6 fast-growing flowers to sow in May to quickly fill the gaps in your borders and containers with summer colour

It’s well and truly time to crack open those seed packets, and if you’re hoping to fill the gaps in your garden with colourful blooms, there are plenty of fast-growing flowers you can sow in May.

This week, I’m starting my list of things to plant in May with flowers – more specifically, annuals and perennials that will bloom this summer. These varieties have so many merits in the garden, from warding off pests in a vegetable patch to sprucing up sparse garden borders.

Here’s a list of fast-growing flowers to sow in May to get you started – including a few I’ll be sowing this week!

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1. Marigolds

I say it every year: marigolds are superhero flowers in the garden, and I don’t think I’ll ever garden again without them.

There are two types: Calendula, also known as pot marigold (you can order Calendula officinalis seeds from Crocus), and Tagetes, which includes other garden marigolds (xxx is a popular variety). Both make fantastic companion planting ideas alongside vegetables and other pest-prone plants in the garden – they’ll attract beneficial insects like ladybirds, which feed on aphids, for example (and many garden plant pests tend to hate the smell of marigolds, too).

‘For gardeners wanting something vibrant in a short space of time, Calendula is a brilliant option,’ says Liam Cleary, managing director at The Old Railway Line Garden Centre. ‘It germinates very reliably in May and tends to flower within a matter of weeks once temperatures are mild.’

Where to buy marigold seeds:

• Amazon: Fothergill's Marigold (French) 'Dwarf Double Mix' seeds produce beautiful double blooms.
• Thompson & Morgan: Buy a pack of 100 Marigold 'Solan' (French) seeds for just 99p.

2. Cosmos

If you’re looking for low-maintenance, fast-growing flowers to sow in May, you can add cosmos to the list. There are so many varieties to choose from – personally, I love chocolate cosmos (you can order plug plants from Crocus for even faster results) – and because they produce so many flowers, they offer plenty of bang for your buck.

‘Cosmos are really popular plants in the UK because they’re so easy to grow and give a lot back for very little effort,’ says Chris Bonnett, founder of GardeningExpress.co.uk.

‘This makes them great for beginners. You can grow them from seed, they germinate quickly, and they don’t need much fuss or special care.’

Where to buy cosmos seeds:

• Gardening Express: Cosmos 'Special Striped Mix' produces stripy pink and white flowers.
• Crocus: The pretty pink blooms of Cosmos bipinnatus 'Apricota' are described as 'almost iridescent'.

3. Nasturtiums

Nasturtiums are another fast-growing flower I sow in May, because the benefits of this flowering annual are huge.

When I first grew nasturtiums from seed, I was stunned by how quickly they grew into mature plants. I tend to use them as sacrificial plants next to my brassicas, as nasturtiums will often lure slugs, snails and caterpillars away from other crops. It’s one of the tricks I learned when I first started my raised vegetable bed.

‘If you’re wanting bold colour fast, nasturtiums are incredibly rewarding and very forgiving for beginner gardeners,’ agrees Liam. ‘Nasturtiums actually flower better in poorer soils, so there’s no need to overfeed them.’

Where to buy nasturtium seeds:

• Marshalls Garden: For trailing or climbing plants, sow Nasturtium 'Tall Mixed' seeds.
• Amazon: This Fothergill's Nasturtium Collection is a great seed starter pack.

4. Lobelia

Lobelia can be annual or perennial, depending on the variety, but Chris recommends annual lobelia (those that last one season) for speed. They’re also brilliant plants for hanging baskets and pots, because they’ll spill over the edge of a container.

‘Annual lobelia varieties are the fastest to spread in summer, whereas perennial types are more upright and don’t spread rapidly in quite the same way,’ Chris explains.

If you’re keen to plant lobelia straight into the garden this month, you can order Lobelia 'Fountain White' plug plants from Thompson & Morgan for white blooms.

Where to buy lobelia seeds:

• Marshalls Garden: Lobelia 'Crystal Palace' seeds produce deep blue flowers and near-metallic foliage.
• Amazon: Fothergill's Oh Sow Simple Lobelia 'Blue' are simple but beautiful.

5. Love-in-a-mist

Love-in-a-mist is as pretty as it sounds: a fuss of star-shaped blue blooms with a ‘mist’ of delicate foliage around the petals.

‘Nigella damascena, often called love-in-a-mist, is excellent for softening gaps between borders because it creates an airy but natural look really quickly,’ says Liam.

It’s one of the best fast-growing flowers you can sow in May, but you’ll need to plant the seeds in their final growing location, because Liam says Nigella really dislikes being transplanted.

‘A light rake over the soil after scattering the seed is usually enough,’ he adds. ‘Burying it too deeply can slow germination considerably.’

Where to buy love-in-a-mist seeds:

• Crocus: Nigella damascena 'Miss Jekyll' is one of the most popular varieties.
• Thompson & Morgan: Nigella damascena 'Persian Jewels' seeds produce multi-coloured blooms.

6. Cornflowers

Cornflowers aren’t a far cry from love-in-a-mist in terms of appearance, but they’re my favourite wildflower. You can sow these fast-growing flowers in May for a profusion of blooms that pollinators go wild for (bees are especially fond of them).

‘For pollinator-friendly spaces, cornflowers are fantastic in May,’ says Liam. ‘They establish quickly and cope well with the unpredictable spring weather.’

Liam recommends sowing cornflowers little and often over a few weeks to keep a steady display of flowers throughout the summer.

Where to buy cornflower seeds:

• Suttons: Cornflower 'Polka Dot Mix' is so pretty.
• Amazon: Sow the classic Wild Cornflower Seeds from Fothergill's.

There are plenty of drought-tolerant shrubs you can plant in May ahead of the warmer weather, too.