Thompson & Morgan Gardening Blog

This sedum provides welcome colour in late-summer borders
Image: Sedum spectabile ‘Brilliant’ from T&M

Perennials are beautiful and versatile plants that fill gardens with interest for many, many years. Often chosen for their spectacular free-flowering colour, you can also plant perennials for height, evergreen structure, scent, ground cover and more. We asked experienced gardener, Carol Bartlett, to share her knowledge about growing these hard-working favourites. Here’s her tried and tested advice…

Browse our full range of perennial plants for inspiration.

What is a perennial plant?

A perennial is a non-woody plant which lasts for more than two years. Hugely popular, they come in all shapes and sizes including tall, scented, climbing, winter-flowering and evergreen perennials. Most are hardy perennials, but a few are half-hardy. Some are also herbaceous, which means they die back in late summer, eventually becoming bare earth over winter before producing fresh new foliage in spring.

If you’re looking for colour in your garden, perennials provide the full rainbow from pale pastels to hot reds. They include garden favourites like bright red/orange/yellow crocosmia, hardy and dependable geraniums, cottage garden plants with tall spires like lupin and delphiniums, long-flowering heleniums, bold coneflowers and the ever-popular clematis. Perennials offer great value for money. Once planted, they return year after year.

When and where to plant perennials

Alstroemeria blooms in patio containers from June through to November
Image: Alstroemeria ‘Indian Summer’ from T&M

Spring is traditionally considered the best time to plant perennials, but you can plant them at any time from spring to late autumn. Many perennials are sold during the summer as garden-ready plants and add instant colour to your border. Good watering is essential to establish new plants, so spring and summer planted perennials will require regular top-up watering. Autumn is often overlooked as a planting time, but it’s ideal. The soil is still warm, and we get more rain.

Perennials are suitable for growing in borders or containers, adding colour and scent. Some of the best perennials for containers are agapanthus, dahlia, lilies, and lavender. A large container of hostas can also look lush and stylish, particularly in those shady corners.

The key to growing perennials successfully is “right plant, right place”. All perennials have preferred growing conditions, which could be full sun, partial sun, shade, moisture-retentive soil, or dry conditions. For a plant to thrive, you need to grow it in a spot in your garden that best matches those growing conditions. If you’re not sure about the conditions required, check our perennials resource page for information and advice.

How to plant perennials

This compact clematis is a hardy perennial that is perfect for patio containers
Image: Clematis ‘Guernsey Flute’™ from T&M

As a rule of thumb, most perennials require humus-rich soil with good structure. Perennials, especially herbaceous perennials, put a lot of energy into growth and flowers each year and are best planted in good soil. There are always exceptions. Lavender and nepeta will thrive on thin soil, as will achillea.

Here’s how to plant a garden-ready perennial:

• Dig a hole slightly larger than the pot, say 5cm extra all around.
• Ease the perennial out of the container and free the roots so the plant is not compacted.
• Place in the hole at the same level as the garden earth.
• Backfill the hole, and firm well to ensure no air pockets. If roots grow into an air pocket, they can’t draw nutrients or water, and the plant may not thrive.
• Look up each plant to find out its height and spread. This helps you to gauge where in the border to plant, and also how close to surrounding shrubs and perennials.
• Water, water and then water again.

There’s one important exception. When planting clematis, ensure the plant is below the soil surface by about 10cm. Clematis prefers to be planted a little deeper.

How and when to prune perennials

The aromatic silvery grey foliage of nepeta is suitable for the Chelsea Chop
Image: Nepeta x faassenii from T&M

You can prune herbaceous perennials in the autumn once the top growth has died back. Alternatively, leave the top growth in place which will afford the plant (and insects overwintering in it) frost protection, and prune in the spring.

During the summer growing season, you can also cut back many perennials after they finish flowering to encourage a second flush of fresh foliage and flowers. Ideal for this treatment are hardy geraniums, alchemilla mollis, centaurea, lavender, heuchera, and nepeta. Deadhead your perennials regularly to encourage new blooms throughout the season.

If you want to make your perennial plants bushier, you could also try the Chelsea Chop. Carried out in May, the Chelsea Chop is simply a light prune of perennials, before they flower, reducing the plants by about one-third. This light prune makes the plant produce more shoots. It’s especially useful for plants which tend to sprawl, like nepeta. You can vary how you apply the Chelsea chop by cutting back some plants and not others. If you have lots of the same variety, this will stagger their flowering period. You can also chop back part of the plant, or chop the whole plant to reduce its height in the border. Adapt the technique as you wish to get the best out of your perennials. Plants suitable for the Chelsea Chop include nepeta, aster, monarda, lavender, echinacea, artemisia, sedum, phlox, helianthus, and rudbeckia. As a guide, it’s suitable for multi-stemmed perennials.

How and when to divide perennials

Hemerocallis has large root balls
Image: Hemerocallis ‘See You Tomorrow’ from T&M (© Plantipp, Visions BV Netherlands)

Ideally, you should divide your perennials every 3 years or so. You’ll know it’s time to lift and divide them if your plants aren’t flowering as well, or have bald patches in the middle. The best time to divide perennials is either in the spring or autumn. Here’s what to do:

• I always use a fork to lift the plant out of the ground to minimise damage to the roots.
• Inspect the roots. With such a wide range of plants, roots vary significantly.
• With smaller plants, such as geraniums, you can pull the root ball apart with your hands. Separate it into 2 – 5 parts depending on the plant size.
• For larger plants, such as Hemerocallis, the root may be quite large. This type of root is best separated by putting two forks back to back and prising the root ball apart.
• There are large plants with very tough, fibrous roots such as Agapanthus which require more force. Once the plant is out of the container, saw it into several parts using an old bread knife or a hand saw. It may seem brutal, but the plant will recover and improve after being divided.
• Replant in appropriate growing conditions, and water well.

Carol Bartlett’s five favourite perennials

The frilly flowers of L. ‘Crazy Daisy’ are one of Carol’s all-time favourites
Image: Leucanthemum ‘Crazy Daisy’ from T&M

I love perennials and grow many in my garden for colour and scent. A few of my favourites are:

1. Leucanthemum because I love daisies and so do the bees. Easy to grow, and long flowering, I especially like the varieties with frilly petals such as L. ‘Crazy Daisy’
2. Agapanthus for its large, spikey flowers in bold shades of blue and white. Agapanthus grows well in containers making it ideal for a patio. Agapanthus are also long flowering and belong to that exclusive group of plants whose flowers look good as they fade.
3. Geraniums are a garden stalwart flowering reliably year after year. They come in many sizes and colours and you’ll find varieties tolerant of almost all growing conditions. There’s always space in my garden for geraniums that add colour throughout summer and into October.
4. Sedum is a late-summer and autumn-flowering perennial. I’ve never had a garden without sedums. I love the rosy red and white varieties, which are wildlife-friendly and a magnet for butterflies and pollinators.
5. Lavender, which I find irresistible for its intense shades of blue and calming scent. I like to plant lavenders next to a path to release the perfume, or on the patio so I can reach out and touch the flowers which are so tactile and wonderfully scented.

We hope this has given you plenty of help with choosing and planting perennials. Check out our top ten perennial plants for more inspiration and, perhaps try sowing your own perennial seeds for a satisfying challenge.

Carol Bartlett

Carol Bartlett has been gardening for over 30 years creating many different types of gardens. Her website, The Sunday Gardener, was set up over 10 years ago to provide practical down-to-earth advice, especially for new gardeners, no matter what size their garden or vegetable plot. She also aims to inspire others to enjoy gardening and reap the benefits of being outside with nature.

Carol writes the website information herself, and most of the images have been taken in her gardens or those she has visited. The idea is to show plants in an authentic garden setting. Her philosophy is that gardening is a great combination of the therapeutic and practical. It is a cliché, but still true, the garden is a haven in which to escape day-to-day pressures. You can follow Carol on Facebook and Instagram and watch her helpful videos over on YouTube.

www.sundaygardener.co.uk/