Thanks to: Burramys Parvus
A greenhouse is any structure with a covering (usually glass) which is used to control temperature and humidity in the cultivation and protection of plants.
When you have a greenhouse, you effectively have your own little micro-environment that you can control. You become the God of your own little patch of land! Greenhouses dont always have to be made out of glass. Many modern greenhouses are made of plastic, or specialized materials. These materials come in a variety of color, size and thickness. Choosing materials carefully will ensure you have the best greenhouse for your situation.
Greenhouses can be simply DIY jobs using some timber and a drill, or poly tunnels using poly tubing and some polythene sheets. They can also be large custom built structures, using metal and concrete. For most people, a small DIY greenhouse (about 10-20 square meters) will do just fine.
What are some advantages?
Ability to control temperature
The greenhouse design lets light in, and when this light is absorbed by objects inside the greenhouse and turns to heat energy, it is not permitted to escape. The air temperature in the greenhouse will exceed the outside temperature. If it gets too hot, all you have to do is open up some of the ventilation panels (or just open the door, depending on the design) and the temperature will drop. Greenhouses are able to regulate temperatures; temperature fluctuations can stress plants and slow growth.
Ability to control pests
As most greenhouses have a pretty good covering over all the structure, pests can't get in as easily as they could if your plants were just out in the open. This also applies to seeds and even pollen from unwanted plants (such as weeds).
Ability to control humidity
The air-tight covering on a greenhouse causes it to become quite hot and humid inside during the day time. The moisture evaporating from the soil, and the moisture given off by photosynthesizing plants (transpiration) fills the air. Once the air is very humid, it becomes harder for plants to lose water through evaporation, and likewise with the soil. This helps to keep everything from drying out on a hot sunny day. Therefore, it is essential to have air circulation to exhaust excessive humidity and regulate air exchange.
Nosey neighbors will have their view obstructed by your greenhouse if you choose to use slightly shaded glass/plastic, which still lets enough light through for strong plant growth, but is opaque enough as to obscure vision from the outside.
Protects your plants from adverse weather conditions
Storms can't blow your plants over and tear them to shreds when they're safely inside your greenhouse! Also helps protect plants in areas where frosts are common.
Excessive fall rain can cause powdery mildew and mold that can effect the final crop.
How does all this environmental control help?
How do higher temps help my plants?
Every plant is made up of cells. Every cell has hundreds of chemical reactions taking place inside it at every moment of the day. These reactions would be very slow, if it wasn't for enzymes. Enzymes act as catalysts for all the chemical reactions taking place in cells (including photosynthesis).
Enzymes work best in certain pH's and temperature ranges, depending on the type of enzyme. Too high or low pH, and the enzymes will work slower than normal, or might even denature (die). Too low a temp, and the enzymes will also not function to capacity, and too high results in them being denatured. Warm -but not hot- temperatures usually result in maximum enzyme efficiency, which means faster plant growth. Having perfect temperatures is like super-charging your plants!
How does higher humidity help my plants?
It helps slow the rate of evaporation from soil and plants, as the air already has a large amount of water suspended in it. Plants use water during photosynthesis (6H20 + 6CO2 (plus sunlight)= C6H12O6) so they need a constant water supply during sunlight hours. Having to battle with high evaporation rates is something they don't want! By lowering this, it enables them to photosynthesis more without losing water to the point of their leaves wilting.
A greenhouse is a great idea for outdoor growers who live in cold climates, or areas where pests and horrible acts of god are a common problem. It also helps hide your crops from anyone who may be having a quick look over their fence. If you want maximum control of your outdoor grow, a greenhouse is an absolute essential part of your garden!
Are there any disadvantages to growing in a greenhouse?
Well, no not really! Growing in a greenhouse is more-or-less just like a regular outdoor grow, except you have much more control over it. The one thing I would say might be a bit disadvantaging would be the humidity. It may increase the risk of mold on your buds. This is easily fixed by opening the greenhouse up a bit and letting some more fresh air in. If you're paranoid about mold attacking your plants in a greenhouse, you can use Potassium Silicate to protect your plants, which is easily available at most gardening/hydroponic shops.
NOTE - Depending on the size, and construction of the greenhouse, you may or may not need permission from local authorities. If they want an inspection, it's probably best to have it cannabis-free for a month or so.
If they want further inspections, it might be a good idea to grow all your 'special' plants in pots, and keep the greenhouse door LOCKED. When someone comes around for an inspection, simply hide the plants somewhere else until the inspection is over. I have never heard of a medium size suburban greenhouse needing to be inspected once, or on a frequent basis. Although, it may be best to check this with local authorities, just to be safe.
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