When is best to plant summer bulbs? Gardening experts share timing guidelines for lilies, gladioli, and more

Our guide on when to plant summer bulbs is a must-read for a show-stopping display. While those long, hazy days are still a while off, now's the time to prepare, especially if you want to make an early start by planting some under cover.

When we think of plants you can grow from bulbs and tubers, the spring-blooming types often come to mind first. But, summer-flowering beauties offer a whole other range of fabulous colours and forms, from perfumed lilies to dahlias the size of dinner plates. There's a variety suited to every garden trend.

Below, you'll find timing tips for six summer-garden stars, including plenty of insights from the experts.

When to plant summer bulbs for a spectacular show

Knowing when to plant spring bulbs, such as daffodils and crocuses, is key for an easy show early in the year. These types are hardy, which means they can put up with the winter cold. In fact, it's necessary for them to bloom.

On the other hand, summer bulbs tend to be less tolerant of chilly temps, and most should be planted in the spring. As Angela Slater, a gardening expert from Hayes Garden World notes, while they arrive in garden centres from the end of January, this is too early to plant them outdoors, as the cold and wet weather can prove fatal.

"You need to have a warm, light, frost-free greenhouse or porch where you can grow them in pots, for putting out into the garden later," Angela explains. Alternatively, you can play the waiting game, and plant them directly outside once it's warmed up.

Similarly to spring bulbs, it's important to note that summer-flowering bulbs are generally toxic to pets – and often to humans, too. Be sure to wear gloves when handling them, and always keep them well away from pets and children.

Angela Slater

Angela has worked for Hayes for 33 years in various guises. She was consistently outdoors from a young age, helping her father in the garden who would grow his own vegetables. Angela now owns a smallholding where she grows a selection of vegetables and soft fruit and has a degree in Conservation and Land Management.

When to plant lilies

While the majority of summer-flowering bulbs should be planted exclusively in the spring, most lilies are winter-hardy and offer a bit more flexibility.

"Lily bulbs can be planted in either autumn or spring, offering a second opportunity if we get busy in autumn and miss the first," says Dickon Harding and Becky Crowley from the park and gardens team at Raby Castle. The end of March or early April is generally considered the cut-off point for getting these plants into some soil.

"With so many varieties available, there’s a lily to suit every garden and taste," the team continues. "The delicate, downward-facing blooms of martagon lilies have an understated charm, while others such as the towering Lilium 'Honeymoon' boast large, richly scented flowers on stems up to two metres tall." For rich, deep-red colour, try the "Claude Shride" martagon lily from Crocus. Or, try their white lily collection – a perfect match for the quiet luxury gardening trend.

"One of the best things about lilies is that their bulbs are perennial, meaning they can be left in the ground year after year, growing stronger over time," the team adds.

Dickon Harding

Dickon Harding is the gardens and landscape manager at Raby Castle, Park and Gardens in County Durham. He is responsible for overseeing the operation of the newly remodelled 18th-century walled gardens and the role the five-acre space plays within the wider visitor attraction.

Becky Crowley

Becky Crowley is the assistant head gardener at Raby Castle, Park and Gardens and will work alongside the team there to ensure the vision of world-renowned garden designer, Luciano Giubbilei, is fulfilled in the remodelled walled gardens.

When to plant Abyssinian gladiolus

"A more delicate option, acidanthera (also known as the Abyssinian gladiolus) requires a little extra care as it’s less tolerant of frost," shares Dickon and Becky. However, they assure that the effort is "well worth it", highlighting the flowers' slender stems, white petals, and burgundy throats. They are also a good pick if you love fragrant plants, as they emit a sweet perfume.

For a head start, you can plant these bulbs – which are actually corms and slightly different in their makeup – in pots under cover in early spring (March or April). At Raby Castle, the team plant three per 9cm pot and grow them on in their glasshouse and cold frames. "Once the soil has warmed in May and the risk of frost has passed, they’re ready to be planted out." Alternatively, you can plant them directly outdoors in May.

Acidanthera bulbs can be purchased from Sarah Raven.

When to plant gladioli

"While acidanthera offer subtlety, gladioli bring drama and vibrancy," says the Raby Castle team. According to Angela, they are making a comeback after largely falling out of favour, appearing regularly at the Chelsea Flower Show. "They are available in a huge range of colours and they make a great addition to the cutting garden, lasting a long time in the home," she says. Apply our tips on how to make cut flowers last longer to prolong their display even further.

Like acidanthera, they technically grow from corms rather than bulbs and you can start them off in a frost-free place in early spring. From May to July, they can be planted outside.

It's possible to extend their flowering season by staggering your planting to every couple of weeks, as the Raby Castle team highlights. "This successional planting provides a continuous display from mid to late summer, right into autumn."

Angela recommends planting them in fertile, well-draining soil in full sun and sheltered from the wind. "They can be grown in containers, provided they are heavy enough to counterbalance their height," she adds.

Try the velvet gladioli collection from Sarah Raven for a mix of lime-green and purple blooms.

When to plant dahlias

Technically, dahlias grow from tubers rather than bulbs or corms, which are similarly underground, fleshy structures that store nutrients to help the plants grow. They are some of the best plants for a riot of late-season colour. Sarah Raven has a fantastic range available to buy.

"From the bold and brassy to demure and subtle, there is a dahlia to suit every garden," says garden designer Isobel Spandler. While tubers are available from early spring, she warns against the temptation to plant them out too soon; "dahlias need protection from freezing temperatures and any growth caught by a frost will die back."

To get ahead, Isobel recommends planting them in pots from March to keep under cover until late May to June, when they can be moved outdoors. "Otherwise, wait until the frosts are over and plant just under the soil surface." Mulching over the top will help to protect any early growth that could be caught by a late frost, she adds.

Top tip: If you want to learn how to grow dahlias but missed the window to buy dry tubers, Angela notes how they are usually available as potted flowering plants in summer. However, this is often a more expensive option, she points out.

Isobel Spandler

Isobel has been gardening since her father gave her a small space in the garden to grow strawberries. From balconies in London to container gardening in Army quarters, Isobel has continued to garden and, after studying at the English School of Gardening, founded Wiltshire Garden Design in 2004.

When to plant non-stop begonias

"Non-stop begonias are the huge, many-petalled plants which give such a fantastic summer show in troughs and containers," says Angela. Like dahlias, these plants grow from tubers.

You can shop for nonstop begonia tubers from Thompson & Morgan, and start them off under cover from February. "Plant up your containers and place them in a frost-free greenhouse or porch until all danger of frosts has passed, usually late May or early June, depending upon where you are in the country," Angela says. "Make sure your container has plenty of drainage holes as if they’re left sitting in cold soggy compost, they will rot."

Once they can go outside, she recommends keeping them in partial shade, out of the full midday sun. "Keep deadheading to encourage a procession of flowers."

Angela shares another useful gardening tip: "Remove the female flowers, they are substantially smaller than the large showy flowers; if they are left, they will start to put their energy into producing seed and not more blooms."

When to plant crocosmia

Crocosmia are a striking late summer flower with arching stems of fiery-hued blooms. Plant the corms in your borders from April to June (after frosts have passed) in a sunny and well-draining spot. Or, get an early start by planting them indoors from February onwards. We love the look of the award-winning crocosmia collection from Crocus, which includes three different varieties.

Crocosmia are often sold as pot-grown plants, too, for planting throughout the growing season of spring to autumn.

FAQs

What is the latest you can plant summer bulbs?

Angela recommends planting summer bulbs by the start of summer at the latest, otherwise, they will be late flowering. "If the autumn weather is particularly cold and wet, they may not flower at all."

Can you leave summer bulbs in the ground all year?

"Most summer bulbs need to be taken into a dry, frost-free environment over winter," says Angela. "They need to be stored where they get a good airflow but out of the reach of rodents.

"If you have a light, very well-draining soil in a mild climate, you may get away with leaving your dahlias and gladioli outside all winter," she adds. If you decide to try this, be sure to add a thick layer of mulch to protect them.

Lilies and crocosmia tend to be on the tougher end of the spectrum. They can stay in the ground in most regions of the UK, although can also appreciate a layer of mulch.

Knowing when to plant summer bulbs will help bring your garden to life throughout the warmer months, but don't forget there are lots of other types of plants to consider. For instance, growing sweet peas from seed is an easy way to fill your outdoor space with colour and scent. And don't forget to add plenty of plants for pollinators to help your local wildlife, too.