There are many different types of mustard that are perfect for growing in a polytunnel. Which mustard or mustards you will choose will depend on what you want from your crop. Various mustards are grown as a fast-growing and hardy salad crop – one of the easiest things to grow in your polytunnel and useful over the colder months and for filling the 'hungry gap' in the spring. Other mustards are grown for their seeds, which are ground for use in the traditional condiment. Still other mustards are used as a 'green manure' – sown with brassicas in crop rotation and dug into the ground to add nutrients and replenish the soil.
Mustard is a perfect crop for children or beginner gardeners as it is so easy and quick to grow. Sow mustard leaves for salad throughout the summer and into autumn in your polytunnel and you can be harvesting cut and come again leaves within only a few weeks. As the plants begin to grow, you may wish to thin them out to a spacing of around 20cm. You can eat the plants you thin. Over winter, there are many type of mustard that will cope with the temperatures – just add extra protection if you live in a particularly chilly part of the UK. Mustard will not grow as quickly over winter but if you planted some in late summer then you could have some plants from which you will be able to harvest all through the coldest months. Successional planting will allow you to enjoy fresh and spicy salad leaves pretty much all year round.
If you are sowing mustard for seed then it is usual to sow the seeds around three weeks before your last frost date in Spring. Plant seeds roughly an inch apart and then thin to a spacing of around 5 inches once the seedlings emerge. The spacing is more important for plants for seed since the plants will be getting much bigger before they are harvested and will therefore use more resources in terms of the soil and water they require. Water well to prevent a check on growth that can cause plants to bolt in warm weather, since the seeds produced in such circumstances will not be as good as those produced in the natural order of things.
Mustards are often also used as a green manure or 'catch crop'. It is important to take care of the soil in your polytunnel beds and mustard is often sown after summer crops are removed and dug in after around six weeks to enrich the soil. Be sure to dig in the mustard before it goes to seed, or mustard will be popping up everywhere!
Simply harvest mustard leaves as you want them for salads and stir fries and other recipes. Collect seed pods when they are fully ripened but before they drop seed and dry these in or on paper before using or storing. Seed can be used fresh or dried and stored for later use.