Gardening in May

Plants in greenhouses are at risk of scorched leaves as the sun gets stronger this month. To prevent greenhouses from overheating, shading is a must. Paint-on solutions are easily applied to roofs and sides with a paint brush – just make sure that glass is clean and dry before application. To improve air flow, open roof vents and louvres in the morning, but close before dusk to retain warmth.

If you can’t be at home 24/7, install automatic roof vent openers. A maximum-minimum thermometer will help you to keep track of temperature highs and lows. During hot spells, dampen greenhouse floors with water to increase humidity.

Sweet, juicy home-grown strawberries are one of the joys of summer, especially if they’re ripe in time for Wimbledon. Trouble is, slugs and birds are partial to the fruit, too, while grey mould can be catastrophic. To keep rot at bay, place straw under flowers and swelling fruits, so they’re not in contact with damp soil. Take care when watering, aiming water on the soil or compost without splashing fruit. If birds are a problem, use netting raised up with canes to protect crops, but make sure birds can’t become trapped underneath.

Hanging baskets add great colour to your garden and they are easy to plant up yourself.

• Sit your hanging basket on a bucket and line the wire frame with moss.
• Fill with a compost specifically for pots and baskets, as it’ll retain moisture for longer, reducing the risk of baskets drying out.
• Push trailing plants through gaps in the moss from the outside, topping-up the compost as you go. 
• Basket favourites such as busy Lizzies, trailing fuchsias and petunias should be planted into the top. Also consider bacopa, trailing begonias, calibrachoa, nasturtiums and lobelia. 
• Firm plants in and water well.

Ant infestations are common as temperatures rise. Ants make a beeline for your food or drink if you’re dining al-fresco, and push soil up between gaps in paving, leaving a mess. Applying ant powder at the entrance to nests will control the problem, but if you prefer not to use chemicals, flood the area regularly with cold water.

Where ants infest lawns, don’t kill nests by pouring boiling water into the entrance. It may wipe out the ants, but scalding water will kill the grass too! Repeated flooding with cold water is the best way to see off ants in lawns.

Towards the end of May, tender flowers and veg in greenhouses or on window sills will need to be acclimatised to cooler temperatures and wind outdoors – a process known as hardening off – before being planted out in their final growing positions. This helps plants to avoid a shock that can stunt growth.

The easiest way to harden off is to stand trays of plants outdoors during the day, when weather is fine, and then bring them back indoors at night, carrying on with this procedure for a fortnight.

Sweetcorn is one of the most rewarding veg to grow: cobs are intensely sweet and flavoursome. Sow the seeds about an inch deep in small pots or modular seed trays filled with seed and cuttings compost.  You can do this up until mid-May, keeping the pots somewhere warm, around 20°C, until seed germinates. Young plants can’t go out until June, when the danger of frost is well and truly over, so in the meantime, prepare the final growing site.

Choose a sunny, sheltered spot, and dig in well-rotted manure. When planting out, position young plants in blocks 18-20 inches apart, rather than in rows, as sweetcorn is wind-pollinated. For extra-sweet cobs, sow varieties such as ‘Swift’ or ‘Lark’.

May is a great month to tidy-up evergreen hedges in time for summer, but before you reach for the shears or hedge trimmer, remember that it is against the law to disturb nesting birds, so examine dense growth to check that no nests are present first. 

Hedges such as box, hornbeam and beech respond well to spring trimming. Leyland cypress hedges should also be given a haircut to prevent these vigorous growers from becoming unruly.

Find out more about hedge care and maintenance >

Primroses and polyanthus can look unsightly now that flowering is over. Left to their own devices, clumps can flower poorly in future, so dig plants up and divide them. The process of division gives plants a new lease of life, resulting in better flowering next year. If space is needed for summer bedding, primroses can be grown on in a quiet corner of the garden after division, then replanted in their flowering positions in autumn.

Greenhouse tomatoes may be big enough to plant into their final growing positions. If you plan to use growing bags, buy the biggest, fattest bags as they’ll allow maximum root development and retain moisture. Where tomatoes are to be grown in big pots, fill with John Innes No3 or multi-purpose compost. Cordon tomatoes, which have one central stem, will need canes for support. Tie stems to canes regularly using string or twine and pinch out side-shoots that emerge between the stem and leaf stalks. Once the first truss of fruit has set, liquid feed with tomato fertiliser.

As temperatures rise and grass grows faster, weekly mowing may be required. If your mower’s blades were set high for the first cut last month, they can now gradually be lowered. However, avoid mowing too close, especially in dry conditions, as it can weaken grass and even risk scalping the lawn. Lawns that are cut too short will need additional feeding and are more susceptible to drought, while bare patches can be easily seen. New lawns, laid from turf last autumn or sown this spring, may need additional watering in warm conditions until established.

Get more maintenance tips for a flawless green lawn >

It wouldn’t feel like May without the spectacular sight of rhododendrons at their finest; shrubs that are simply unbeatable for springtime flower power.

Growers say there are around 900 rhododendron species and more than 25,000 named rhododendron hybrids, so there is no shortage of choice, but gardeners often regard rhododendrons as woodland plants, fearing they could take over smaller plots. While some can indeed grow into monsters, compact hybrids will grow happily in big pots, so even owners of tiny gardens need not miss out on their spring-time spectacle.

Rhododendrons thrive in dappled shade but must get their roots down into acidic soil, so use a soil test kit to determine the pH of your soil.  Instead of using peat, improve soil ahead of planting rhodos by digging in organic acidic matter such as leafmould, composted tree bark or decomposing pine needles.

Experts’ recommendations for small gardens are always welcome and Rhododendron yakushimanum ‘Koichiro Wada’ topped a list of 100 favourites chosen by members of the Rhododendron, Camellia & Magnolia Group. It loves dappled shade and will hit a height and spread of no more than 1.5m after 10 years.

What if you don’t have acidic soil? Inkarho lime-tolerant rhodos are a modern marvel that’ll grow on neutral soil (up to pH7). Or simply pick any rhododendron of your choice and grow it in a large container filled with John Innes ericaceous compost, which is blended for acid-loving shrubs.

One final tip: don’t plant a new rhododendron too deeply, or it may be reluctant to flower. Set new shrubs in the soil so the top of their rootball sits level with the surface.

Growing sweet corn in UK gardens is a doddle, and there’s no reason why novices with little grow-your-own experience shouldn’t be rewarded with a decent crop. 

Varieties proven to perform include ‘Swift’, an F1 hybrid that’s regarded as the holy grail of sweetcorn – vigorous, extra tender, high in fibre and bred to crop as early as August. ‘Lark’ is a dependable F1 hybrid that offers a less chewy texture than conventional super sweet varieties.

Sow sweetcorn seeds indoors in pots of compost as early as possible in May, for planting out in June when the risk of frost has passed. Sweetcorn seed is large enough to handle easily: sow each seed 1.5cm deep in small pots or modules of seed and cuttings compost. Keep moist and at a temperature of 15-20°C and seeds will germinate within a fortnight. Towards the end of the month, place young plants outdoors by day and bring back under cover at night – a process known as hardening off, which gradually gets plants used to cooler conditions outdoors and avoids the risk of a sudden shock to their system.

In early June, sweetcorn plants will be ready for their final growing positions outdoors. Choose a sheltered, sunny spot with fertile soil and plant in blocks instead of rows – because sweetcorn is wind-pollinated – setting young plants at least 40cm apart. Avoid planting more than one type of sweetcorn together, because some varieties need to be grown in isolation, and keep plants well-watered – especially during hot spells and while they’re flowering.

Now here’s the fun part. When plants’ tassels turn brown, peel back the leaves surrounding a large cob and use your fingernail to pierce a kernel. If the liquid that squirts out is watery, the cob isn’t yet ripe; however, if the liquid is creamy, the cob is ready to harvest. Don’t leave cobs on plants once they’re ripe, because they’ll soon be past their prime and lose flavour.

The Judas Tree is magical in May when its naked skeleton is transformed by a profusion of pea-like, mauve-pink flowers. It’s an awe-inspiring spectacle and one that reminds us that the warmer months are just around the corner. Flowering is followed by distinctive, large, heart-shaped leaves. If this sounds like a tree that’ll look glorious in your garden, all you need is an open, sunny spot with well-drained soil.

Judas trees can reach a height and width of 4x3m (12x10ft) but will take 20 years to reach their mature size.