Welcome to Top of the Crops! Today, you will learn about growing butternut squash in your polytunnel. And for more advice on growing squash in the UK and other crops, check out our blog Polytunnel Gardening.
There are many different types of squash that can be grown in a polytunnel garden. Butternut squash is one of the most popular. Though it is not the easiest of squash to grow, butternut squash can be grown successfully in a UK garden, especially with a polytunnel for warmth and protection.
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Butternut squash are winter squash of the species Cucurbita moschata. These elongated, pear-shaped fruits commonly used as vegetables are prized for the delicious, sweet, nutty taste of their flesh. The seeds can also be eaten.
This squash is a modern squash hybrid, which originated in a cross between gooseneck squash and Hubbard pumpkins. It was developed in Massachusetts by Charles Leggett in 1944. Today, it is grown all over the world. It will do best during a warm summer here in the UK.
If you would like to grow butternut squash, one of the most important things to do is make sure that you provide the right growing conditions. Butternut squash need:
Full sun, a sheltered position, and as much warmth as possible.
Plenty of space – these are vigorous vining plants.
Some kind of support structure up which to grow.
A fertile, humus-rich and moist yet free-draining, acidic or neutral soil.
To grow butternut squash you will need:
Butternut squash seeds.
Seed trays/ pots/ soil blocks and seed starting growing medium.
A propagator or a sunny windowsill.
A suitable growing location (in the ground, raised bed, or a large container).
Facility to water your crop.
A trowel for planting out.
Organic matter for mulching/ fertility.
Possibly a support structure for vertical growing.
Butternut squash has many of the same growing requirements and needs as other squash and pumpkins do. To grow it successfully you do need to make sure that the growing needs are met. Getting started is relatively simple – these squash are fairly easy to grow from seed sown indoors in the spring. Once planted out, watering and feeding are key concerns for these 'thirsty' and 'hungry' plants.
Sow butternut squash seeds indoors from April into seed trays, small pots or soil blocks. The growing medium should be a good quality, peat-free seed starting potting mix – either one you have purchased or one that you make yourself at home.
Seeds should be sown flat on their side around 1cm deep. Temperatures of between 18 and 21ºC. are required for successful germination. You may be able to get the seeds to sprout successfully on a sunny windowsill though being able to use a heated propagator will often make this easier and mean that seeds germinate more quickly.
From May, after the last frost date where you live, the seedlings can be transplanted to larger pots, hardened off, and planted out into their final growing positions.
If you prefer, you can also try direct sowing these squash in late May or early June. However, germination can be patchy and you may encounter other issues so it is generally easier to start the seeds indoors in the UK.
Providing that you have taken good care of the butternut squash you have planted out, you should be able to harvest the fruits some time between September or October and November if the area where they are growing can be kept frost free.
The fruits can vary in size upon harvesting but are ready when they turn orange in hue and their skin cannot easily be dented with a fingernail. Harvest by cutting through the stem a short distance above the fruit, being careful not to cut too close and damage the fruit or it will not store as well.
It is best to cure your butternut squash before use or storage. This simply means letting the fruit dry out in a warm, bright, dry location. This develops the flavour of the butternut squash and thickens and dries out the skin so that it will keep better for longer.
Once you have harvested and cured your butternut squash, you will easily be able to make use of them. There are plenty of excellent butternut squash recipes to choose from.
Whether growing in the ground or in containers, the right soil or potting mix really can make all the difference to how successful your growing efforts will be.
Remember to make sure that you prepare the soil or growing medium to provide the fertile, moist yet free draining and neutral or acidic soil that these plants require for best results.
When planting out your butternut squash, make sure that you plant them out promptly to avoid any check to their growth. Place the plants in planting holes you have prepared, firm the soil or growing medium gently back into place around their roots, water them in well and mulch with organic matter. Typically, these squash should be planted at least 1m apart from one another.
Butternut squash are typically vigorous vines, that will do well if provided with some form of support. You can use a traditional trellis structure, a cage, or another type of support. But to make the most of space, vertical growing is a good idea. The squash will take up a lot of space if allowed to sprawl over the ground, and having fruits up and off the soil is also beneficial.
Prune off excess shoots and leaves to focus the plant on fruit production and to allow light to get to and ripen the squash already on the vine.
Consistent moisture is important for butternut squash plants. Make sure that you water deeply and on a regular basis – aiming for the soil at the base of the plants and trying to avoid wetting the foliage, flowers or fruits.
When growing in the ground, be sure to mulch well with organic matter to provide plenty of nutrients and improve the soil. When growing in containers feed with an organic, balanced, liquid plant feed every couple of weeks through the growing season.
Remaining vigilant for pests and disease is important when growing butternut squash. Look out for slugs and snails while the plants are still small.
Companion planting can help with a range of pest issues, from aphids to squash bugs, and vine borers, helping to keep the garden ecosystem in balance.
Look out for fungal issues such as powdery mildew or downy mildew, and reduce the chances of these taking hold by ensuring good airflow and watering correctly.
Timing things right is important. Don't be tempted to sow or plant out too early. Do not sow or plant outdoors when temperatures are still too low but equally, don't leave sowing too late or your squash won't have the chance to grow fully before the end of the growing season and the first frosts.
Remove the odd leaf shading fruits to allow these to ripen. Aim to allow fruits to ripen on the vine. If fruits form too late, these should be removed to give other fruits the chance to mature fully.
There is a lot you can do to increase the chances of a successful butternut squash harvest. Just remember that these squash like heat – and since our summer in the UK is not always obliging, some years can be better than others when growing this crop.
Growing varieties best suited to our climate and the conditions that we can provide can go some way to increasing your chances of success. Some varieties of butternut squash recommended for UK growers include:
Aside from the problems with pests and diseases mentioned above, butternut squash may also encounter problems simply due to the weather.
Growing these squash in a polytunnel can help by insulating them from weather patterns to a degree, but some summers, the lack of sunshine and warmth can lead to disappointing yields for those trying to grow butternut squash in the UK, even when the gardener has done everything just right.
Learning how to store squash, as well as different fruits and vegetables, is important. Many varieties of butternut squash will keep for up to three months or so, some for even longer. In a relatively cool storage location, you can keep butternut squash to enjoy throughout the winter months.
Grow vertically in a polytunnel to make the most of the space available. Provide support for the butternut squash to keep it confined in the space it takes up. Support fruits up off the ground to prevent potential problems with rotting.
Cover young plants with cloches early after planting if cold weather threatens.
Companion plant squash with legumes for nitrogen fixation, radishes and nasturtiums for pest control purposes, and a range of other flowers and herbs to attract beneficial insects and further aid in repelling, confusing or distracting pest species.
Creating polycultures of plants that aid one another can help you to grow squash more successfully.
We cannot change the weather we get over the summer, and that will affect the size of your yield and the success of your crop. But practicing good organic gardening methodologies should help you to grow butternut squash or other squash successfully where you live.
Bonvie, L., (2023) The Largest Butternut Squash Ever Grown Weighed as Much as a Hellfire Missile. A-Z Animals. [online] Available at: https://a-z-animals.com/blog/the-largest-butternut-squash-ever-grown-weighed-as-much-as-a-hellfire-missile/ [accessed 14/12/23]
BBC Good Food. (n.d.) Butternut squash recipes. BBC Good Foods. [online] Available at: https://www.bbcgoodfood.com/recipes/collection/butternut-squash-recipes [accessed 14/12/23]
