May - What To Sow, Grow & Do In The Greenhouse & Garden

May is when everything comes together. With frosts behind us, mostly, you can plant outdoors with confidence and use your greenhouse for tender crops that love the heat. From beans and carrots outside to tomatoes and cucumbers under glass, this is the month to fill every inch of growing space

What to grow in May

We've spoken to our gardening expert friends Ellen Mary and Ruth Darrah and here is everything you need to sow, grow and do in your greenhouse in May

Vegetables to Sow Outside in May

With warmer soil and longer days, May is the time to get beans, carrots, radishes and sweetcorn directly into beds and containers outdoors.

Beans

Runner, French or fava beans all thrive in May warmth. Plant directly in your veg patch, raised beds or containers. Train them onto supports to save space and make harvesting easier. Harvest before pods mature to keep the plant producing throughout summer.

See our Planting Companion Guide for the best plant combinations.

Carrots

Carrots can be sown in April but if you haven't gotten around to it yet, don't worry, it's not too late!

Sow small batches every three to four weeks for a steady crop through summer. Place seeds in drills in watered soil. Be patient with germination; it can take two to three weeks. Protect young seedlings from slugs and avoid planting where carrots grew recently to prevent carrot fly problems by covering crops with row cover fabric. Try some of the colorful varieties for a bit of garden theater.

Radishes

Quick from sowing to harvest, radishes can be ready within a matter of 3 weeks! In the ground in May, on your plate in June. Perfect for children or the impatient gardener.

Sweetcorn

Needs a warm, sunny spot with shelter from strong winds. Plant in a grid rather than rows for better pollination. Different varieties offer varying levels of sweetness, so choose based on your taste preferences.

Vegetables to Sow in Your Greenhouse in May

Your greenhouse is the perfect home for heat-loving crops like tomatoes, peppers and cucumbers, as well as tender herbs that will flourish under glass.

Tomatoes and Peppers

Although April or even March are common months to sow tomatoes and peppers, May is still a great time to sow from seed for a late-summer crop. The sun is stronger now, so maintaining moisture levels becomes crucial. Water carefully and regularly to prevent stress on developing plants.

Read our guides on how to grow tomatoes and how to grow chilis in a greenhouse.

Cucumbers

Like tomatoes and peppers, cucumbers can be sown in April but May is also not too late. These are worth growing for the flavor alone. Not only great in salads, but superb pickled! Cucumbers love warm conditions and produce delicious, crisp crops under glass. They climb beautifully when trained up supports, saving valuable floor space.

Find out more in our cucumber growing guide.

Basil, Cilantro and Dill

These warmth-loving herbs are best grown from seed in your greenhouse in May. Keep a healthy supply flowing into the kitchen all summer long. Pinch out growing tips regularly to encourage bushy plants.

Flowers to Sow in May

With no frost to worry about, May is ideal for sowing show stopping flowers both inside the greenhouse and directly outdoors.

Sunflowers

A firm family favorite and easy to grow. Choose a slightly sheltered spot and support tall varieties with a bamboo cane or sturdy stake if needed. Sunflowers can go straight in the ground in May and will quickly establish.

Who will win the sunflower growing contest in your household?!

Marigolds

Bright, cheerful and easy from seed. Marigolds are garden workhorses. Pair them with tomatoes to help deter nematodes and whitefly. Plant in a sunny spot with well-drained soil for best results.

Nasturtiums

Easy, beautiful and entirely edible. The flowers, leaves and seeds are all delicious and add a peppery kick to salad bowls. Blooms often continue well into October, providing months of color and interest.

Cosmos and Zinnias

Long-flowering plants that are great for pollinators. They are often paired together in borders and containers. Plant them where they have room to grow, and they will reward you with months of delicate blooms.

For advice on encouraging bushier plants, see our guide on when to pinch out.

May Greenhouse and Garden Tips

Hardening off, pest patrol and the Chelsea chop are all on the agenda this month.

Hardening Off Your Plants

By the end of May, it is generally safe to plant out tender seedlings. Begin by moving young plants outside during the day, then bringing them back inside at night for about a week. Gradually leave them out overnight once they have adjusted.

Plant on a mild day without strong direct sun to reduce transplant stress. Water well after planting to settle soil around roots.

Keep Your Greenhouse Cool

The sun is getting stronger now and temperatures in the greenhouse can soar. You may need to use greenhouse blinds or shading to protect seedlings from too much direct sun, which can scorch delicate leaves. Good ventilation is key to managing temperatures and preventing disease.

See our article on how to keep your greenhouse cool in summer for practical tips.

Watch for Pests and Diseases

After a wet season, keep a close eye out as the weather warms. Blackfly on fava beans can be dealt with by spraying with a hose, pinching out the top shoots, or simply leaving them for ladybugs to feast on.

Red lily beetle will eat through lily foliage quickly. These bright red insects are unmistakable but evasive. Check for chafer grubs if your lawn looks yellowed and patchy in places.

Visit our guide to keeping your greenhouse pest-free for more prevention strategies.

The Chelsea Chop

Gardening consultant Ellen Mary says

"It's such a busy but enjoyable month in the garden. Don't forget to give some larger perennials the Chelsea chop if needed and mostly"

What is the Chelsea Chop?

This involves cutting back a proportion of stems by about a 1/3 to a 1/2, which encourages bushier growth and extends the flowering period. This is a useful technique for plants that tend to become tall and leggy.

Keep Weeding

Stay on top of weeding before it runs away from you. Despite being a chore, it can be quite therapeutic once you get stuck in. Leave beneficial plants like daisies and dandelions where they are not in the way, as they support pollinators and wildlife.