The great thing about growing your own food in an indoor or outdoor greenhouse is the control you have over the environment — as long as you use that control wisely. Here are some basic ideas and techniques to get you started on a lifelong love affair with greenhouse gardening.
Even for accomplished gardeners, the idea of stepping up from outdoor gardening to year-round growing in a greenhouse, grow tent or grow room can be intimidating. But it doesn’t have to be an all-consuming quest. You can get great results on a small scale, and build from there if you want.
First, decide what type of growing structure suits your needs and capabilities. It helps to go over some terminology here, because greenhouse variations are called by many different names.
Outdoor Greenhouse vs Cold Frame vs Indoor Greenhouse vs Grow Tent
In this blog, by “greenhouse” we mean a structure with the capability to mechanically adjust light, temperature, humidity, and airflow.
This doesn’t include “cold frames,” which are usually glass or plastic covers over plants in the ground, heated only by sunlight without any powered equipment.
An outdoor greenhouse typically has transparent or translucent walls and ceilings to make use of existing sunlight. In these structures, plants can be in containers, in the ground, or both. They can be free-standing or attached to a building.
We refer to the Eco Garden House as an “indoor greenhouse,” because like its outdoor counterpart, it has the ability to mechanically adjust light, temperature, humidity and airflow. It’s a relatively closed system, however, where you create an optimal growing cycle without the sun.
Some might call this a “grow tent,” which is fine. But it’s quite different than the simple shelving units with inexpensive clear plastic covers — among other variations — which are also called “grow tents.”
So for the following advice regarding greenhouse growing, let’s stick with the terms “outdoor greenhouse” and “indoor greenhouse.”
Here are 5 basic elements of greenhouse gardening, based on Rodale’s Organic Life magazine’s basic guide about outdoor greenhouse gardening, and on some of our experience with indoor greenhouses:
1. Temperature: Heaters, Vents, and Fans Are Essential
To maintain good growing temperatures in an outdoor greenhouse, you’ll need the capability to augment — or to counteract — the amount of sunlight you’re getting, and the outside temperature.
On a cold winter day, for example, you may need to add or subtract heat, depending on the amount of sunlight and on your outdoor greenhouse’s degree of solar efficiency.
The Organic Life article explains:
“On a sunny day, even at 20°F below zero, greenhouse air can heat up well beyond healthful levels. If the greenhouse is attached, you can move this hot air into your home. But in a freestanding unit, hot air must have a way to exit, and cool air a way to enter.”
“Passive vents allow for this sort of movement, as do thermostatically controlled exhaust fans and intake vents. Manually operated vents are relatively inexpensive, but you’ll need to check them at least twice a day, and open or close them as necessary. Automatic ventilation systems are more costly, but they save time and reduce the chances of excessive cooling or heating.”
A good indoor greenhouse should include electric vent fans controlled by a thermostat and an automatic timer. This way, you can set your unit to automatically maintain the optimal temperature for your plants.
Shade cloths and screens also help you regulate interior temperatures for an outdoor greenhouse. Remember that simply keeping sunlight out — as you might with a black shade cloth over the top of our outdoor greenhouse — might not lower the temperature enough.
Shade cloths with certain reflective exteriors can keep temperatures lower than a dark shade cloth.
With an indoor greenhouse, you’re making your own “sunlight,” so you don’t have to adjust anything to accommodate the weather. And if your indoor greenhouse is in a spare room or corner of your living space, you won’t have to worry about the ambient temperature.
If, however, you install an indoor greenhouse in a garage, breezeway, etc., that can get very hot or very cold, look for a model that has additional ventilation ports, so it’s simple to hook up a heater or air conditioner in certain seasons.
The optimal temperatures inside your greenhouse will vary widely according to what you’re growing. (Get yourself a vegetable germination/growing temperature guide such as this one from the Colorado State University Extension for outdoor “hobby” greenhouses.)
2. Air Circulation Replaces Carbon Dioxide, Strengthens Stems
In a closed system, plant growth can slow significantly if you allow the carbon dioxide they need to make sugars to reach low levels. In short, plants need fresh air every day.
Again, this is where vents and fans come in. Organic Life also suggests the low-tech solution of installing screened windows and/or doors in outdoor greenhouses:
“By opening a window on one end and the door panel on the other, you’ll have cross-ventilation. Positioning windows at the top and bottom of the greenhouse walls allows warm air to rise and escape from the upper windows, and cooler air to enter through the lower ones.”
Another important reason for good circulation in your greenhouse to simulate the breeze that some plants require for proper growth and/or pollination.
If you’re growing plants, such as tomatoes, that require help from wind, birds, and insects to get pollen from Point A to Point B, a small fan trained on those plants at the right time can do the trick. You may also need to gently jostle the flowering limbs.
Watch this four-minute video of hand-pollinating tomatoes from Young Urban Farmers.)
Some seedlings also grow better with an occasional breeze to strengthen their stems so they don’t become leggy. This is especially important if you’re using your greenhouse to start seedlings for transplant to an outdoor garden.
3. Humidity: Avoid the Rainforest Effect
For most vegetables and herbs, a super-humid environment (90% or higher) usually isn’t healthy. They can bolt early, sending up flowers that will claim the plant’s energy instead of the leaves or fruits you want.
Harmful fungal diseases also grow better in hot, humid conditions. Once they take hold, these can spread quickly from plant to plant.
High humidity is another reason for vent and exhaust fans. Keep the air moving and don’t over-water.
The same goes for indoor greenhouses, although they’re generally more protected from high humidity by your home’s heating/cooling system. An indoor greenhouse, however, may be susceptible to higher humidity is it doesn’t have proper drainage.
For example, a simple sheet of plastic as the floor may not do the job, especially if it has low spots where water collects. This type of floor is also prone to leaks, which can seriously damage the floor underneath.
Look for a drainage system that channels “gray” water out of the growing space. The Eco Garden House floor directs overflow into a tank beneath the floor. The tank has a pump that expels waste water through a standard garden hose attachment when the tank’s indicator shows that it’s full.
4. Light: Supplemental Light Can Be Crucial
Depending on where you locate an outdoor greenhouse — its orientation to the sun, the shade patterns cast by nearby buildings and trees, etc. — at certain times of year you’ll likely need to give sunlight a boost with artificial lighting.
The Organic Life article says:
“Fluorescent lights are very useful when you’re growing spring seedlings, particularly in cloudy regions. They can also give a boost to midwinter greens and the last of the fall-fruiting crops. Ornamentals will also be healthier and more attractive with supplemental lighting.”
“Shop-light setups suspended over raised benches are easy to install. You can choose fluorescent bulbs designed for plant growth, bulbs that mimic sunlight, or simply pair cool and warm bulbs in your fixtures. For intense light in a smaller area, another option is a compact fluorescent setup.”
Full-spectrum fluorescent lights are also an excellent option for indoor greenhouses. In this environment, the “sunlight” is entirely artificial, which means you have complete control over growing cycles.
Fluorescent bulbs such as the T5 high output lights used in the Eco Garden House emit a full spectrum of light but very little heat, so you can position them a couple of inches above plants at crucial stages — such as at the seedling stage — without burning their tender leaves.
In both indoor and outdoor greenhouses, it’s also helpful to have supplemental lighting from the sides of the structure, not just from above. You can more fully replace sunlight, which strikes plants from the sides as well as overhead during its daily arc.
Keep in mind, however, that your artificial lighting pattern must also allow the plants to rest in darkness every day. They need this time for the crucial food production stage of photosynthesis. That’s why a timer system for your lighting is so important.
5. Soils and Watering: Keep Your Soil Loose and Your Watering Schedule Tight
For a hobby-sized outdoor greenhouse, it’s probably easier to plant your vegetables in containers than in the ground. And the potting mix you use for containers should be less dense and more fertile than your basic garden soil.
According to the Organic Life article: “Good soil mixes drain fast, hold moisture well, contain balanced and slow-release organic nutrients, and have a slightly acid pH.” Organic potting mixes are the best choice for achieving this, and you have plenty of options from gardening stores and online..
OrganicLife recommends adding some compost and other amendments such as vermiculite each spring and fall, and a mid-season booster shot of nutrients such as:
*Liquid fish emulsion
How much to water vegetables in a greenhouse varies greatly according to the type of plant and your overall environment. One good tip is to use an automated system whenever you can.
Relying on your memory — and assuming that life won’t throw a few monkey wrenches into your daily routine — could cost your plants dearly. A drip irrigation system controlled by a timer delivers moisture to the roots of your plants, where it’s needed, at regular intervals.
Whether you choose an outdoor or an indoor greenhouse, you’re making a short-term investment for a long-term payoff of fabulous vegetables grown year-round by your own hand. If you’re an outdoor gardener also, you’re giving yourself an annual head start on vigorous seedlings to plant.
And of course, your greenhouse gives you more flexibility to experiment and learn in a controlled environment — an environment you’ll love to visit each day, if only to glory in the whoosh of rich green and earthy air.
Are you ready to grow your garden year-round?