How to Extend The Growing Season: Greenhouses, Polytunnels & More Tips

In this gorgeous mid-summer weather, it is wonderful to admire the successful rows of plants you seeded in spring and just water and weed where necessary.

Over the years, I have learned some tricks to beat the colder nights of autumn and winter approaching and late in the winter, I try to gain a few extra weeks by sowing seeds earlier so they are ready to be planted outside, as soon as the last frost is gone in spring.

Everything depends on your available space, the light available for plants, the soil temperature, how well your plants can adapt to colder weather, and what you can do to cover them up.

You may need to move some plants into a more suitable, late autumn location so that they can continue to thrive and provide leaves, fruit, and flowers for you to enjoy. Here are 6 ideas so that you can extend your growing season a bit longer.

6 methods to help extend your growing season.

1. Know Your Weather

This is a very obvious first choice and it means simply keeping an eye on the weather forecast. If there is a heatwave, you need to water more. If a storm or thunderstorm is due, some plants will drink up the extra rain but you may want to protect more delicate ones like aubergines (eggplant in the US). Early on in the season, you may need frost protection like bubble wrap covers, cloches, or cardboard and the same goes for later on in the season.

Squash that is still producing flowers in September can go on to produce that fruit, provided no frost damage kills it off. Do not try to plant exotic plant seeds outdoors which are meant to grow in India or Brazil and expect the same results in your UK back garden. I always try one new exotic each year (with varying success) but my advice for new gardeners is to stick to seeds they know will grow well in their area.

2. How to Get More Heat: Cover and Wrap Plants!

• Buy a greenhouse or polytunnel. It depends on whether your winters tend to be harsh with cold winds, frost, and snow or whether you live in the sunny south of the UK where frost is not as frequent. You can protect wintering greenhouse plants by insulating the whole area with bubble wrap, to keep draughts out. Make sure you keep the door closed when frost is predicted. You can also use heated propagation mats to keep seeds at a steady temperature. These can be timed to come on at night time for a few hours or if the temperature drops below a certain temperature.
• In winter, warm up the soil in an outdoor area, where you will plant in spring by placing a layer of polythene, fleece, or even cardboard on the area. Allotments used to place old carpets down as cover but this has now been forbidden because many carpets contain poisonous substances, which can leak into your soil, and therefore your food. I use manure bags, compost bags, and any recycled clothes from home.
• I dig the patch over in the autumn first, and then add some homemade compost, and then lay the chosen cover.
• I cover this with comfrey leaves for my first crop of spring salads or peas. Just leave this weighed down with stones for the winter. When spring comes you can crumble the leaves directly into the soil to release their goodness.
• An added benefit is pest control because when you lift the cover, you will find a fantastic selection of slugs and snails which have used the cover all winter to evade the frost. Gather them all in a bucket in one go and feed them to the compost heap or the chicken coop!
• The ground underneath will be weed free and much warmer than the soil nearby. Perfect for early rows of lettuce and leafy vegetables!

• Use a cloche. These can be bought in garden centres or online or you can make one yourself. A cloche (pictured left) is usually a tunnel made from wire with fleece or polythene covering the tunnel to keep the heat in. Some gardeners use these to keep butterflies and moths away from cabbages and also to protect a new row of seeds from birds. I use them to keep a row of plants warmer for longer. On allotments, they have the dual purpose of keeping a particular plant warm in the cooler months and helping to promote early germination in spring. The word cloche comes from French and it refers to glass jars made to place over delicate plants. Nowadays we use washed-out milk or yogurt containers to do the same job but if you have an unlimited budget, look for some glass designs online. There are some beautiful designs!
• Cover up delicate trees: Some trees or delicate shrubs will benefit from frost protection in a very harsh winter. This is particularly important for almonds, apricots, and peach trees which tend to flower early. If a late frost comes, the whole crop may be ruined so use fleece or build some fences around the trunk to shade these shrubs from the worst of the weather. Drape blankets or fleeces, if snow is predicted.
• Protect plants on cold windowsills indoors. The coldest air from outside is next to your windowsills when you pull the curtains in the evening. To help keep that cold air out, place a piece of recycled bubble wrap around the pot and also next to the window to keep that cold away!
• Have you heard of hot beds and cold frames? These are ways to nourish and protect plants from harsh weather or to give them early protection in the spring. See more about hot beds and cold beds below in FAQs.

3. Know Your Growing Area

• Plan for the sunny areas. This is where you will place hungry feeders like peas and beans, sunflowers, squashes, and so on. You can heat up the ground by covering plots with fleece or cardboard after the last produce has been picked. In winter you can put a plant in a pot here to add some colour, then it is easy to remove this plant and the cover after the last frost has gone. Your new plants will get a head start, especially if you dig in some leaf mould, some manure, and some comfrey leaves to enrich the soil.
• Plan for the shady areas. Under protection from a fruit tree is often a shady spot where you will not find it easy for heavy feeders to grow. Plant mint and herbs that like shade and you cannot go wrong! Russian Tarragon is a great plant to self-seed in shady areas but do not plant sun lovers here. Remember that many Brassica plants (the Cabbage family – sprouts, cabbage, kale, etc) adore shade so when the leaves from autumn fill the ground, this is when your shady patch will come alive.
• Plan by season. Have you got a plant that dominates in one season but is not there in another?  When I know that my grape vines and my almond and cherry trees will cover the entire driveway in summer, I plan an early treat of flower bulbs and Honesty for a lovely spring display. Once they have finished, then I let the trees and the vine compete for light later. When the vine dies back completely in winter, then my snowdrops, grape hyacinths and cyclamen take over when the temperatures are cooler.

• Plan using a crop rotation system for your garden. If you want to keep your soil healthy it is not a good idea to keep growing the same crop on the same soil, as it depletes the area of the same nutrients. So if you have potatoes the first year, plant peas or beans the next year to add some nitrogen to the soil. Follow these up with cabbage family plants and finally, roots like carrots, parsnips, and swedes. Beans do not really tolerate frost whereas the cabbage plants are just fine with cold weather. In this way, plan for each area to have something to follow it so that you use the ground well and keep your soil healthy.

4. Get As Much Light as Possible for your Plants:

• Cut back overgrown shrubs, that keep the sun from getting to certain corners of the greenhouse and also those that shade your windowsills. It is really important to do this in the autumn, winter, or spring so that light can easily reach plants behind them or just close by.
• Big shrubs in the ground outside also benefit from pruning to keep them in shape and to make sure that plants underneath them have enough moisture and nutrients. The established shrubs have deeper roots than lettuce or herbs so to help these along, trim the branches and put them to dry for firewood or make them into mulch.
• Windowsill plants will love the extra light and will help your new seedlings to develop well.
• Place your greenhouse in the sunniest location. This is easy on paper, but in my garden, this meant either moving a gooseberry bush or trimming back a bay tree. If you and your family are sun worshippers, you can have seats dotted all over the garden to catch the sun but the greenhouse plants need steady sunlight, water, and nutrients and they can’t hop and move into the sun so make sure you give your greenhouse the sunniest location. You can always put a seat in there so the family can enjoy it too!
• Indoors, check that furniture is not shading indoor plants when you want as much sun as possible. Move it for the summer if it stops light getting in.
• Vertical gardeners or balcony growers can use mirrors and paint the wall black to attract more sun. This work really well if you paint a wall black behind a pallet shelf for example. The mirrors can also help to increase the feeling of space in a small terrace but be warned that increased heat in summer may need to water more.

5. Harden Off Plants Before Placing Them in Permanent Positions

This means that you allow greenhouse plants to get used to the weather outdoors by placing them out in the sun for a while each day as soon as it feels warm enough.

This means you can plant more delicate seeds like aubergines and courgettes earlier than in the soil, and use the greenhouse protection to have bigger plants ready earlier. This is the time of year when gardeners are keeping an eye out for blustery winds, frost, or anything that their new plants will not like.

All the plants need to go back indoors before Jack Frost visits though, so I usually have a table outside my greenhouse and move them out early in the morning and back in at sunset. It takes a while each day but the plants like being outdoors during the day and get used to it gradually.

6. Plant a Second Set of Seeds at the End of the Summer

When I first started my allotment, Mick a more experienced grower, told me to always plant more seeds in August. I remember looking at Mick blankly and thinking “But it’s nearly the end of the season?” He let me into a big secret.

In Kent, the frost may keep away until December and the squash plants will keep going but you can plant lettuce, rocket, parsley, and even beans in August so that you get a second run of greens by October and right up to the time when the frosts hit.

Obviously, if your frosts start in October, this is not necessarily good for your area but you can think about which seeds might survive indoors on a windowsill or in your greenhouse if you give them some protection in colder spells.

• Certain seeds are better planted in August because they do not bolt so easily. “Bolting” means the plant gets too hot and tries to flower and go to seed, like beautiful white basil flowers but lovely as they are, the whole plant then tries to go to seed and dies off.
• Herbs like Rocket are fabulous because they will grow outside and you can pick green leaves all winter. Garlic Mustard is another hardy green salad that I eat all year round. “Winter Density” is a Cos lettuce that performs really well in colder temperatures. Coriander has been another success story for me in my winter greenhouse and Winter Savoury is another delicate taste to add to salads in the cooler months. Placing a cloche over these late seeders will conserve nighttime warmth too to ensure germination takes place. You can remove the cloche if the weather is mild.
• Transfer less hardy plants like Basil into the greenhouse or on a sunny windowsill and try to keep them cooler, remove the flowers (you can eat basil flowers), and they last longer too.
• Tarragon and Lovage must be protected from frost and both tend to die back when the weather gets cool so just make sure their pots are protected indoors and plant them back outdoors when the weather gets warmer.
• Take cuttings from herbs before winter. This is just in case the frost kills any you have left outside. We all remember years of heavy snow and bitter cold and some herbs just give up in this weather. If you have taken cuttings, you can still enjoy them indoors and wait for spring to plant them in the soil the following spring.

What is the difference between a cold frame and a hot frame (or bed)?

A cold frame is a box with a removable (or adjustable lid), full of soil on a slope, usually close to a building that can provide some warmth. This could be a conservatory, a greenhouse, or a shed. Some gardeners use hay bales stacked to provide some insulation from the cold and then fill the inside with soil and add the lid.

The idea is that it provides new seeds a chance to grow in a warm, frost-free environment. The soil can be made from homemade compost, leaf mould, old plant pot soil, etc. You place your seeds in pots on top of the soil and as the weather improves, you can lift the lid and harden them off until the weather gets warmer.

The advantages are that you can start seeds a lot earlier than if they are in cold, wet soil and you can also protect them by covering them up at night or when frost or cold weather is forecast. The seedlings are usually in pots and then transplanted into the growing bed as soon as the danger of frost is gone.

A hot bed is a box similar to the cold frame but usually placed on top of a warm area, such as a large compost heap. If you have ever felt the top of a plastic composting bin, you will be surprised at how warm it is.

This is due to the rotting and decomposing of the material in the heap. Heat rises so the placing of a grow box on the top will mean these plants benefit from a really cosy atmosphere. Plants grown like this include melons (which are super fussy about heat) or aubergines, which adore the heat and being placed on top, they can greedily absorb every bit of available sunshine.

The hot bed has a lid too and you can lower or raise the lid to make the plant happy. In mid-summer, the heat continues to work and you can completely remove the lid if the temperature is stable.

Both hot beds and cold frames benefit from facing south, to make the best of available sunshine. Insulation can be provided in winter using straw, wood piles, old planks, recycled cloth, bubble wrap, or polythene to increase temperatures.

They need soil to slope for stability and to allow many plants access to the sun. Watering is essential too as the hot bed, in particular, can reach very high temperatures in summer. You can DIY either of them using your outdoor compost heap to provide heat and a free wall to build a cold frame.

They both offer the gardener the chance to both protect vulnerable plants in cold weather and boost growth before the plant can normally go directly into the soil.

Conclusion

Hopefully, by now you will have discovered a few ways to extend your growing season wherever you live. Cloches, greenhouses, and sunny windowsills offer protection to delicate plants and their cheerful scent indoors will fill your space too.

Hot beds and cold frames provide additional space and weeks in the growing calendar. Covering up delicate plants with fleece, cardboard, or carpet can save trees and shrubs that don’t enjoy the snowy weather.

Your greenhouse can have a dual function of protecting more delicate herbs and also allowing you to experiment with hardier herbs and salads that will happily grow throughout the year so that you can have salad all year round. Wrap up, keep warm and bring the delicate ones indoors. Winter does not have to be a gardener’s holiday!

Useful websites

• https://www.gardeningknowhow.com/special/greenhouses/cold-frames-for-spring-seedlings.htm
• https://agriculturistmusa.com/difference-between-hot-bed-and-cold-frame/#:~:text=1.,glass%2C%20Fiberglas%2C%20or%20plastic.