May is a great month for flowers. The soil is warm, the days are long, and most things you plant now will be flowering by July. Whether you have a border, a veg patch, or just a few pots outside the back door, there is plenty you can get going in May.
One thing to be careful of in early May is frost. It can still happen, especially in the north and on higher ground. Tender plants put outside too soon can get damaged overnight even when the days feel warm. From late May the risk mostly disappears and you can plant out far more freely.
This guide covers what flowers to plant in May in the UK, kept as simple as possible. For everything else going on in the garden in May, What to Plant in May in the UK has the full picture.
Easy Flowers to Sow Directly Outside in May
These are seeds you can put straight into the ground outside in May. No pots, no indoor starting, no fuss. They grow quickly in warm soil and most will be flowering in about 8 weeks.
Nasturtiums are the easiest flower you can grow. Push the seeds into the soil where you want them, water them in, and that is pretty much it. They come up within a week and flower all summer. They are also really useful near vegetables because they attract aphids away from other plants. You can eat the flowers and leaves too.
Cornflowers are also very easy. Sow them thinly, thin them out a bit once they come up, and deadhead them regularly to keep the flowers coming. They produce blue, pink, and white flowers from July right through to the first frosts.
Calendula (pot marigold) is bright, cheerful, and hard to get wrong. Sow directly outside, give them a bit of space, and keep deadheading. The petals are edible and work well in salads.
Sunflowers can go straight outside in May once the risk of frost has passed. They do not do well if transplanted so put them where you want them to grow. Give them a sunny spot and some shelter from wind if you can, as tall varieties can fall over before they get established.
Annual poppies just need scattering onto prepared soil and a light rake over the surface. They do not like being moved so sow them where they are going to flower. They come up quickly and give a good show through summer.
Borage is a brilliant flower to grow near a vegetable patch. Bees love it, it is very easy from seed, and once it is happy it tends to come back on its own each year.
Phacelia is less well known but bees love it even more than borage. It grows fast from seed sown directly in May and is one of the best flowers for attracting useful insects to the garden.
For the full vegetable sowing guide to go alongside your flowers, What to Sow in May in the UK covers every crop worth getting in the ground this month.
Bedding Plants to Plant Out in May
Bedding plants are flowers that were started indoors earlier in the year and are now ready to go outside. The main thing to know is that these plants cannot cope with frost. Do not put them outside for good until late May, when overnight temperatures are reliably above 10 degrees.
Before you plant them out permanently, it helps to harden them off. This just means putting them outside during the day and bringing them back in at night for a week or two. It sounds like extra work but it really does help them settle in much faster once they are out for good.
If you are not sure when your last frost is likely to be, UK Last Frost Dates by Postcode is useful.
Marigolds are one of the most popular flowers for May planting. French marigolds are compact, flower for months, and are great to grow near tomatoes. Plant them out from late May.
Cosmos are tall and airy with flowers in pink, white, and deep red. They are great for cutting and flower right through summer and autumn. Plant out from late May and keep deadheading.
Antirrhinums (snapdragons) are reliable and colourful. They like a sunny spot and well-drained soil. Plant out from late May.
Petunias are one of the best flowers for pots and hanging baskets. They produce masses of flowers all summer. Plant out from late May. They need regular watering and a weekly liquid feed from June to keep performing well. How Often to Water Plants in the UK has more on keeping container plants happy.
Busy lizzies are ideal for shady spots where most flowers struggle. They flower all summer and do well in pots. Plant out from late May and keep them well watered.
Summer Bulbs to Plant in May
May is the main time to plant summer-flowering bulbs. These are different from spring bulbs like tulips which go in during autumn. Summer bulbs get planted in May and flower later in the year.
Dahlias are worth every bit of effort. They flower from July right through to the first frosts and the range of colours and shapes is huge. Plant the tubers about 10cm deep in a sunny sheltered spot from late May once the frost risk has gone. Do not rush them out early as they are not frost hardy.
Gladioli are good for cutting and easy to grow. Plant the corms about 10cm deep in a sunny spot. Planting a small batch every couple of weeks through May and June gives you flowers over a longer period rather than all at once.
Begonias (tuberous types) can go outside in late May in sheltered spots or containers. They are one of the better options for partial shade and flower well into autumn.
Flowers for Pots and Containers in May
Most of the bedding plants above work well in pots and containers. Petunias, marigolds, cosmos, and busy lizzies all do well in pots on a sunny patio or doorstep.
The main difference with container growing is watering. Pots dry out much faster than the ground, especially in warm weather. In summer you will probably need to water every day. A weekly liquid feed from June also makes a big difference to how many flowers you get. Without feeding, plants in pots run out of nutrients quite quickly and the flowering drops off.
Flowers to Grow Near Vegetables
If you grow vegetables as well, it is worth planting a few flowers nearby. It is not complicated. A few simple choices make a real difference.
Nasturtiums are the most useful. They attract aphids away from beans and tomatoes and bring in insects that eat aphids. They are also good for pollinators.
Marigolds are popular near tomatoes. Whether they actually deter whitefly is debated but they do attract useful insects and look good.
Borage and phacelia both bring large numbers of bees into the garden which helps all your flowering crops. If slugs are causing problems, How to Get Rid of Slugs in the Garden covers the best methods. Young flower seedlings are just as vulnerable as vegetable seedlings in the first few weeks.
Do Not Rush Tender Plants Outside Too Early
The most common mistake in May is putting tender plants outside before the cold weather has properly gone. A sunny warm day can make it feel like summer but one cold night can set plants back badly or damage them completely.
Check overnight temperatures, not just daytime ones. Below 5 degrees is risky for most tender plants. If frost is forecast after you have already planted out, How to Protect Vegetables from Frost in the UK covers what you can do quickly. The same methods work for flowers.
FAQs
What flowers can I sow directly outside in May in the UK?
Nasturtiums, cornflowers, calendula, sunflowers, annual poppies, borage, and phacelia can all be sown straight into the ground in May. Most will be flowering within 8 weeks.
When can I plant bedding plants outside in May in the UK?
From late May in most parts of the UK once overnight temperatures are reliably above 10 degrees. In Scotland and northern England this may be closer to early June. Harden plants off gradually for a week or two before leaving them outside permanently.
Can I plant dahlias in May in the UK?
Yes, from late May once the frost risk has gone. Plant tubers about 10cm deep in a sunny sheltered spot. Do not plant them earlier as they are not frost hardy and cold wet soil will rot them.
What flowers are best for pots in May in the UK?
Petunias, marigolds, cosmos, and busy lizzies all do well in pots. Plant out from late May. Water regularly and feed weekly from June to keep the flowers going through summer.
What is the easiest flower to grow from seed in May?
Nasturtiums. Push the seeds into the soil where you want them, water in, and they will be up within a week. Calendula and cornflowers are close behind for ease.
A Sensible Place to Start
May is one of the best months for flowers. Hardy annuals like nasturtiums, cornflowers, and calendula can go straight in the ground right now. Bedding plants like marigolds and cosmos follow in late May once the frost risk has gone. Summer bulbs like dahlias go in now too and will be flowering by mid-summer.
If you are not sure where to start, a few nasturtium seeds pushed into the soil near your vegetables is genuinely one of the best things you can do in May. They come up fast, look good, help the garden, and ask for almost nothing back.
For more on what is worth doing in the garden this month, May Gardening Jobs in the UK and What to Sow in May in the UK cover the full picture.