August is traditionally holiday-time, so you might need to enlist the help of friends and family to look after the garden while you are away.
Sowing and planting:
Plant spring flowering bulbs. (Our selections of bulbs are due to arrive here during the third week of August.)
Towards the end of August sow hardy annuals directly into borders. They will overwinter and flower next summer.
Propagate perennials by dividing once they have finished flowering, but only in areas with some rain and duller weather, to avoid drying out problems.
Take cuttings of tender perennials such as Pelargonium and Osteospermum, as soon as possible. A greenhouse, cool conservatory or a light windowsill are ideal to bring them on until they are established.
Rock garden plants, such as Helianthemum, Aubrieta and Dianthus can be propagated from cuttings at this time of year.
Semi-ripe cuttings can still be taken to propagate many common garden shrubs (e.g. Ceanothus, Box, Lavender).
Cutting back, pruning and dividing:
Don’t neglect hanging baskets and planted containers – deadheading, watering and feeding will help them last through until autumn.
Hardy geraniums can be cut back a little to remove tired leaves and encourage a new flush of growth.
Hebes and lavenders can be given a light prune after flowering.
Rambling roses can be pruned now, once they have finished flowering.
Prune Wisteria. (Click here to visit the RHS website for how to prune Wisteria.)
Raise the blades on the mower before cutting fine lawns. This will help reduce drought stress.
Mow lightly and frequently so that short grass clippings can remain on the lawn during hot summers to act as a moisture-retentive mulch. Excess thatch can be scarified out during autumn maintenance next month. Mulching mowers cut the clippings even finer than normal rotary blades, making the mulch less visible.
Avoid using lawn weedkillers in late summer – they will be more effective in the cooler, damper autumn weather.
Browning of the lawn is very common at this time of year. Don’t water the grass unless absolutely necessary. It will green up when the autumn rains arrive. Browning can be partially prevented next year by ensuring that the lawn is well scarified, aerated and drained later in the autumn and that any soil compaction underneath is remedied before the following growing season.
Feed containers, and even tired border perennials, with a liquid tomato food each week to encourage them to bloom into the early autumn.
Keep picking flowers from the cutting garden to encourage more flower buds to form and open.
Alpines that have developed bare patches of die-back, or have become weedy can be tidied up by in-filling the patches with gritty compost. This will encourage new growth as well as improving their appearance.
Most perennial weeds are best dealt with when in active growth. If necessary, apply a weedkiller.
Continue to deadhead shrubs such as roses, to extend flowering into early autumn. Spindly specimens that have lost leaves can be cut back a little further when deadheading to encourage new growth.
Pest and disease watch:
Powdery mildew can be prevalent at this time of the year. Treat with an approved chemical at the manufacturer’s rates.
Set traps to reduce earwig damage on Dahlias.
Don’t be worried by bright green, heavily-armoured looking insects on your plants – these are harmless shieldbugs (pictured below) which do not require control.
Mid to late August is a good time of the year to apply biological controls for vine weevil. Grubs will be starting to hatch and soil temperatures are now suitable for the nematodes to be effective.
Black spot on roses is very common at this time of year, and spraying will no longer be effective. Clear fallen leaves and burn them to prevent spread.
In the greenhouse:
Damp down greenhouses on hot days to maintain humidity levels. Ventilation and shading is also vital.
Many greenhouse pests will be active during the summer months. Check plants regularly for signs of glasshouse whitefly, leafhopper, red spider mite, mealybugs and scale insects.
Clean up fallen leaves and spilt compost from benches and floors to prevent pests and diseases spreading.
Harvest sweetcorn and other vegetables as they become ready.
Continue cutting out old fruited canes on raspberries.
Lift and pot up rooted strawberry runners.