Greenhouse Growing

Greenhouse growing is perfect for gardeners that want to extend their season and expand their growing opportunities.

Many gardeners find themselves restless in late winter, eager to get back into the garden. Utilizing hoop houses and greenhouses is a fantastic way to extend your growing season, your growing options and excite even the most seasoned gardeners.

High tunnels and Hoop Houses

These simple structures are easily built with PVC piping, rebar, 2x4’s and greenhouse plastic, with minimal carpentry skills required. A great introduction to greenhouse growing, high tunnels are an affordable way to provide plants with solar gain, as well as protection from excessive rain, snow, or wind.

Optimize high tunnels with multiple succession plantings starting in late winter with greens, radishes, and peas. Transition to summer cropping, heat-loving vegetables, such as tomatoes and peppers, and end the season with winter cropping vegetables like sprouting broccoli, chard, kale, and other greens. High tunnels can also offer protection from pests, and they are excellent at protecting tomatoes from late season blight. Building a potting bench in a hoop house greatly enhances the space by creating a peaceful retreat from winter rains, and an excellent place to start seeds in spring.

Glass Greenhouses

Glass greenhouses are more durable than hoop houses and high tunnels. A more permanent and structurally sound building, with more heating and ventilation options than a high tunnel, greenhouses offer even more season-extending abilities. Generally, the cost of greenhouses per square foot is greater than that of high tunnels, but with year-round growing capabilities a greenhouse can offer green thumbs and plant geeks the opportunity to delve deep into horticultural pursuits. Propagation, over-wintering tender plants, orchid-growing, and growing vegetables year-round are just some of the possibilities.

Things to Know Before You Grow

We are about to get scientific here. Don’t let that scare you off though. Remember that gardening is all about experimentation and learning by doing.

Humidity

Excessive humidity in the greenhouse promotes the germination of fungal pathogen spores such as Botrytis and powdery mildew. Condensation in greenhouses can make plant surfaces wet, which makes it easy for pathogens to spread from plant to plant. To suppress diseases by keeping plant foliage dry, managing humidity is essential. Here are our top tips for managing humidity:

Use drip irrigation instead of hand watering. Drip irrigation ensures water only goes where it needs to, the soil.
Ensure proper ventilation and heating. Ventilation allows moist greenhouse air to exchange with drier air from outdoors. Heating can help to bring outdoor air up to optimum growing temperature, and also increases the capacity of the air to carry moisture, thus avoiding condensation.
Air movement is another important consideration when managing humidity. When there is enough air movement, the air along the surface of foliage is prevented from cooling to below the dew point (forming condensation). Small fans can be very effective at moving air through a greenhouse. Adequate air movement around the plant occurs when the leaves move slightly.
The optimal relative humidity level for the majority of plants is around 80% at 27 degrees Celsius. Temperature affects humidity. Higher temperatures mean more evaporation and lower humidity. The opposite is true for low temperatures.
A hygrometer is a device that measures the relative humidity in the air. It’s a great tool to help you know when it’s time to take steps to change the humidity.
More information

Temperature

Temperature goes hand-in-hand with humidity in creating the ideal environment for plants in a greenhouse. The challenge is keeping temperatures down in the summer, but warm enough in the colder months.

The first thing we recommend is a max-min thermometer, which can tell you the lowest temperature reached overnight, the highest temperature reached during the day, and the current temperature. Max-min thermometers are designed to be reset every 24 hours. There are many digital smart thermometers and hygrometers available now as well. With this information it will be much easier to when to adjust the temperature in your greenhouse, and here are the best ways to regulate and alter greenhouse temperatures.

Keep it Warm

Reemay is a gardeners best friend. It has excellent insulating properties and can protect tender seedlings or be used to wrap borderline plants in the winter.
Thermophilic compost piles have the ability to produce a substantial amount of heat. Consider putting some compost in your greenhouse. Be cautious of attracting rodents if your compost has a lot of kitchen waste.
Heat mats or electric heating are often a last resort for home gardeners, but if used sparingly supplemental heat can drastically affect a gardeners ability to utilize a greenhouse in the colder months.
Plastic domes are ideal for young seedlings, and can provide just enough protection to prevent seedlings from being frosted.

Keep it Cool

Shade cloth can be essential in the summer months to reduce solar gain.
Water stored in rain barrels or IBC tank can help to regulate temperature in both the summer and winter.
Ventilation and fans are essential in regulating temperature. Roof vents are ideal in glass greenhouses, and roll up sides are ideal in polytunnels
Plant strategically. Planting basil at the base of tomatoes will prevent basil from sun scorch. Plant more heat tolerant plants on the west or south side of your greenhouse, and less sun tolerant plants on the east or north side of your greenhouse.

Troubleshooting: Greenhouse Pollination

The walls of a greenhouse have the potential to keep more than the elements out. It’s important to be aware of pollination in a greenhouse to have maximum fruit set. Inside the greenhouse, it is advisable to grow some plants that will benefit pollinators. Read our article here on the best plants for pollinators.

Tomatoes:

Tomatoes are the most likely greenhouse crop to be affected by a lack of pollinators. If left alone, tomatoes will have a 20% – 30% fruit set rate without the help of pollinators or hand pollination.
Tomatoes can be pollinated by hand using a paintbrush pollination technique. Simply rub the paintbrush on the inside of the flower lightly back and forth. This encourages the pollen to drop down to the female portion of the flower. Flowers will wilt and fruiting will begin when pollination is successful so keep a keen eye out for this.
A breeze is also enough to assist with tomato pollination so sometimes all that’s needed is to give the plant a good shake or to turn on a fan.

Cucumbers:

Most cucumber varieties are monoecious with unisexual flowers—have separate male and female flowers within the same individual— and require pollination for reproduction.
Some varieties are mostly or totally gynoecious (produce only female flowers) and can produce fruit through parthenocarpy. These cucumbers are often marketed as ‘greenhouse’ or 'glasshouse types' of cucumbers are usually seedless as they set and develop fruit parthenocarpically (without pollination).
If you are growing a field or garden cucumber and do not have pollinators visiting your plants, hand pollination may be required. It’s a little more complicated to do than hand pollinating tomatoes. Click on the link for an excellent description of this process.

Our Favourite Vegetables to Grow in a Greenhouse

Cool Season Crops

Radishes: Seeds can be sown when temps are as low as 10 degree Celsius, and radishes grow at lighting speed.
Peas: Choose cold-resistant for early producing peas.
Lettuce: Can be grown all winter long in a greenhouse. May benefit from being covered with reemay on the coldest nights.
Asian greens: Fast growing greens like Tah Tsai and Yu Tsai Sum mature in 35 - 45 days and prefer cool temperatures as long as there is no frost.
Carrots: Seeds can be sown in raised beds in greenhouses in February for an early spring harvest, or sown in late summer for a winter supply

Warm Season Crops

Tomatoes: Grow indeterminate (vining) tomatoes in a greenhouse and train with string to make the most of your space.
Peppers: This vegetable thrives in the heat. Hot temperatures also make spicy peppers even spicier
Sweet Potatoes: Plant a slip into a large black pot in the spring and harvest in the fall. Foliage acts as a great groundcover and weed suppressant as an added bonus.
Basil: Grow under tomatoes to prevent leaf burn in the heat of summer.
Cucumbers: Can be trained on a trellis, or grown directly on the ground.
Nasturtium and Marigolds: Great annuals to underplant vegetables with to invite pollinators into the space.

Why We Love Greenhouse Gardening

Experimentation: Want to try to grow cardamom, ginger or luffa gourds? Greenhouses are a great place to experiment with growing plants that need more humid conditions and a longer season.

Starting seeds: For those short on indoor space a greenhouse or polytunnel can be a great place to start seeds. Get yourself a grow light, heat mat, timer, reemay, and some plastic domes and you’re set.

Borderline hardy plants: Greenhouses are great places to overwinter agaves, cactus, subtropical plants, and citrus. Even without supplemental heat, the insulation of reemay or blankets on cold nights can be enough to prevent plants from having cold damage. Putting Christmas lights around citrus is a tried and true way of keeping temperatures at their preferred minimum of 3 degrees Celsius.

Propagation: For those ready to take the next step in their horticultural journey a greenhouse is an asset when propagating plants as it allows you to control the environment more than the great outdoors. Controlling moisture and humidity is essential for cuttings, and a greenhouse can provide a much more humid environment.