Sowing and planting:
Sow biennials now including polyanthus, wallflowers, sweet Williams and winter pansies. Hellebores can also be sown at this time of the year and will give a whole range of fine colours. They are best sown just before the seeds ripen, so you have to be quick off the mark. Remove seed capsules from the flowers before they dry and split. Crack the capsules open and take out the seeds. Sow on a surface of deep 15cm (6in) pots filled with moist soil-less compost and cover with a thin layer of coarse grit. Put the pots in a cold frame on a layer of sand or grit and cover them with slates or something similar that will keep them moist, humid and cool. Check the pots from time to time to make sure they don’t dry out. The seeds should germinate from October to the end of the year, although this often depends on which species their parents have come from.
Continue planting perennials. These delightful plants give excellent value for money, flowering year after year and can also be divided to make more plants as they grow.
Plant up summer hanging baskets and containers or choose ready made from our displays.
The warm weather doesn’t just wake up plants and get them growing – it also sends a signal to all the pests that might be lurking, telling them there could be rich pickings in your vegetable garden. There are two pests in particular that we need to keep an eye open for at this time of the year. Carrot fly will lay eggs at the base of carrot, parsnip, parsley and celery foliage. The eggs then hatch out into tiny grubs that make brown tunnels in the tops of the roots. A few carrot varieties such as ‘Fly-Away’ and ‘Resistafly’ are ninety per cent resistant, but the best way to foil the fly is to hide it’s target. Alternate rows of carrots with onions, which have a stronger smell that confuses the fly, or plant them among flowers, or cover/surround them with a sheet of horticultural fleece. Cabbage root fly attack young brassicas such as cabbages, cauliflowers and Brussels sprouts, causing them to collapse completely. Take precautions at planting times by preventing the adult fly from getting into the soil near the stem, where she lays her eggs by using cabbage collars that fit snugly around the stems of the plants.
Pinch out the laterals (sideshoots) that appear between the leaf and main stem. Any greater than pencil-thickness should be cut off as they often don’t snap cleanly.
Harvest lettuce, radish, other salads and early potatoes.
Runner/climbing beans, dwarf French beans, carrots, chicory, endive, lettuce, kale, kohl rabi, peas, beetroot, winter cabbage (early in the month), radish, swedes, sweetcorn, turnips, salad leaves, spring onions.
Broccoli, Brussels sprouts, summer cabbages, cauliflowers, peppers, kale, leeks, celery, celeriac, cucumbers, squashes, tomatoes, sweetcorn, strawberries.
Regularly mow lawns to keep them in shape – removing ‘little and often’ is the key to a good quality sward. Continue cutting lawn edges with a half-moon edging iron to ensure they are neat.
Add grass clippings to the compost heap in thin layers (too much grass all at once is likely to be very wet and poorly aerated, resulting in smelly slime rather than compost).
Apply a high nitrogen summer lawn fertiliser if not done last month to encourage a healthy-looking lawn – always follow the manufacturer’s instructions carefully, as any over-use or runoff can cause water pollution. If moss is a problem, choose a combined fertiliser and mosskiller when feeding the lawn. Selective weedkillers are available for lawns, which will kill the weeds but not the grass or any naturalised bulbs (providing they’ve died down). However, they will kill wild flowers.
Ensure new lawns (either from turf or seed) do not dry out during hot weather, as turves will shrink if allowed to dry out and fail to knit together.
Plant out summer bedding. Make sure they are well watered in and keep moist during dry weather. From elaborate public garden designs and street planters to the smallest front garden, bedding plants provide a temporary decorative seasonal display for beds, borders, containers and hanging baskets. Click here to view a selection of our bedding plants.
Keep tubs, hanging baskets and alpine troughs well watered. Use collected rainwater, or recycled grey water wherever possible.
Liquid feed containerised plants every two to four weeks.
Plant out cannas and dahlias once danger of frost has passed.
It is not too late to direct sow a few fast growing, late-flowering hardy annuals such as Calendula, Godetia and Clarkia.
If you want to grow your own spring bedding for next year, many (including wallflowers, pansies, and Bellis perennis) need to be sown between May and July in order to flower next spring.
Polyanthus primulas are best sown only when temperatures are reliably warm, as they need a constant temperature of around 15°C (60°F). A sheltered cold frame in June or July provides the right environment.
Winter bedding plants for the following winter can also be sown from May until July. Attractive choices include ornamental cabbages, kales and winter pansies.
Ensure newly planted trees and shrubs do not dry out. Water with rain, grey or recycled water wherever possible.
Sprinkle fertiliser around perennials, shrubs and roses.
Weeds will be growing at a rapid rate and are best tackled before they become out of control. Hoe borders regularly to keep the weeds down.
Watering can be a never ending job (especially in the drought conditions we appear to be having locally). There are plenty of products nowadays designed to reduce to need for hours of carting backwards and forwards with a watering can such as automatic watering systems and watering storing granules. Prevent evaporation by mulching beds, borders and pots with any suitable material such as gravel, slate chippings or bark etc. Consider purchasing water butts (available in many sizes), ready for the summer. Rainwater is particularly useful for watering acid-loving, ericaceous plants (tap water is often slightly alkaline).
Shade greenhouses to keep them cool and prevent scorch. Good ventilation is also essential. Click here to visit the RHS website guide to greenhouse ventilation and shading.
Stake tall or floppy plants. Perennials in borders often put on strong lush growth that makes them vulnerable to collapse, especially after heavy rain or strong winds. Staking them early in the season will help avoid disaster. In particular; tall plants and hybrids with large flowers require additional support.
Prune many spring-flowering shrubs. Click here to visit the RHS website guide to pruning early flowering shrubs.
Pest and disease watch:
Aphids multiply rapidly in summer. Remove early infestations by hand to prevent the problem getting out of control. Aphids can transmit viruses, as can other sap-sucking insects. Approved insecticides are necessary for more serious attacks.
Vine weevil larvae can be a serious pest of containerised plants and become active this month. There are various chemical and biological controls available.
Continue to protect lily, delphiniums, hostas and other susceptible plants from slugs and snails.
Inspect lilies for the scarlet lily beetle whose larvae can strip plants in days.
Check roses for signs of blackspot, aphids and leaf-rolling sawfly damage.