Marijuana Horticulture: The Indoor/Outdoor Medical Grower's Bible

than glass. During the day, this can help keep plants cooler, but at night the heat loss requires the use of an artificial heat source. Poly Weave™ is a plastic fabric made of 8-mil polyethylene reinforced with nylon mesh. It transmits up to 90% sunlight, can be sewn or taped, and has a lifespan of up to five years.

Two months after planting, the grower used black plastic to induce flowering with a 12/12 day/night photoperiod.
    Polyvinyl chloride (PVC) is two to five times more expensive than PE but can last five years or longer. Polyvinylchloride is pliable, transparent or translucent, and comes in four to six foot (1.2-1.8 m) widths which can be sealed together to provide a super-wide piece. Ultraviolet inhibited corrugated plastic panels provide another option. The panels can be used in cold frames, propagation houses, and greenhouses to provide excellent wind and snow protection and optimal solar heat collection. UV inhibited corrugated plastic also has insulating properties (2.5 R insulation/3.5 mm panels, 3.0 R/5.0 mm panels).

The long 12-hour nights induce visible signs of flowering in about two weeks. These plants have been flowering for almost a month.
    Corrugated fiberglass is lightweight, strong, and comes in eight to twelve foot (2.4-3.6 m) panels. Poor grades will discolor, reducing light penetration, but a good grade of clear fiberglass can cost as much or more than glass. Its lower weight is an advantage, and it is more difficult to see through!
    Lexan™ http://www.geplastics.com/gelexan/ is a thermoplastic that lasts for years and transmits almost as much light as glass while retaining heat. Clear panels like those in glass or Lexan™ may require shading during the heat of the day. Again, there are anumber of options. You may select a roll-up shade of wood or aluminum, or a shading compound that is painted onto the outside of the glass. Vinyl plastic is a flexible film that installs easily against wet glass inside the structure and is reusable.
    Framework and covering are only the beginning. Growing plants in a greenhouse is often more demanding than growing plants indoors. Air temperature, humidity, light, and air quality must all be controlled in relation to a constantly changing greenhouse climate.
Climate Control
    Even the best greenhouses will lose heat through radiation, conduction, convection through glass, walls, and floor (or soil), and also through vents, doors, and cracks. To counteract external variables, the internal structure of the greenhouse is, in some ways, more complex than the selection of framing and covering materials.
    All greenhouses need ventilation and most need fans. Look for an extraction fan with the capacity to change the air once every minute. Capacity refers to the amount of power needed to circulate the air volume of your structure.
    Calculate the volume by multiplying the square footage of your greenhouse by the height. Multiply the volume by sixty air changes per hour to get the cubic feet per minute (cfm) capacity of the greenhouse.
    For example a greenhouse with the following:
    8 × 12 × 7 feet (2.4 × 3.6 × 2.1 m) greenhouse requires a fan with a cfm of 40,320 8 × 12 × 7 feet × 60 minutes = 40,320 Here is a similar metric example: 2.5 × 3.5 × 2 = 17.5 m 2 × 60 minutes = 1050 m 3
    The combination of louvers and fan will force the hottest, most humid air out while protecting the plants from draft. See Chapter Thirteen, “Air,” for more information.

Big strong buds are a few weeks from harvest.
    Vents control temperatures in all seasons and improve growing conditions. Hand-operated roof vents will require frequent checks, or you may install automatic vents with an electric motor and thermostat that will respond to conditions around the clock. Venting is important with a cold frame, too. The high-end models have wax-filled vents that operate automatically, opening when theheat rises in the frame and contracting as the temperature cools. You can find the paraffinfilled “Optivent” and many other greenhouse supplies at www.charleysgreenhouse.com .

This good looking ‘Mekong Haze’ is an outstanding sativa cross.
    Heating systems are important to keep plants healthy during cold nights. Cannabis grows well with night temperatures of 60-65°F (16-18°C), but colder nights will require an additional heat source for sustained growth.
    You can turn a cold frame into a hot frame by insulating it with manure or heating it with steam, hot water pipes, or