If you love homegrown vegetables like myself, but live in less temperate climates than most, the most effective and sensible way to be self-sufficient when it comes to veg is by using a glass house or cold frame. This can extend your gardens growing season and is exactly what I have done this past year and I’m well-happy with the results. In this article, you’ll learn how to use a greenhouse to extend your growing season!
Unlike professional growers, I’m not inclined to heat my greenhouse. However, I do find that by making full use of my glasshouse and homemade cold frames, my garden’s growing season is vastly extended. I do use a couple of small heated propagators to start off my seedlings and this seems to work pretty well. Best of all, it’s without the expense and effort of heating my entire glasshouse.
Are you looking to extend your garden’s growing season by a few months? Or plant crops the whole year through? Either way, a greenhouse or cold frame is an invaluable year-round growing environment.
Where to Site Your Greenhouse
There are many aspects to take into account when siting your greenhouse and cold frames. These all have a huge impact on your garden’s growing season.
Let’s take a look at some of the main points to remember:
- You want to site your glasshouse or cold-frames in an area that receives at least 6 – 8 hours of sunshine per day, preferably south facing.
- Good ventilation is paramount to avoid environmental conditions where pests and diseases can quickly spread.
- Cold frames need to have a hinged, liftable lid, or a removable lid to allow for air circulation.
- Avoid exposed and windy sites—ideally choosing a more sheltered outside area
- Although your plants will need sunshine, they’ll also need shading from the sun’s strongest rays. Make sure they have shade throughout the middle of the day to avoid scorch damage and drying out.
- Make sure you have a good hard standing base, at least for your walkways of your greenhouse. These are easier to clean, keep tidy and are safer to walk on.
- Try to site your greenhouse where you have good access to an outdoor tap
- Leave pathways wide enough to allow for a good sturdy wheelbarrow to travel through
Although many of these points listed may seem like common sense, it is quite common to overlook important “ease of use” factors.
How to Use a Greenhouse or Cold Frame
An unheated greenhouse will usually be too cold for early crops. Regular glasshouses provide a warm environment for tender plants all year round and for starting off seedlings. Generally, once seedlings have gained vigor and temperatures outside are warmer, we move our plantings outdoors. This, however, isn’t always successful due to temperature fluctuations, but problems can be avoided by using cold frames.
Cold-frame greenhouses are different from traditional ones, and certainly much smaller. These use energy from the sun (solar energy) and rely on insulation to create a mini microclimate within the garden. Not only is this growing environment great for growing early seedlings and overwintering tender plants, it also acts as a halfway house for plants destined for outdoor growing.
Without great fluctuations in temperature, the cold frame allows you to gradually acclimatize your plants to outdoor temperatures. As a result, your plants will be stronger, happier and healthier.
Further benefits of using cold frames include:
- Solar energy keeps planting soil warm
- Seeds can be started off earlier in the season
- An extended growing season for many cool-season, winter vegetables
- They provide a sheltered environment, regardless of the weather outdoors
There are a great selection of greenhouses and cold-frames around with dimensions suitable for every space and aspect.
Now let’s look at Winter Crops to Grow in your Greenhouse
To grow a wide variety of winter crops you will need to provide an extra heat source. This is commonly known as “forcing” and is the process of making a flower or plant fruit out of its natural growing season.
Early Forced Vegetables
Vegetables may be forced in a heated frame, or in borders and boxes in the warm greenhouse. You’ll see that many choice crops can be grown in this way. Below, I’ve outlined a good selection, with a rough guide to get you started.
Sow in pots or boxes, using a quality fibrous compost, added leaf-mold, and well-rotted fine manure. Place these in your greenhouse or cold frame in January and February with a temperature of 50 F or below. Later batches can be hardened off in the cold frame and set out outdoors. Take a look at varieties like “Aquadulce Claudia” and “Violetto”. These both withstand cooler temperatures and produce early, plump crops.
To produce the earliest possible crops, sow in monthly intervals through December, January and February. Use a good-quality loam compost with a little sharp sand and well-rotted manure. Fill 6-inch pots and grow three seeds per pot. After watering sparingly, place your pots a foot apart on staging close to the glass. Your ideal temperature needs to be above 60F. “Hunter” is a good and reliable early cropper.
An early cropping of the household’s favorite home-grown beans can be achieved by sowing in seed boxes in April. Use equal parts loam compost and leaf-mold, with a little coarse sand. These can be planted outdoors in late May. My favorite varieties are “Polestar”, “Scarlet Emperor” and “Czar”. All tasty and delicious.
Sow in seed boxes, placed in the warm greenhouse or heated propagator from late winter to early spring. These can be planted out when warmer weather arrives. There are many recent F1 seed varieties available that are worth looking at, resulting in tight sprouts and great disease resistance. “Braemar” and “Brodie” sit at the top of my list.
Choose early varieties like “Early Nantes” or “Amsterdam Forcing” for growing in January. These can be sown in shallow drills in 4 inches of fine soil or compost, around 10 inches apart. You can sow these at intervals until the first outdoor sowing is made. By springtime, you’ll have tender crops.
Grow early seed, such as “Autumn Giant” or “Aaslmeer” varieties in seed trays with a bottom heat source in February and early March. Try to place these directly under glass, thinning them out when they are strong enough to handle. Plant the seedlings singly in 3-inch pots, planting them out in a warm and sheltered site in April.
Alternatively, you can sow seeds in cold frames from August and September, pricking out the seeds as soon as they are strong enough. Autumn-sown plants can be potted individually and forced under glass for an early supply.
In February or March, use seed trays filled with light and sandy soil, sowing your seeds under glass with a bottom heat of around 70 degrees F. Once strong enough, take your seedlings out singly, placing them in 3-inch pots and place them in a cold frame or unheated greenhouse. Should you need to, these can be potted on before planting outdoors in early May.
Cucumbers can be grown at any time by sowing your seed in pots with bottom heat, or alternatively under glass. These take around three months to mature.
For an early supply, thinly sow your seed in fine, sandy compost in 3-inch-deep seed trays towards the end of January or early February . Keep them at a moderate hat of about 55 to 60 degrees F. When three inches in height, pick your seedlings off into trays, 2 to 4 inches apart. Place in an airy and well-lit position, transferring them to a cold frame about mid-April. Once hardened off, transfer them outdoors in May.
Mustard and Cress
You can grow a continuous supply by sowing in seed trays in the greenhouse from October through till March. Very good in sandwiches too.
Sow onions 3 inches apart in a loam, leaf mold, and sandy compost mixture in January. Give them heat of approx. 60 F. Once they’re fit to handle, reduce heat and ensure they get lots of light and air. Transfer to a cold frame in April to harden them off and plant out in early May.
Sow peas in December, January or February. Put 5 seeds per 3-inch pot in a compost mix of soil, sand and leaf-mold. Place these directly into the cold-frame and plant outdoors in a warm, sheltered position in early March. “Douce Provence” is an old-fashioned pea variety that overwinters well and is a reliable cropper.
Plant early sprouted seed potato varieties in boxes, tubs or pots towards the end of January or early February. Use a loam and leaf-mold soil mix. Plant them 4 inches deep and 9 inches apart, giving them heat of around 50F.
Sow your seed in drills from early February time using a deep seed tray. Leave 6 inches or so between drills, placing the tray on a hotbed. Once the seedlings are fit to handle, thin them out to 3 inches apart and harden them off in the cold frame. Plant these out in May.
As you can see, a heated greenhouse gives a whole new meaning to “year-round vegetables”. That said, you can still achieve winter crops with a standard, unheated glasshouse and cold frames.
Standard Unheated Greenhouse Growing
Using an unheated greenhouse makes you more attuned to seasonal vegetable growing, rather than forcing growth out of season. It really depends what you’d like to achieve.
From around April onwards (in the U.K.), depending where in the world you are, seedlings will often thrive in a glasshouse with no extra heat. Just place your filled seed trays on staging or benches, keeping them well up from the floor.
Remember, the floor will be the coldest spot in your greenhouse. On the other hand, you can even start your seeds off indoors on a warm and sunny windowsill rather than using heated propagators. You just have to accept the growth rate will be a little slower.
For all of us with non-heated cold frames, take a look below for winter planting inspiration and make the most of your garden facilities.
At the very beginning of the year, sow frost-tolerant plants like Brussels sprouts, spinach, cabbage, lettuce and broccoli in seed trays in your unheated glasshouse. These plants tolerate much lower temperatures than most.
Moreover, they can be planted outside up to a month before the last frost date when evening low temperatures are above 30F. Similar winter plantings include kale, snow peas, mangetout, broad beans, turnips and early-pull carrots. Make the most of your resources and plant them in your greenhouse without the hassle of frozen ground and frost- bitten fingertips.
This is a rather apt time to talk about outdoor thermometers, which can make life for the year-round gardener much easier than relying on incorrect forecasts. Consider investing in a quality thermometer that not only gives you today’s temperature, but also last night’s “low” too.
Spring Grown Crops
Spring is the “official” planting season as we know it. Now is a good time to sow early crops like lettuce, carrots and herbs, and tender crops such as melons, cucumbers, and squash. Once the last spring frosts have passed, these can be transplanted outdoors.
Come summertime, all of your previous hardy seedlings can be planted outdoors. This frees up more growing space in the greenhouse for our mid-summer, heat-loving crops like aubergines, peppers and tomatoes. Don’t forget to grow your tender herbs like basil and coriander. Remember your greenhouse will need to be well-ventilated to allow sufficient air movement from now on. Maybe consider investing in a ventilation kit?
Once the balmy days of summer are over and with cooler weather on the way, move all your pots of heat-loving, summer crops into your greenhouse to finish growing, including plants in the Brassica family and oriental greens. Furthermore, you can start sowing seeds for late autumn and winter salad leaves.
When your outside allotment is spent, now is a good time to break up the soil, adding nutritious organic matter as you go. Leaf-mold, seasoned garden compost or well-rotted farmyard manure are all excellent soil improvers. Remember: the more you work the soil now, the less you need to do later.
Once dug over well, you can start planning for your next hardy cool-season crops.