One of the secrets to cultivating orchids successfully in this part of the country is to put the plants outside for the summer. Night temperatures usually don't fall below 60 degrees F during the months of June, July, and August and the humidity is high. Epiphytes thrive in these rainforest-like conditions. What confounds hobbyists, however, is how to provide proper light levels.
As the elevation of the sun changes during the year, it becomes a challenge to diffuse the light for outdoor plants. A partially shaded tree with orchids underneath may be ideal right now but, next month, the blistering rays may bypass the branches. Few orchids can take the intensity of 100% sun.
The effect of solar overexposure on orchids is well documented. The first sign is that the leaves are warm. Moving the plants to a shadier spot quickly fixes this problem, but if the excess sun continues, foliage starts to yellow. Soon after, expect various shades of brown and black. The plants might not perish but they will likely be set back for years.
One growing technique that is commonly practiced in southern climates is the use of a lath house which filters the light regardless of the position of the sun.
Lath houses became popular in the early 19th century with the invention of the circular saw which could cut thin narrow slats by the thousands. Originally used to hold plaster to walls, laths were later used as bars for venetian blinds and window shutters. Horticulturalists enjoy slats made into lattice which not only protect mature plants from sun and wind but also ease young seedlings and rooted cuttings into the world gradually.
Designs of lath houses vary but most are rectangular and tall enough to walk inside. The corners are held up with strong posts - typically 4"x4" wood beams. The cross boards are 2"x4" or 2"x6" pieces of assorted lengths. The key element in lath house construction is the slat which determines how much sunlight gets in the building.
Optimal slat spacing can be calculated but is usually determined through trial and error. Factors which affect the light transmission include not only the spacing between slats, but also the slat width and thickness. Slats are commonly milled from 2"x4" boards which have an actually width of 1 1/2". Cutting every 1/2" gives a uniform slat of 1/2" thick x 1 1/2" wide.
Growers have determined that spacing this uniform slat every 2 1/2" provides about 50% shading which accommodates the higher light orchid genera such as Cattleyas, Dendrobiums, Oncidiums, and Vandas. (Phalaenopsis are prone to crown rot from nighttime rain and are not recommended for lath houses.) Slats are attached not only to the roof but also to any side which might get direct sun. Inside the structure, plants may rest on benches or hang from the ceiling.
There are many types of wood from which a lath house can be constructed. The rot resistant trees of cedar, cypress, and redwood are beautiful but may be expensive. For the price conscious consumer, pressure treated pine may be used. (Since 2003, copper has been used to treat pine and the wood does not carry the health concerns of earlier preservatives.)
A lath house floor can be as basic as weed barrier fabric or pebbles. Watering is accomplished with regular rainfall and supplemented with a garden hose during dry periods.
By the end of the summer, growers will be amazed at the renewed vigor of their orchids, many of which are starting to bloom.