Growing broccoli: when, how & the best companion plants

Broccoli, the delicious wonder-veg that you can grow in your own garden. Growing broccoli is worth the effort and it doesn’t take long to reap the rewards.

Broccoli (Brassica oleracea var. italica) is a delicious vegetable that is packed with vitamins. As a brassica, it is closely related to other veg like cabbage (B. oleracea var. capitata), brussels sprouts (B. oleracea var. gemmifera), cauliflower (B. oleracea var. botrytis), and kale (B. oleracea var. acefala). Typically, what we buy in the shops and eat is the flowering head and stem of the broccoli plant, but the leaves are also a delicious and hearty green, and the seedling sprouts make a wonderful addition to any meal. Growing broccoli for yourself can be easy, and brings satisfying results in a relatively short period of time. Not only that, but it can also be harvested throughout the year in the UK. This article will take you through the ins and outs of planting broccoli, including how to grow broccoli, when to sow broccoli seeds, and when to transplant broccoli, with lots of helpful tips.

Growing broccoli: where and when

To grow a successful crop of broccoli there are two important factors you need to consider: the right place, and the right time of year.

Where to grow broccoli

Since it is a cool-season crop, the UK offers the perfect climate to grow broccoli. Cooler weather with plenty of sunshine will produce beautifully dense flower heads without open flowers. As a rule, broccoli plants need plenty of space. That being said, they can certainly be grown in your garden, or even in a medium-to-large pot, a minimum of 19 litres or larger, on your patio or terrace. Be sure the space you choose has good air circulation. The more space a broccoli plant has to spread its leaves, the larger the first flower head will be able to grow. Broccoli plants, similar to tomatoes, are heavy feeders, needing lots of nutrients for healthy growth, especially calcium. Therefore, when planting in containers or raised beds, it is important to use a well-drained, balanced, and nutrient-rich soil such as our Plantura Organic Tomato & Vegetable Compost. This 100% peat-free soil is ready to use and is primed with the essential nutrients that are required for a heavy feeding plant such as broccoli. If you are planting broccoli directly into the garden, make sure your soil drains well and is enriched with nutrient-rich compost. If your soil has a low pH and low calcium levels, it is a good idea to add some lime to your soil to increase the pH of your soil and improve calcium levels; this supports healthy growth and reduces the risk of club root forming in your broccoli plants.

If you live in a particularly hot and dry area, know that growing broccoli may not be as successful. There are plenty of better suited plants to grow in these areas.

Organic Tomato & Vegetable Compost, 40L

(5/5)

• Perfect for tomatoes & other vegetables such as chillies, courgettes & more
• For strong & healthy plant growth as well as an abundant vegetable harvest
• Peat-free & organic soil: CO₂-saving composition

£16.99

Shop now!

When to grow broccoli

The growing and harvesting times varies for different varieties of broccoli. In the UK, long-season varieties should be started in September and left to overwinter to produce an early harvest. On the other hand, short-season, or fast-growing varieties can be started indoors or in greenhouses in the spring, from February until April, and then transplanted outdoors after April. Alternatively, you can sow broccoli seeds directly into the garden in April. Take care with young broccoli seedlings as excessive periods of cold can trigger early flowering. Also, avoid keeping your broccoli seedlings in the greenhouse after April, as it can be too hot. Warmer temperatures can trigger bolting, which is when the plant shoots up open flowers to produce seeds. These fast-growing open flowers, while edible, tend to be quite bitter. The leaves and stems also become bitter, as the plant is focusing its energy and sugars on the fast-growing flowers for reproduction.

Tip: the optimal temperature for growing broccoli is between 16 to 24 °C. To avoid bolting and ensure good flower head formation, always use a variety that suits the planned sowing and cultivation time.

How long does broccoli take to grow?

Short-season and early varieties of broccoli can be harvested 7 to 12 weeks after sowing. Long-season or overwintering varieties are best started in September and left to overwinter in the garden. They can be harvested in 4 to 5 months, the following February through April.

Sowing broccoli seeds

Growing broccoli from seed is an easy way to ensure they will be ready to harvest at the right time. Three are two options for sowing of broccoli seeds: in trays/modules indoors or directly into your garden. With either method, you can speed up the germination time by soaking the seeds in lukewarm water for 12 to 24 hours before sowing, a process called priming the seeds. The general rule for planting depth of seeds is 6 times the width of the seed. So, with small broccoli seeds, a light dusting of soil or planting media up to 1cm thick is perfect.

When sowing broccoli seeds module trays, it is best to sow 2 seeds per cell. Seeds will germinate and pop up within 4 to 11 days of sowing. There is no need to use a heat mat for broccoli seedlings, as they prefer cooler soil temperatures of 5 to 15 °C.

When sowing broccoli seeds directly in the ground, you should use the same method. Sow 2 to 3 seeds per hole with 30 to 60cm spacing between plantings.

The first leaves to develop are heart-shaped and called cotyledon leaves. The next set of leaves are the first true leaves, which are markedly different in shape from the heart-shaped cotyledons. The first true leaves actually look like tiny broccoli leaves. When your seedlings reach about 5cm tall, and have at least two sets of true leaves, it is time to thin your seedlings. Leave the strongest-looking seedling in each cell or planting hole and remove the rest. Thinning is important as it ensures that your seedlings get good light and air circulation and avoid becoming leggy and weak. It is important to transplant your broccoli starts when you see roots coming out of the bottom of their pots, or when they are taller than the height of the cell trays that they are growing in. Keeping plants in pots that are too small can trigger early flowering. This can be prevented by either transplanting them into larger containers or if temperatures permit, planting them out into your garden.

Planting broccoli

When it comes to planting broccoli starts, whether shop-bought or ones you have grown from seed, it is essential to harden them off. This allows the plants to adjust to the outdoor conditions and reduces the risk of “transplant shock”. If they are going outside from an indoor, temperature-controlled environment with artificial light, you need to acclimatise them to the colder temperatures and to full sun. This can be done by leaving them outside for part of the day at first, avoiding direct sunlight at the hottest part of the day. If it is still cold outside, you could start by leaving them out for the daytime and bringing them in at night. This avoids damaging the tender seedlings with cool night air. If you buy your seedlings from a shop, be sure they are not root bound and leggy, as this can trigger premature flowering.

When to plant out broccoli

For short-season varieties, sow indoors or in greenhouses between February and April or directly in the ground from April onwards. For long-season varieties, sow or transplant seedlings in September, or at least 4 full weeks before your first frost. Here are a few of our favourite varieties of short-season and long-season broccoli.

Short-season broccoli varieties

• Brassica oleracea var. italica ‘Calabrese’: Produces a large central floret and abundant lateral florets after the central floret is harvested. Can be harvested 65 days from when it was sown.
• Brassica oleracea var. italica ‘Spring Rapini’: A wonderful short variety that can be harvested in as few as 60 days. All parts of this plant are edible and delicious.
• Brassica oleracea var. italica ‘Summer Purple’: This delicious variety produces abundant small florets, all the way through to the autumn.

Long-season broccoli varieties

• Brassica oleracea var. italica ‘Claret F1’: Abundant side florets are produced once the central floret is harvested.
• Brassica oleracea var. italica ‘Red Fire’: This variety produces bright purple florets.

Both short and long-season varieties

• Brassica oleracea var. italica ‘Purple Sprouting’: This is a very versatile variety as it can be sown in the spring for summer harvests and/or sown in autumn for winter and spring harvests. It produces abundant small florets.
• Brassica oleracea var. italica ‘Santee’: Another versatile variety, as it can also be sown in the spring for summer harvests and/or sown in autumn for winter and spring harvests. It produces green stems and purple buds.

Tip: covering your plants with horticultural fleece or fine-meshed nets can protect them from frost damage and help to reduce damage from pests like the swede midge (Contarinia nasturtii) and large cabbage white butterfly (Pieris brassicae).

How far apart to plant broccoli

As was mentioned earlier, broccoli plants need plenty of space, both for their leaves as well as for their roots. If you are growing broccoli in containers, either choose dwarf or sprouting varieties, or plant in containers larger than 30cm deep, with a minimum volume of 19 litres. These containers can then be spaced out enough to allow ample room for the leaves. When planting in the ground, or in raised beds, space them at least 30 to 60cm apart. Plant your starts in the soil at the same depth at which they are growing in their starter containers. Planting them deeper could cause their stems to rot.

Tip: varieties like Brassica oleracea var. italica ‘Kabuki F1’ produce shorter plants and smaller florets that are great for container gardening on a patio.

Broccoli companion plants

If your garden soil is very rich in nutrients and calcium, it can be a good idea to combine the heavy feeder broccoli with other heavy feeders such as tomatoes, aubergines or corn. After all, these plants share similar nutrient preferences.

However, if you have a limited supply of nutrients in your soil, two nutrient-hungry plants would be competing for the same resources and neither would produce good yields. For poorer soils, we recommend companion planting broccoli with herbs, since herbs will not compete with the broccoli for nutrients and can even keep pests away with their aroma. For example, the annual herbs dill and basil, or onions, garlic and spring onions all make great neighbours for broccoli.

Common pests encountered when growing broccoli

Common pests vary by season when growing broccoli in the UK. Slugs, snails, and birds love to eat young broccoli plants and various caterpillars will feed on young and developed leaves. You may also experience club root formations and have cabbage root fly larvae infect the roots of your broccoli plants. Club root formations can be avoided by adding lime to your soil before planting and by using resistant varieties like ‘Tall Green Curled’ or ‘Kilaton F1’. For caterpillars, simply removing them by hand can be effective. Covering your plants with horticulture fleece or mesh will protect them from birds and many harmful insects. It’s necessary to rotate your crops and avoid planting brassicas year after year in the same space – this practice actually prevents many diseases.

While all parts of the broccoli plant are edible and delicious, so are the seedling sprouts. Read our article on how to grow broccoli sprouts.