Thompson & Morgan Gardening Blog

Lupins and alliums are classic cottage garden plants for May – their contrasting shapes work beautifully together
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May is a fabulous time in the garden – fresh spring foliage is injected with colour from early herbaceous perennials, tulips give way to alliums and iris, whilst numerous flowering shrubs such as rhododendrons, viburnum and weigela launch into bloom and the air is perfumed with lilacs. At the end of the month the world’s greatest flower show returns at Chelsea, inspiring gardeners for another busy season.

To keep up with your garden tasks, turn to my jobs to do in May blog . But before you roll you sleeves up and get stuck in, let’s have a look at some of this month’s best plants!

Lilac ‘Palibin’ (Syringa meyeri ‘Palibin’)

Lilac ‘Palibin’
Image: Canva

This dwarf lilac is an old cultivar dating from around the early 1900s where it was already a popular garden plant in China before its discovery by Western collectors. ‘Palibin’ is quite different from the large and domineering common lilac. Slow growing, it will take up to 10 years to reach its maximum height and spread of 1.5 metres but will flower in abundance at an early age, making a perfect specimen for a container or small border. It lends itself to growing as a standard or bush form.  Unlike the large, heart-shaped leaves of common lilacs, ‘Palibin’ has small, delicate, slightly leathery oval leaves which give it added value throughout the season. In spring, deep purple buds open to lavender-pinkish airy panicles. The individual flowers are small but numerous and intensely scented.

Clematis macropetala

Clematis macropetala
Image: Canva

It’s easy to be seduced by the large, showy flowers of later clematis, but the early varieties have exquisitely delicate blooms, are a valuable source of nectar for pollinating insects emerging from hibernation and don’t require any routine pruning. The nodding, semi-double blooms of Clematis macropetala are borne in abundance and make an exceptionally dainty feature scrambling up a wall or trellis.

Geum ‘Totally Tangerine’

Geum ‘Totally Tangerine’
Image: Thompson & Morgan

Geums are valuable perennials for their early flowering and I love the way their ruffled flowers hover on long stems, dancing in the breeze. ‘Totally Tangerine’ wins my vote for the best variety. It is extremely long flowering and after its main May/June performance it continues to produce more flower buds below the earlier spent flowers. The flowers are sterile and so they don’t waste energy on seeding or demand dead-heading. The blooms are not a brash orange but a soft-apricot with subtle variations in tone. This means that they are easily combined with absolutely any colour – although they look especially stunning with purple alliums.

Aquilegia ‘Nora Barlow,

Aquilegia ‘Nora Barlow’
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Aquilegias are one of those happy plants which look incredibly delicate but are as tough as old boots, growing in sun or shade and in any soil type. Their nodding flowers are held on long, graceful stems above a mound of feathery foliage. They don’t take up too much space and can be easily dotted in between other perennials to fill gaps. ‘Nora Barlow’ is an old variety with fully double flowers flushed red, pink and white. If allowed, it will throw up seedlings which will generally come true.

Astrantia major ‘Moulin Rouge’

Astrantia ‘Moulin Rouge’
Image: Sempra

A perfect partner for aquilegias, and equally well-behaved, are astrantias. If you haven’t grown astrantias before then you are missing out. The flowers of Astrantia ‘Moulin Rouge’ are distinctive from other flowering perennials. This is not plant which shouts for attention, but deserves close appreciation of its intricately detailed flowerheads, with maroon papery bracts surrounding a central pincushion. The deeply lobed leaves are also a handsome feature. Prefers a rich, moist soil.

Rhododendron ‘Blue Tit’

Rhododendron ‘Blue Tit’
Image: Morley Nurseries

May is the time to visit woodland gardens when showy rhododendrons are out in force. These large specimens may be beyond the scope of most of us, but we can still enjoy the dwarf varieties. Rhododendron ‘Blue Tit’ is smothered in masses of clear, violet-blue flowers and is perfect for acid borders in sun or partial shade or as a feature in spring containers. Its dark, evergreen leaves provide useful background foliage for the rest of the season.

For more plants which are looking fabulous this month, see Looking Good on The Nursery.

Zinnia varieties come in deliciously fruity shades
Image: Canva

I’m an unabashed lover of zinnias. There is something about the luminosity of their colours and their unique shape – a ruff of stiff petals surrounding a central cone which is crowned with star-shaped golden florets – which I find irresistible. But when I mention zinnias so many people shake their heads and say they have no end of trouble growing them from seed. But they’re actually easy. All you need to do is observe one simple rule!

How to grow zinnias

Zinnias like this ‘Purple Prince’ are members of the daisy family. The centre of the flower is composed of hundreds of tiny, golden, star-shaped florets. These tiny flowers open from the outside towards the centre, forming a raised central cone encircled by stiff petals.
Image: Canva

Zinnias come from Mexico. They are hot season plants which loathe chilly days and nights as well as fluctuations in temperature between the two. As they are half-hardy annuals, many people sow them in February or March, thinking that they need a long growing season and that they will get ahead by sowing them early. What actually results is sad-looking seedlings which just sit and sulk. Before long their leaves curl up in disgust and go brown with botrytis (grey mould) infection.

Ipomea (Morning Glory) are similar in this regard. They resent fluctuations in day and night time temperatures and like it warm throughout the day and night. Expose them to cool nights and they soon develop sickly white patches on their leaves.

Not all zinnias are jazzy, they also come in delicate pink and peach shades such as this charming ‘Zinderella Peach’ which features a large ruffled centre.
Image: Thompson & Morgan

To grow zinnias successfully from seed, always delay sowing until the end of April or first half of May. It may seem late but the seedlings will germinate very quickly – within one to two weeks – and will grow on strongly. Seeds sown earlier will still be flailing whilst later ones rapidly catch up and overtake them.

Sow one or two seeds into separate modules filled with free-draining seed compost. They resent root disturbance and so modules are the best method. You can sow them direct outdoors but I find indoor sowing more successful as slugs love the seedlings. If you sow 2 seeds per plug and both come up then carefully remove the weaker seedling whilst holding the other good seedling in place by pressing your finger alongside it.

Once the roots of your zinnia seedlings have filled their plugs, pot them on into larger pots – I use 9cm ones. Don’t bury the stem deeper in its new pot – this reduces the chances of stem rot. The timing of potting up is crucial. Too early and the plugs won’t drop neatly out of their modules. This will disturb the roots and put a check on their growth. Too late and your seedlings will become pot bound. Pot bound seedlings fail to recover, remaining permanently stunted for the rest of the growing season.

When your seedlings are a few inches tall, pinch out the growing tips to encourage bushy plants. As soon as the root system has comfortably filled up the pot and is just beginning to poke out of the drainage holes, they are ready to plant out. Again – don’t leave it until they are pot-bound! If you sow them late April to early May, then they’ll be ready to plant out in June by which time all risk of frost will have passed. Plant them into freely-draining soil in a sunny position about 23-30cm apart. Water them well afterwards. The large-flowered varieties may require staking.

Zinnias as cut flowers

Zinnias make long lasting cut flowers
Image: Shutterstock

Zinnias make fabulous cut flowers. They have long, sturdy stems and long-lasting blooms. The single coloured varieties make excellent matches for the jewelled hues of snapdragons (antirrhinums), salvias, dahlias and salpiglossis.

Zinnias in the veg garden

Image: Canva

Zinnias look at home in the veg garden, providing welcome pops of colour amongst leafy vegetables and associating well with other potager annuals such as sunflowers. The single flowered varieties are also attractive to beneficial insects such as hoverflies.

Single varieties of zinnia attract plenty of pollinators
Image: Shutterstock

Zinnias in borders and containers

Zinnia ‘Red Spider’ has smaller, finer flowers
Image: Shutterstock

Zinnias bring intense colour in late summer and autumn and are good for filling gaps in borders or extending displays after most perennials have finished. They also lend themselves to hot-themed tropical borders alongside the luxurious purple leaves of the castor oil plant (Ricinus communis), dahlias and cannas.

Compact varieties such as ‘Red Spider’ are also available. These have smaller, more delicate flowers which are good candidates for edging borders and associate well with ornamental grasses. Zinnias also perform well in container displays alongside other summer bedding.

Combine zinnias with other hardy and half hardy annuals to make gorgeous vase displays. Here they are displayed with Antirrhinums (Snapdragon), marigolds, Celosia cristata, Rudbeckia and Ageratum
Image: Shutterstock

Problems

Zinnias like it hot and dry and will succumb to grey mould if it is wet and humid. There isn’t much to be done about this except keep your fingers crossed and hope for a good summer!

Take a look at our selection of zinnia seeds and get sowing now. They will hold their dazzling flowers through summer and autumn, providing zingy colour when everything else looks drab. Keep dead-heading them and you will get armfuls of colourful blooms for indoor displays too. 

Image: Canva

The garden is really racing ahead and it’s a job to keep up with it. Getting on with tasks now will enhance your summer displays, but don’t forget to give yourself some time to relax outside and just enjoy it! 

Bedding

Summer bedding is perfect for plugging gaps in borders
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Spring bedding will be fading and it’s time to switch to summer varieties. If you live in cold parts of the UK and you don’t have a greenhouse or cold frame then you will need to resist the temptation to buy these frost-tender plants too early. Tender bedding plants need to be gradually acclimatised to outdoor conditions after purchase, but you can save yourself the hassle with our Garden Ready plants which have already been hardened off for you. Our pre-planted hanging baskets are also great time-savers.

Greenhouse

Open up the doors and vents in your greenhouse as much as possible
Image: Canva

It can get really hot in the greenhouse so ventilate it as much as possible. Invest in a maximum /minimum thermometer and keep an eye on temperatures. Ideally, you don’t want them to climb above 80 °F or 26 °C. When a greenhouse gets too hot plants stop growing, become stressed and succumb to pests and disease. Aphids, whiteflies and red spider mite will quickly take hold and multiple rapidly in these high temperatures.

In an overheated greenhouse small seedlings can be frazzled incredibly quickly. Seedlings which have just been pricked out are especially vulnerable and it’s important to have a cool, shady spot for them somewhere. Cool down your greenhouse and protect your seedlings by painting some greenhouse shading on the glass. I tend to paint shading onto one side of my greenhouse, which I use for seedlings, but leave the other side for my sun-loving succulents. The other way to cool an over-heating greenhouse is by ‘damping down.’ This simply means flooding the floor with water which will cool the greenhouse quickly by evaporation.

Feeding

New trees and shrubs will appreciate a spring feed with granular fertiliser
Image: Shutterstock

Container plants

At this time of the year winter rains have washed a lot of nitrogen out of the soil and plants may start to look hungry and exhibit yellow leaves. Specimens which have been permanently growing in containers will have used up all the nutrients in their compost and can struggle to put on new, leafy growth. Top dress them with a balanced fertiliser which includes trace elements such as Vitax Q4.

Bedding

When potting up seasonal containers and hanging baskets, professional gardeners always incorporate slow-release fertilisers into the compost. This saves an enormous amount of time on applying liquid fertilisers. Liquid feeds can be useful as a top up but are more wasteful as much of the feed simply drains out of the pot. They also tend to result in inconsistent feeding, promoting surges of soft, sappy growth. Try using a controlled release product such as Incredibloom instead – it will continue to feed your plants for about 7 months.

Roses

Feed hungry roses with a proprietary rose feed now. These contain high levels of potassium to promote flowering.

Pelargoniums (Geranium)

Pelargoniums will also benefit from a flower-boosting high potash feed. Avoid giving them nitrogenous feeds, which will encourage the production of leaves over flowers and create soft growth which is more vulnerable to fungal diseases.

Plant supports & ties

Tall plants like Delphiniums are best supported early on
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Herbaceous borders

Plants are growing rapidly and it’s important to get them supported as quickly as you can. If you are like me, you probably tell yourself this every year, promise to do it next weekend, and then, before you know it, the plants have shot up, the first rain storm hits and everything falls over into a mess! It’s a wise idea to purchase some supports early on so you can avoid getting yourself into this pickle.

Climbers

All your other climbers, such as climbing roses and clematis, will need regular tying in. Sweet peas left to trail along the ground not only grow poorly but are targets for slugs and snails so tie them onto their canes or obelisks early on. I prefer natural jute twine as it’s biodegradable and blends in unobtrusively. Tie the twine in a figure of 8 allowing room for the plant stems to expand. I like to put the string around the plant stem first and then wind it around the support twice. That way you can hide your knots on the back of the support and the tie won’t slide up and down.

Watering

New trees and shrubs should be ‘puddled in’ on planting and watered frequently during dry weather
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Springs have become increasingly dry and the recent weather has been warm but windy. This combination of high aerial temperatures and wind really licks the moisture out of plants and can quickly desiccate them.

Before you plant new plants, soak the root ball by plunging them into a bucket of water. After planting, water them regularly in dry weather. It is best to give plants a generous soak at longer intervals rather than frequent but sparse watering which only encourages surface rooting. Once the root balls of containerised plants have dried out underground they tend to repel water and can remain surprisingly dry after watering attempts, so aim to keep them moist.

Pruning

Cut out a proportion of the old stems from early spring flowering shrubs such as this Ribes (flowering currant)
Image: Annelise Brilli

Early spring-flowering shrubs

Prune established spring- flowering shrubs such as forsythia, Ribes (flowering currant), Spiraea ‘Arguta’ (bridal wreath), Exochorda (pearl bush) and Kerria japonica. These shrubs flower on growth produced in the previous year and so they need to pruned immediately after flowering.

You can take out up to one third of the oldest stems, cutting them back to a shoot/bud near the base. Pruning out the oldest stems will encourage the formation of new ones so that the wood is continually renewed. With Kerria japonica you can cut back all the previously flowered stems.

You can also trim back the top of stems to keep them tidy but bear in mind that these shrubs have an arching habit which you don’t want to ruin. Always try to prune them back to a convenient shoot lower down rather than leaving stubs.

Japanese Quince

Chaenomeles (Japanese quince) has a twiggy rather than arching habit. Shorten the new growths to encourage the formation of lots of short, flowering spurs.

Evergreen shrubs

Inspect evergreen shrubs and prune out any frost damage.

Pyracantha

Wall trained pyracanthas can be pruned back now, reducing outward-facing shoots to a few inches and removing completely any shoots growing into the wall. This will inevitably remove some flowers but it will promote the formation of short spurs which will become crowded with future flower buds and berries.

Dahlias, cannas and other tender perennials

Dahlia tubers should be shooting well by now
Image: Shutterstock

Harden off your tender plants which you have been overwintering in a frost-free place. Plant them out when all danger of frost has passed. This is always a gamble but generally you should be safe by the end of the month.

If you don’t see signs of growth on your tender perennials don’t give up on them too soon -they are slow into growth. All the top growth of tender Salvias may completely die off but don’t throw them away in haste – many varieties will re-sprout from the base.

Chelsea Chop

The Chelsea Flower Show is a reminder to do the ‘Chelsea Chop’
Image: Shutterstock

Enjoy the Chelsea Flower Show which runs from 24-28th May this year, and use it as a reminder to try out a pruning technique called ‘The Chelsea Chop’.

If you want to keep your herbaceous perennials shorter, sturdier and more compact, you can chop them back by up to a half in late May or early June. This pruning technique will produce plants with shorter, self-supporting stems and more (but usually smaller) flowers. Not all perennials will respond to this treatment but those which do include: Asters, Echinacea (coneflower), Helianthus x laetiflorus (perennial sunflower), Hylotelephium (Sedum), Monarda (Bergamot), Nepeta (Catmint), phlox and rudbeckia.

Seed sowing

Sow flowering annuals in rows the same way as you would with vegetables
Image: Canva

• The soil is lovely and warm now and hardy annuals can be sown directly into the ground. Rake the soil well to make a fine tilth and sow them in rows. Sowing in rows makes it much easier to distinguish between your flower seedlings and weed seedlings. Once the seedlings have grown and you have thinned them out, you won’t notice that you originally sowed them in straight lines.
• Thin out/pot on any annuals sown earlier.
• It’s a bit late now to sow most tender annuals – best to buy them as ready-grown plugs.

Biennials such as Wallflower ‘Scarlet Bedder’ can be sown this month
Image: Canva

Don’t forget to sow biennials for next years bedding plants. Wallflowers like the sumptuous ‘Scarlet Bedder’ and dainty forget-me-nots will make fabulous partners for next year’s tulips. Pansies sown now will provide vibrant colour in the winter, whilst Bellis perennis ‘Pomponette’ will grace window boxes and border edges with bright pom poms next spring. For cut flowers, you can’t beat clove-scented Sweet Williams (Dianthus barbatus).

Foxgloves are cottage garden staples and great for bees
Image: Canva

If you are looking for some flowers for the wildlife garden, foxgloves will provide plenty of food for foraging bees. They should be sown now on the surface of trays of fine compost or in a patch of spare ground. The tiny seeds are like dust and shouldn’t be covered – they won’t come up if you do! Sweet Rocket (Hesperis matronalis) will also attract pollinating insects with its sweetly fragrant blooms.

Lawns

A drop spreader is a useful tool for fertilising large lawns
Image: Shutterstock

Fertilise

Apply spring fertiliser to lawns. If you only have a small to medium family lawn ecofective Lawn Feed is an organic treatment which will treat up to 150 square metres and is safe for pets and children.  For large lawns, Sportsmaster Spring & Summer is a high nitrogen, slow-release formula which you can use all growing season.

Try to keep your use of weedkillers to a minimum. Remove deep weeds by hand or spot treat them. If you are going to resort to chemicals, it is much more effective to use a separate fertiliser followed by a weedkiller product, such as Weedol, rather than a combined treatment. Fertilising first boosts the grass in its competition with the weeds, enabling it to quickly grow into gaps after treatment. Meanwhile the weeds will put out more leafy growth, which will absorb more weedkiller.

Mow

Keep mowing your grass every week, gradually reducing the height of cut as the weather gets warmer. Better still – stop mowing and weedkilling altogether! And join in with No Mow May – see below.

Pest control

Avoid using chemical pesticides which are not only harmful to your enemies but also your garden allies such as this aphid-munching ladybird larva
Image: Canva

• All this warm weather means that pests are rapidly multiplying. Use environmentally friendly sprays such as ecofective Bug & Mildew Control to keep on top of aphids, black fly and red spider mite.
• Order biological nematodes to control a range of pests which rapidly emerge in spring including slugs and vine weevil.
• Whitefly in greenhouses can quickly become a real problem, so introduce the parasitic wasp, Encarsia formosa, before pest populations build up.
• Treat roses with a preventative anti-fungicidal spray before blackspot, rust and mildew take hold.

Clematis

Keep tying in clematis but use natural twine, which biodegrades, rather than this plastic coated wire.
Image: Shutterstock

• Regularly tie in clematis to its support.
• Clematis montana does not need regular pruning but if it’s getting out of hand in a confined space you can trim it back now and remove any dead or diseased growth.

Plant propagation

Softwood cuttings should be about 3-5 inches (7.5-12.5cm) long
Image: Canva

• Take softwood cuttings from hardy perennials, shrubs and tender perennials such as pelargoniums and osteospermums. Although softwood cuttings should be pliable, the very softest growth will often wilt quickly and is less successful. Wait until the material has firmed up slightly and gives a little snap when you bend it.
• Once they have finished flowering, cut back and divide spring flowering perennials such as Pulmonaria (lungwort), Doronicum (leopard’s bane), Brunnera (Siberian bugloss) and primulas.

Rock plants

If you don’t prune aubretia after flowering it will become straggly and bare at the base
Image: Shutterstock

Spring flowering rock plants such as alyssum, arabis and aubretia should be cut back hard now to keep them trim and tidy.

Early spring bulbs & flowers

Dead head tulips and daffodils so that they don’t waste energy on developing seed.
Image: Annelise Brilli

• Continue dead-heading daffodils and tulips but leave the foliage intact for at least 6 weeks after flowering. It is not worth doing this to ‘bedding’ tulips, which decline after the first year, but some tulips, such as Darwin hybrids, are reliably perennial and will continue to flower again next year.
• Once the foliage is yellow, lift congested daffodils which didn’t flower well this year and replant them.
• Fading spring bulbs which you want to keep for next season can be lifted and transplanted elsewhere to make space for summer bedding. Feed all your spring bulbs now, preferably with a low nitrogen, high potash feed.
• Pulmonaria (lungwort) and Brunnera (Siberian bugloss) invariably get mildew at this time of the year. Simply cut all the foliage off and give them a good water and they will reward you with healthy new foliage.

Wildlife Gardening

Plants such as this Calendula (pot marigold) are important sources of food for foraging insects
Canva

• Buy or sow insect attracting summer plants such as Nicotiana, (tobacco plant), Calendula (pot marigold), cosmos, sunflowers, salvias, heliotrope, Agastache (giant hyssop), wallflowers, Lobularia maritima (alysum).
• Gently clear the pond of pond weed leaving the debris on the side so pond creatures can easily crawl back in.
• “Join legions of gardeners and say “no” to the mow this May to help our bees, butterflies, wildlife and us!” says Plantlife. You don’t have to do much. Nothing in fact. Just relax in a garden armchair and let the wild flowers in your garden provide a feast for hungry pollinators. You might be amazed at what pops up in your lawn when you stop mowing. Instead of telling yourself that there’s nothing you can do to help the planet, do just this one simple thing and help to reverse the drastic decline in our insect populations.