Expert Gardening Tips & Advice for May 2024. Apr 29

Heritage Gardens Exclusive Gardening Tips for May 2024.

After one of the wettest Aprils for years we hope to start May on a warmer note in a few days time. As soon as we get a few days without rain with many plants & vegetables growing rapidly it is very important to keep watering especially at the roots.


Check plants regularly to see if they need watering. Most established plants can be left to fend for themselves, but those in containers, growing close to walls and anything recently planted may be in need of a drink. It is better to give one good soaking every seven to 10 days, rather than watering little and often.

Liquid feed plants growing in pots and other containers or use a controlled-release fertiliser if you haven’t already added some to the compost. Flowering plants benefit from a high potash feed.

Pot on plants showing signs of being pot bound. Carefully tip out plants that are looking unhappy to see if they are pot bound or if they are suffering from some other problem.

Use all your weeds (apart from the roots of perennials), lawn clippings and other garden debris and household vegetable waste to make garden compost. Mix grass clippings with coarser material to ensure your heap doesn’t turn into a sticky mess. Large pieces should be chopped up first to speed up the composting process.

Ventilate greenhouses and conservatories from early in the morning on sunny days. It may still be chilly outside but seedlings can fry in the bright sun.

Frosts are still possible this month, so make sure tender plants like young bedding plants and tomatoes are protected from the cold. Always have horticultural fleece on hand that can be thrown over the plants at night when frost or low temperatures are forecast. Fleece can even be used to give cold protection for plants in greenhouses, frames and cloches.

Weeds can soon take over the garden, looking unsightly, smothering your prized plants and stealing valuable water and nutrients from the soil. Make sure you keep them under control before they become a problem. Annuals and small weeds can be hoed on a dry or windy day. Make sure the blade of the hoe is sharp as the aim is to cut through the weeds’ stems.

Larger infestations and all perennial weeds, especially those with creeping roots, are best treated with a weedkiller based on glyphosate, which will kill the roots as well as the top growth. You’ll get better results if you spray in the evening. A thick mulch of organic material or using a planting membrane when planting up new beds will help keep the garden weed free. As temperatures rise, pests and diseases will be stirring in the garden. Keep on top of any problems that do occur to ensure they don’t get out of control – early eradication is the key to success.

Ready-to-use sprays do away with fiddly mixing and ensure you have something on hand to deal with the problems as soon as they are seen.

Slugs and snails are a major problem at this time of the year. Make sure you protect susceptible plants. – especially hostas, delphiniums and vegetables.

Sprinkle slug pellets, place barriers or water on a liquid slug killer. Vine weevil larvae can be a serious pest of containerised plants. Inspect the root ball of suspect plants, looking out for the C-shaped, creamy, orange-headed grubs. They are very destructive, but there are various chemical and biological controls available.

Grow Your Own - Fruit

Remove any coverings used to protect against peach leaf curl on peaches, nectarines and apricots. In most areas it’s now safe to remove frost protection from fan-trained peaches, nectarines, apricots, almonds and figs. In cold regions wait until the end of the month or check for frost forecasts. If frost is forecast it pays to give some protection by covering with horticultural fleece.

Wall-trained plums and cherries can be pruned this month, as long as the weather is warm enough for them to be in growth. If pruned while still dormant, they run the risk of getting silver leaf or bacterial canker diseases. Any branches coming out from the support should be removed entirely.

Pinch out the growing tip of the branches of wall-trained sweet cherries once they have produced six new leaves. Remove wayward shoots and tie in better placed ones on fan-trained peaches and nectarines and all wall-trained cherries, plums and gages.

Keep wall-trained fruit, especially stone fruit, well watered during fruit set and fruit development. Shorten leaders and side shoots on over-vigorous wall-trained apples and pears; this will divert their energy to fruit production rather than leafy growth.

On indoor vines, allow only one flower truss to develop on each main side shoot coming off the central rod; remove the rest. Keep flowering laterals short by pinching out their tips at two leaves past the flower truss. Non-fruiting side shoots can be kept a bit longer, pinching out the tips after five leaves have formed. Any side shoots coming off the main laterals should be stopped at one leaf. Gently tap or run your hand over indoor grape vine flowers to help pollinate them.

Ensure there is good air circulation in the greenhouse to reduce the risk of botrytis, powdery mildew and downy mildew diseases.

Thin out crowded raspberry canes. This ensures there will be sufficient air and light between the branches to help reduce disease problems and ensure the plants can ripen all the young fruit to maturity. Hoe off or pull out raspberry suckers appearing between the rows.

It’s usually best to remove strawberry runners before they start to creep along the ground. If you leave them, they will reduce the yield of fruit. If you need runners to produce new plants for next year, pinch off the flowers from a couple of plants, which will encourage them to produce runners at the expense of flowers and fruit. Peg down the young strawberry plants into small pots of compost. The runner can then be severed once the plant has rooted.

Early strawberry crops kept under glass, fleece or cloches, should be uncovered, or the greenhouse doors opened, to give pollinating insects access. Put down slug controls and straw around outdoor strawberries to keep the developing fruit off the soil. 

Place growing-bags in the greenhouse to warm up for two weeks before planting with indoor melons. Sow melons in a heated propagator for growing outdoors.

Net soft fruit bushes as the fruit begins to ripen to prevent bird damage.

There are a number of fruit pests you should keep an eye out for this month. Plum sawfly normally attacks a week after petal fall. Deal with apple sawfly and capsid bug and prevent blossom wilt if it struck last year. Deal with pear and cherry slug worm. Stay alert for gooseberry sawfly damage and the raised red blisters of currant blister aphid. Deal with spur blight, cane spot and cane blight on raspberries, blackberries and hybrid berries. Deal with raspberry leaf and bud mite and raspberry rust. Blackcurrants are vulnerable to big bud mite (affected buds appear larger than normal). The mites can spread blackcurrant reversion virus. Virus symptoms will be evident once the plant comes into flower, as the flowers look red. Affected plants must be dug up and disposed of. You can grow resistant varieties, such as ‘Farleigh’, ‘Foxendown’ or ‘Ben Hope’.

Hang pheromone traps in apple trees to help reduce codling moth numbers. This is the month when they start to mate, so trapping them will reduce the numbers of eggs. You will need one trap for every three to five trees.

American gooseberry mildew produces a brown crust over the leaves and fruit. Use a protective spray with an approved fungicide. If you don’t like using chemicals, grow a more resistant variety like ‘Invicta’.

Look out for powdery mildew on a range of fruit crops, especially during warm dry spells. Reduce attacks by keeping the soil moist, but the foliage dry.

Apples and pears may need spraying against scab, where this has got out of hand in previous years. Don’t spray pesticides once the flowers have opened as they may kill pollinating insects; sprays should be used before the flowers open or after the petals have faded. It’s better to spray early in the morning or in the late evening when fewer pollinating insects are around.

Grow Your Own - Veg

Continue making little and often, successional sowings of salad crops, to ensure an even supply throughout the summer. Sow thinly within the row to reduce the need for thinning out once the seedlings are growing well.

Sow French and runner beans, squash, outdoor cucumbers and pumpkin seeds directly into prepared beds outside - in colder regions you should wait until the end of the month. Cover with cloches. Keep an eye out for late frosts, and cover young plants with horticultural fleece if frost is forecast.

Take a look at the Heritage Gardens Grow Your Own category - there are lots of products and ideas to help you get your vegetable growing off to a good start. Small vegetable plants do especially well under cover at this time of year and our choice of Cold Frames & cheaper polythene grow media are good value.

Runner beans benefit from well-prepared ground with lots of well-rotted manure and organic matter dug in. They need to be planted alongside suitable supports (often a frame or wigwam of bamboo canes tied together with string) for the shoots to twine around and grow upwards.

Sow cauliflowers and purple sprouting broccoli in a seed bed, for transplanting out into their final cropping position when large enough to handle. This will provide harvests in winter.

Brussels sprouts for next winter should now be ready for transplanting after early or mid-spring sowing. The gaps between plants can be used for short-term ‘catch-crops’ like radishes, salad leaves or lettuces.

Now’s the time to plant out tomatoes, once the first truss of flowers has set. You can grow them in soil, but it’s often best to use other methods, such as growing-bags, 30cm (12in) pots or growing rings filled with potting compost. If you find that growing-bags dry out too quickly – and the bottom of the fruit turns black – then cut the bags in half widthways, stand them on end and put one plant in each half. And don’t forget you’ll need to regularly water, feed and support the plants.

Sweetcorn can be sown outside now or plants grown indoors planted out. Sweetcorn is best grown in blocks with plants spaced 45cm (18in) apart. 

Celery can also be planted out towards the end of the month. Well-prepared soil with plenty of organic matter is essential for good crops. Planting celery in blocks helps self-blanching celery remain white. Alternatively, place a strip of plastic around the edge of the block to exclude some light and further improve the pale colour and sweetness of the stems. Ordinary celery can be wrapped in cardboard or similar tied in place to exclude light.

It is still possible to sow vegetables indoors, especially in colder regions. This will shorten the growing time needed to reach maturity and harvest. Young plants can be planted out once conditions are suitable, and once they've been hardened off (acclimatised to the colder outdoor conditions) for 10 to 14 days.

Earth up potatoes as the shoots grow, covering them entirely if frosts threaten, and finishing when the earthed up ridge is about 25cm (10in) high. Potatoes grown under black polythene do not need earthing up, as the polythene excludes light. If frost threatens, cover the shoots with horticultural fleece.

Leeks can either be sown in rows outside in a seedbed, or indoors in modular trays. They can be transplanted to their cropping site when they reach about 10cm (4in) high. Use a large dibber to make holes (about 7.5cm/3in deep) for each plant. Drop a leek into each hole, and then fill the holes with water. 

Witloof chicory can be sown this month, to have some ready for forcing next winter (when other salads can be scarce). Sow in drills directly outside.

Peas need staking with pea sticks, netting, or pruned twigs from the garden.

Pests such as caterpillars, aphids and carrot fly can be kept off a wide range of crops by covering with fleece or fine woven plastic mesh. Ensure that the corners are tucked in or buried to prevent the insects getting through.

Pinch out the tips of broad beans once they start to flower. This helps to discourage blackfly, which can be a problem on these plants.

Pigeons are serious pests of brassicas and other vegetables, so use crop coverings to keep them away from vulnerable crops.

Promptly remove diseased and yellowing leaves from crops, especially brassicas, to prevent the spread of disease.

Grow Your Own - Herbs

For continual supplies of herbs like basil, coriander, dill and parsley sow a small pot of each every fortnight.

Herbs are not only good to grow for the kitchen – many also have colourful flowers and foliage, so plant some today.

Lift and divide invasive herbs, like mint and lemon balm that have outgrown their allotted space. Growing these herbs in pots will prevent them growing out of control.

Flower Power

Hardy annual seed can be sown outdoors now where you want them to flower. There are lots to choose from including calendula (pot marigolds), echium, escholtzia (Californian poppies), limnanthes, lobularia, love-in-a-mist and nemophila. You can also sow annual grasses, such as Briza maxima, Lagurus ovatus and Hordeum jubatum.

Marking out irregularly shaped seedbeds and broadcasting drifts of different seed gives a more natural look. However, you may find it easier to sow in rows in these shaped areas; this makes it easier to distinguish between flower and weed seedlings, as you know where the flowers have been sown. If you sowed seeds indoors, you can now plant out young plants.

Hardy annuals that were direct sown earlier in the year may now need thinning out. This is best done in two or three stages at fortnightly intervals. The final spacing should be between 10-20cm (4-8in); the lower limit for smaller plants and the upper limit for tall or spreading plants.

Towards the end of the month, clear away spring bedding to the compost heap, making space for half-hardy summer bedding plants.

Harden off half-hardy summer bedding plants that were started under cover, ready for planting out; in cold regions don’t plant out until early June. Put the plants outside for a short period only, at the warmest time of day, and then gradually increasing the length of time they are outside, building up to leaving them out overnight. Do this for 10-14 days before planting them outside, after the risk of frost has passed. By hardening them off you can avoid the cold shock they would experience if moved outside suddenly and permanently.

Containers and hanging baskets can be planted up and placed outside towards the end of the month or early June – after the last frost. Keep some horticultural fleece handy in case a late frost is forecast.

Cut back spring-flowering perennials such as pulmonaria and doronicum to encourage a fresh flush of foliage, which is more attractive than old, mildew-covered leaves. The plants will also remain more compact and flower better next year. A good watering and a light feed will help recovery.

Spreading and trailing plants, such as alyssum and aubrieta, can become tatty and patchy as they get older. Trim them back after flowering to encourage strong, healthy growth and a better shape.

Lift and divide your best primroses and polyanthus once they’ve finished flowering, checking carefully for any vine weevil grubs among the roots. Replant in well-drained, humus-rich soil and water well; keep well watered until they’re established.

Lift and divide overcrowded clumps of spring-flowering bulbs that didn’t flower well after they have flowered and started to die down. Replant at the right depth in well-prepared soil.

Tall and floppy herbaceous plants – especially those in windy and exposed gardens – will need some form of support –such as twiggy shoots or metal supports. Putting plant supports in place early will help prevent problems later on when it’s difficult or impossible to do anything about it and the plants will grow up through them, covering them discreetly. Then carefully tie in the stems as they develop with soft string or similar, criss-crossing the strings between the supports.

Plant out cannas, dahlias and other summer-flowering bulbous plants when the danger of heavy frost has past.

Sweet peas need training and tying in to their supports to encourage them to climb and make a good display.

To grow your own winter and spring bedding for next year you will need to sow between now and July. Good choices include wallflowers, winter pansies, Bellis daisies, primulas and ornamental cabbages.

Remove dead leaves from around the base of alpine plants to prevent them rotting. Then top dress with a layer of grit or gravel.

Take cuttings of tender perennials like marguerites, trailing petunias, pelargoniums and fuchsias. They will provide new plants for later in the summer.

Perennials that are producing new shoots from the crown can be propagated by taking basal stem cuttings. Cut shoots 7.5-10cm (3-4in) long, remove the lower leaves, dip the cut end in hormone rooting mix and then insert in small pots of cuttings compost. Place in a propagator and keep them warm, but out of direct sunlight.

Inspect lilies for the lily beetle as both adults and the larvae can strip plants of leaves and flower buds in just a few days.

Trees, Shrubs, Roses & Climbers

Prune spring-flowering shrubs, such as berberis, Chaenomeles, Choisya and rRibes after flowering has finished. Remove one stem in three from kerria and spring-flowering spiraea, such as Spiraea ‘Arguta’, and shorten the remaining flowered stems to a suitable side shoot.

Winter- and spring-flowering evergreens such as Viburnum tinus can also be cut back or trimmed this month after flowering. Prune out any frost-damaged shoots from affected evergreens.

Cut out overcrowded, dead or diseased stems of Clematis Montana and other spring-flowering clematis species once they have finished flowering. Overgrown plants usually respond well to hard pruning at this time too.

Now is a good time to coppice or pollard eucalyptus, especially E. gunnii.

Gently pull off suckers appearing around the base of grafted trees, shrubs and roses. It is far better to pull them off than cut them off, as dormant buds can remain when cutting and these will produce further suckers.

Remove reverted all-green shoots from variegated evergreens. These shoots, being stronger than the variegated ones, can soon take over and spoil the overall appearance of the plant. Cut them back to where variegated foliage starts or remove them completely.

Prune wall-trained Chaenomeles and pyracantha, removing any shoots coming out from the wall, and shortening other new growth to about 7.5cm (3in). This encourages short spurs, which produce the flowers. Don’t do the same to climbing hydrangeas as these shoots carry the flowers.

Always feed plants with a general fertiliser after pruning to ensure good regrowth and flowers.

Guide the new shoots of clematis and other climbers in the direction you need them to grow. Make sure the shoots provide a good, even coverage of their support. Then carefully tie them in with string or similar material.

Try to tie in climbing and rambling roses, as well as shoots of other climbers, as near to horizontal as possible. Or, if more suitable, into a fan shape. This reduces the growth of the shoot and, as a result, encourages more flowers.

This is a good time to get hedges back into shape, providing all nesting birds have flown. And you can even rejuvenate overgrown evergreen hedges – but not conifers – by cutting them back hard. When cutting, make sure the hedge is wider at the base then it is at the top to prevent it becoming bare at the bottom. Shredded clippings can be added to the compost heap.

Deadhead azaleas and rhododendrons after flowering by carefully snapping off the dead flowers between thumb and forefinger. Be careful not to damage developing buds below. Then feed with a fertiliser suitable for lime-hating plants.

Take softwood cuttings of deciduous shrubs, including forsythia, fuchsia, hydrangea, philadelphus and spiraea.

Layering is a good way to propagate climbers and lax-stemmed shrubs, such as rhododendrons and magnolias. Layers should root by next spring.

Check roses for signs of blackspot, aphids and leaf-rolling sawfly damage. Spray roses regularly with a suitable fungicide to prevent black spot, rust and mildew diseases.

We sell a comprehensive range of high quality bush roses & bare root roses dependent on the season.  

Viburnum beetle grubs start attacking this month, making holes in the leaves and giving plants a tattered appearance. Inspect V. tinus and V. Opulus regularly and spray or pick off the grubs by hand.

Inspect sick looking box and holly trees for signs of blight.

Phytophthora root rots can cause dieback on mature trees and shrubs. Wet winter weather followed by a hot spring and summer can encourage this problem on susceptible woody plants. Unfortunately, there isn’t any method of control, apart from making sure soils are well drained and don’t become waterlogged in winter.

Luscious Lawns

Although it’s getting a bit late to lay new lawns from seed, it can still be done if the soil is moist. But it’s still a good time to lay turf. Improve the soil first with composted bark or similar, remove large stones and other debris, lightly consolidate it by walking over the area with your weight on your heels, rake it level, add some general fertiliser, rake it again and you’re ready to start laying.

Do not walk over or mow newly sown grass until it has reached a height of 5-7.5cm (2-3in). Then give it a light trim with the mower set at its highest setting.

Over-seeding dead patches can still be carried out in May. Prepare the ground for sowing, by cultivating, levelling and lightly firming beforehand.

Grass will be growing strongly now so make sure you mow regularly – probably weekly – aiming to keep the grass around 2.5-4cm (1-1.5in) high. Adjust the cutting height of your mower so that it takes off no more than half the length of the blades of grass at a time. While you’re mowing, take note of bare patches, moss invasion and weeds and deal with any problems you see.

Cut lawn edges with a half-moon edging iron to ensure they look neat and well shaped. Then trim after each mowing with edging shears
Feed lawns with a granular lawn fertiliser or use weed and feed or weed, feed and moss killer where these are a problem.

If weeds are a real problem it would pay to treat them with a liquid lawn weedkiller.

Worm casts can cause problems – providing areas for weed seeds to germinate. Sweep away dry worm casts with a hard-bristled broom or besom to remove them before mowing.

Molehills are often a problem in spring, as the moles tunnels in search of food. Traps are the most effective way to deal with this problem.

House Plants

Water indoor plants regularly as and when they need it. Check plants every few days and water if necessary. Most plants prefer to slightly dry out between watering and overwatering is more of a killer than underwatering.

Feed houseplants every seven to 10 days with a liquid houseplant fertiliser.

If roots are peeping through the holes in the base of the pots repot into a larger size. Flowering houseplants usually perform better if they are kept slightly pot bound.

Make more African violet plants by carefully pulling off individual leaves with most of the stem and pushing this into small pots of fresh compost.

Cut the babies off spider plants and put them into individual pots to make new plants.

Some house and conservatory plants, such as orchids, ficus and citrus can be put outside during warm days, but bring them back inside if cold nights are expected.

Clean shiny-leaved plants with a damp cloth to remove dust.

Regularly inspect plants for aphids, mealybug, red spider mite, whitefly, thrips and other pests. Control with a pest spray or use pesticide pins that can be pushed into the compost and will protect plants for several months.

Where feasible, damp down floors on hot days to reduce the risk of red spider mite. Alternatively, mist the foliage regularly or stand the pots on trays of damp gravel to increase humidity.


Flower Power

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