ksgrower: Yes it can help. About a 50% is what I'd go for. I like the woven black polypropylene because of cost and durability. Some of mine is still functioning after 30 yrs of on and off use. I like the tape and grommets on the edges. The woven type will unravel without edging.

One of my favorite topics, since I love conifers, but have a property with minimal full sun spots. In my experience pungens will punch you in the face if they are placed in shade, yet engelmann will tolerate it rather well (at least when young). We will have to see if it continues to thrive as it ages. The same is true here with pinus strobus, they will grow in hemlock dense shade when young and still look full and healthy, but they are just waiting for an "event" to occur, that will allow them to surge up toward the sun. At some point in their maturity, I assume they begin to decline if no opportunity for sunlight is presented Tom & Dax are both correct about looking to the species natural habitat and response to competition, to gauge how the cultivars of that species will handle shade. Searching the web for "shade tolerance" of conifers will generally yield the same answers about yew & hemlock. To dig deeper, you need to read through some of the scholarly papers that have been written, some of which have actually run controlled experiments on growth rates and overall health of conifers when covered with 30%, 60%, ect... shade cloth. So far, I have had really good luck with Abies concolor in just a few hours of direct sun, then bright shade. I decided to give it a shot after finding one growing in very dense shade near my house. It was a little bit sparse, but considering the amount of deciduous shade it was growing in, I was very impressed. lasiocarpa is supposed to do even better, but I have not yet experienced with this species. For my shaded spots that I don't want to use tsuga chinesis or diversifolia, I plan to use conifers with white/light foliage such as Picea orientalis 'Sulphur Flush', Cryptomeria japonica 'Knaptonensis', & Picea sitchensis 'Bentham's Sunlight'. *Also, I have been planning to get more scientific about how much sunlight my conifers get, as it changes from season. I found this really cool YouTube video on how to use a solar analog alarm clock to record direct sunlight hours precisely. Measure Direct Sunlight Hours Video. That alarm clock can be found on amazon for $14Equity by La Crosse 65901 Hybrid Solar Analog Alarm Clock This post was edited by SC77 on Mon, Nov 17, 14 at 16:48

I dont have anything especially helpful or definitive, I'm afraid, but an idea that you might or might not be able to use. When we lost a huge maple in the center of our yard (I hated and loved it - the kids had their tree house out there and there were swings hanging from the branches, but it was a very messy, greedy tree) I looked into these triangular "shade sails" as a way to maintain some shade in the yard. They are pretty inexpensive, and come in different colors and sizes. I always thought they'd look quite nice in the right setting. This would be a different way to go than covering the plants, but might work for you. Here is a link that might be useful: shade sail at overstock

If you live in a humid area, misting or fogging may not be enough to cool your greenhouse in those hot, humid summer months. When all else fails you can create your own shade by putting shade cloth on your greenhouse. Shade cloth is a commercially available material for hanging over your greenhouse glazing to cool it in summer months. They are usually made of loosely woven polyester or even aluminum and can be found in varying densities or degrees of shade from appx. 5% to 95%. Remember that shading will affect your plant growth so chose as low a density as you can get away with. Most plants will do best with a maximum of 40% - 60% shade. When growing orchids, 75% or higher maybe needed to get correct light levels. Choosing a shade cloth with grommets will make it much easier to install. Use nylon cord or rope to secure the shade cloth to your greenhouse frame. If your greenhouse has roof vents that open you may have to install your shade cloth in sections to accommodate this. Shade cloth is open weave fabric so it can go directly over windows. Its best to attach the shade cloth so it can be rolled up or retracted as needed. There are also compounds available to be applied to your greenhouse glazing. These will wash off if you receive much rain and have to be reapplied. If you get one that is not water soluble then you have to use a cleaner to remove it when summer is over.