Greenhouse growing isn’t just for commercial growers.
Since Roman times, people have used manufactured materials to enclose and warm up growing spaces. The practice provides enough climate control to keep plants alive through cold weather. It also blocks some pests and diseases.
Adding insulation and heat to a greenhouse further improves outcomes, meaning there’s little a gardener can’t grow or keep safe when the mercury dips. Whether glass or plastic, greenhouses are a dream come true.
If you’re new to greenhouse growing, or just looking to get started with cold weather cultivation, this guide is for you.
Before you start
Adding a greenhouse or hoop house to your garden is one of the best decisions you’ll ever make towards increasing your garden harvest. Besides extending the season for cool weather crops and helping to hold tropical plants through the dormant period, greenhouses help you get a jump on the season by providing a sheltered space for seed starting. But there are a few things to consider before jumping into greenhouse growing.
Where should you put a greenhouse?
Location is key when installing a greenhouse. Proximity to tree roots and water, along with the amount of sunshine, will affect your greenhouse’s ability to produce for years to come. In northern latitudes, greenhouses are usually oriented east to west, with a clear exposure on the south side (that means no shade). This helps maximize the amount of sunlight received, as well as the heat gained.
In addition, the best greenhouse location will be level and well-drained. Since rainwater flows down a greenhouse roof the same way it does from a house, it’s important to consider how you’ll deal with the run-off. Gutters attached to rain barrels can easily deal with rainwater disposal while collecting moisture for irrigation. A simple perimeter foundation can also help: dig a trench six inches deep and eight inches wide and fill with gravel.
How big should a greenhouse be?
While commercial growers need ample space to grow and rotate crops for profit, home growers don’t need a greenhouse the size of a small apartment block to supplement their diets. But how big should you go? Consider the following questions:
- What vegetables does your family like to eat? Which ones need summer shelter to improve their harvest rates (e.g. tomatoes and melons) and how many would you like to grow? Which can you produce in your growing zone year round, with winter protection? If growing year round, winter could be the busiest time in your greenhouse, as you hold crops through the dormant season.
- Will you be growing any citrus or other warm-weather fruit in your greenhouse? Fruits like oranges, lemons and avocados will grow as far north as Canada if given the right protections, but they take up more space than annual crops like lettuces.
- Will you use your greenhouse for starting seeds? Overwintering tender, outdoor perennials? If so, you may want shelving space or tables to make access easier.
Ultimately, the size you choose will depend on your plans and ambitions. One rule of thumb says the average family needs 20 to 25 square feet of greenhouse growing space per person for vegetable gardening. Another says get one size larger than you think you’ll need. You’ll never regret it.
What type of greenhouse is best?
Freestanding or lean-to? Glass or polycarbonate? With so many greenhouses on the market, we produced a guide to answer these questions and more. Read more at Greenhouses: How to Choose and Where to Buy.
Preparing to grow
What is the best soil for greenhouse growing?
Most greenhouse sites will come with native soil that can be improved over time. Whether you add raised beds to your greenhouse or grow directly in the ground, you’ll want to amend the soil before planting to give your plants the best chance at producing. Knowing what to add comes down to three simple tests:
- Squeeze test: This simple test will help determine whether your soil leans towards sand, silt, clay or loam. Loam is the ideal base for vegetable gardening. Greenhouse soils made up of 60% loam, 30% finished compost, and 10% bark mulch or coconut coir will support most vegetable plants.
- A pH test: This test will reveal whether your soil is too acidic, too alkaline or too close to neutral. Once you have this information, you can amend your soil with lime (to increase alkalinity) or sulfur (to increase acidity) to achieve your desired pH. Garden vegetables and ornamentals prefer a slightly acidic pH between 5.8 and 6.5. Find out the pH of your soil using a pH test kit or digital pH meter.
- Soil nutrient test: Just like humans, plants have specific needs to thrive. The most important nutrients (also called ‘macronutrients’) that soils provide are nitrogen (N), phosphorus (P) and potassium (K), the same three listed by number on commercially available fertilizers.
Soils also deliver secondary macronutrients–such as calcium, magnesium and sulfur–to plants when available. Use a soil test kit to help determine what nutrients are available in your soil and what you might need to add. Organic fertilizers can usually add whatever nutrients are missing.
What to plant in your greenhouse
As noted above, greenhouses are the perfect place to grow a variety of fruits and vegetables. The most common choices include:
- Seedlings and starter vegetable plants: In late winter or early spring, seedlings that may die outside can thrive in a greenhouse environment. Daily temperatures can be many degrees warmer inside, and tender plants are protected from frost.
- Plants requiring more warm days to mature than your climate provides: Tomatoes, eggplant, melons and even citrus can thrive in a greenhouse when outdoor temperatures are nippy. Greenhouses can extend the growing season by many weeks for these plants.
- Plants requiring protection from pests and disease: Even if a heat-loving plant can grow in your climate, it may still suffer from damage. Greenhouses provide protection from scourges like tomato blight, which ravages otherwise beautiful crops in areas with cool, rainy weather.
- Cold hardy greens to eat over the winter: Lettuce, kale, chard, tat soi, pac choi and more: start these cold hardy greens in late summer to enjoy all winter long. You can add shade cloth where necessary.
- Perennials that need a warm space to overwinter: Tender plants like geraniums, dahlias, gladiolas and, yes, peppers will hold over the winter in many locations, providing the conditions are right. When resurrected in the spring, you’ll have an extra month or more of harvesting and enjoyment.
During the growing season
Once the season is in full swing, pay attention to the following factors to keep your plants healthy.
How often should you water a greenhouse?
Watering plants in a greenhouse is much like an outdoor garden, with a few important considerations. Greenhouses are hotter than outdoors, so soil will tend to dry out more quickly, especially if it lacks organic matter.
At the same time, greenhouses tend to be more humid, so moisture can accumulate in unwanted places (e.g. on plant foliage) if the greenhouse isn’t properly ventilated. A few simple tips can help your plants get exactly what they need:
- Use drip irrigation or soaker hoses to prevent spraying leaves and to reduce moisture loss.
Water early in the day to minimize evaporation and to prevent fungal diseases common in moist environments.
- Water seedlings daily. Their small and shallow root systems can’t reach moisture stored deep in the soil.
- Water established plants a few times each week, depending on their individual needs.
What’s the best way to control greenhouse humidity?
Since greenhouses are enclosed spaces, they tend to trap moisture, resulting in a higher humidity when compared to the air outside. Luckily, most vegetable plants like a humid environment, though too much humidity can lead to fungal diseases.
The easiest way to keep things under control is to keep foliage dry overnight and make sure your greenhouse is adequately ventilated. Side and roof vents are available on most manufactured models. Adjust these as needed. You can also install a humidity meter in your greenhouse if you’re encountering problems and want to monitor things more closely.
How can you make the most of your greenhouse space?
Greenhouses have less square footage than your average garden, so how can you plant to maximize your harvest? The easiest way is to plant successively, or to stagger your crops so one is ready to start when another is just about finished. It’s not as hard as it sounds.
Consider the most common greenhouse plant: the tomato. In many locations, tomatoes wrap up their peak growth in the early days of fall. The fruit tends to be higher on the plant at this time of year, leaving nothing but straggly leaves down below.
Removing these leaves opens up space in the greenhouse for lettuce and spinach transplants to thrive at the tomato’s base. When the tomato is finally finished, it’s easy to snip the plant off at soil level, leaving the bed free and clear for those cold hardy annuals through the winter.
Other ways to make the most of your space include planting intensively, broadcast sowing (instead of sowing in rows), and minimizing paths.
Controlling greenhouse pests and diseases
While greenhouses prevent many pests from taking hold, issues do happen. Thankfully, good ventilation and sanitation are the best ways to discourage pests and disease.
Other tips include:
- Keep your greenhouse clean. Dead plants, leaf litter, old pots or rags all provide havens for pests and spores that can overwinter or take hold.
- Watch for infestations. Colonies of spider mites or aphids can sneak up on you when your plants are big and your greenhouse crowded. Hand pick or use the controls mentioned in our linked guides. Addressing these problems early is the surest way to prevent them from becoming a bigger problem.
- Do a full clean once a year. Scrub your greenhouse from top to bottom, removing any algae or mold. This will also help light pass through more easily.
- Wash your hands when touching diseased plants. You don’t want to make an isolated problem bigger.
A proven way to grow
Getting started in greenhouse growing is exciting and rewarding. There’s nothing quite like eating a fresh salad in the middle of winter, or plucking a ripe tomato from your homegrown vine. With care and attention, growing in a greenhouse can become a favorite part of your gardening year.
Looking to buy a greenhouse? See our full line of greenhouse kits and accessories!