
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