In summer, greenhouses can be cooled considerably by ventilation, but the extreme heat of the sun in some sections of the country makes shading necessary. Even plants that require considerable warmth cannot stand full summer sun; they need shade or semi shade for good growth and flowering. There are various means and materials for shading. Some simple installations are only temporary, others are longer lasting.
Paste or powder shadings can be applied with sprayer, paintbrush, or roller. One gallon of paste mixed with benzine or gasoline covers about 600 square feet of glass. Two applications are necessary one in spring when protection from sun first becomes necessary, the other toward July when the earlier application will have been almost washed away by rain. Because it weathers unevenly, paste or powder shading is always unsightly parts of the greenhouse may be washed almost clean by spring and summer rains while protected areas of glass are still fully covered. A paint roller gives the smoothest application unless you are an expert with a sprayer. Powder shading gives briefer protection than paste, since it mixes with water and washes off more readily. Powder and paste both come in white or green. I think green is less objectionable; white looks like whitewash. (Incidentally, whitewash should never be used on an aluminum greenhouse it contains lime that causes serious deterioration of aluminum. Either shading paste or powder may be used freely since neither contains lime.) Paste is sold by the gallon; powder comes in cartons, usually 25 pounds each. Both are cheap enough, but the cost is recurrent, since the supply has to be replenished yearly and application takes a lot of time, too.
Wood panels or aluminum slats, made in sections to fit the glass panels of most hobby greenhouses, offer a far more attractive means of shading than paste or powder. Aluminum panel slats come in kits for easy assembling to aluminum-painted wood frames. Slotted fasteners slip over the round-head screws already in place on the greenhouse frame. Panels are easy to install in spring, quickly removed when shading is no longer necessary, and of a convenient size to store.
Panels are made for roof, sides, and the ventilator section of the greenhouse. They can be installed overall or only where they are needed, a great convenience if you are growing mixed plants with different shade requirements in the same house. Once panel shades are in place, they are usually left on all summer, and this is something of a disadvantage on sunless days when plants are deprived of much-needed light.
Wood roll-up shades are more practical than panels or slats. True, they are far more expensive, but they do offer the most convenient and a most attractive means of shading. Made of redwood slats linked with non-rusting metal clips, they are factory assembled and come complete with hardware, ropes, and pulleys, all ready for installation. Fastened outside just below roof ventilators, they are long enough to extend to the sill. A system of ropes and pulleys makes them easily adjustable for sun and other weather changes.
These roll-up shades usually come covered with two coats of aluminum paint to reflect the heat of the sun, but they can also be obtained unpainted if the natural redwood blends well with the color of your home, or they can be painted in some harmonizing color more attractive than a stark aluminum finish. As these shades do not cover roof ventilators, you will have to use other shading for this area if your plants require complete protection from sun. Here paste or powder will do, or a vinyl plastic; or you can hang a green netting rather like cheesecloth on the inside under the roof sash. This netting not only cuts down sunlight but if it is installed from the bottom of the roof sash on one side to the bottom of the roof sash on the other side, it also screens out insects.
If you prefer plastic, try the 8 mil flexible, light-green sheet film that is proving so satisfactory. It comes in rolls 29 inches wide by 25 feet long. In my present greenhouse, two gable sections of glass were covered with it, and it still adheres nicely after twelve months even though it was left on all winter. Application is to the inside of the glass, is simple and attractive if carefully handled, and transmission of sunlight is reduced by as much as 65 per cent.
The whole greenhouse may be shaded, even roof vents, and while the plastic reduces the intensity of the sun, it is sufficiently transparent for visibility into or out of the greenhouse.
To install it, cut a piece the exact size of the glass section to be covered. Then moisten the glass with a moderate stream of water from the hose or go over it with a wet sponge, making sure it is evenly moist. Place the sheet of plastic against the inside glass and smooth it down (as you would wallpaper) with a wallpaper brush, squeegee, or plastic scraper. Be sure to get rid of air bubbles and give particular attention to the corners. If the job is done properly, surface tension of water between plastic and glass holds the shading firmly in position.
When days get shorter in fall and shading is no longer needed, loosen the comers of the plastic sheets, peel them away from the glass, wipe off any moisture that remains on the plastic, and store the pieces flat. With a little care in handling, this plastic shading can be re-used for years.
You may not want to buy proper shading your first summer. In that case, use your ingenuity. I have seen cheesecloth, green netting, even discarded dacron curtains, serve very well as temporary inside shading. A wire fastened along the eave line will give support to material stitched or stapled over it. Drapery rings sewn along the top edge of the material permit it to be drawn over the glass or pushed to the sides as desired.
The time for application of shading varies from one section of the country to another, and is also influenced by the location of the greenhouse. Therefore, it is impossible to give definite dates; the condition of your plants will be the best indication. If soil seems to dry out unusually fast or foliage looks droopy early in the day, or seems somewhat paler than it should, it is probably time to protect plants from the gradually warming sun. Timing the removal of shading in fall is easier, for a decrease in the amount of daily sunshine is always evident and a hint to you to get rid of any deterrent to full sun.