Peter Cundall: Tips for helping to stop the rot, such as powdery mildew, over summer
Although summer is one of the harshest seasons of all on plants, a few timely interventions can make all the difference to a bloomin’ marvellous garden, according to PETER CUNDALL
Summer is always a crucial part of the growing season because plants in all parts of the garden are not only growing strongly, but flowering or fruiting steadily. It’s when plants receive an unexpected check in growth in summer that we need to act quickly to rectify the problem.
Powdery mildew is a fungal disease that sucks vitality from plants. For example certain roses are more vulnerable than others. When the atmosphere around the foliage remains stagnant due to overcrowded branches and sunlight is also restricted, powdery mildew can strike. Immediately pruning off all diseased leaves and branches then opening up the plants for good air circulation stops most fungal diseases developing. Any roses in too much shade – they all need full sun – are a target for powdery mildew so be prepared to transplant them to a sunny spot next winter.
Dahlias also need as much sunlight as possible. When powdery mildew moves in, spraying is a waste of time where plants are in shade. The answer is to lift tubers in late May and re-plant the following spring in the sunniest place you can find – even in large pots if necessary.
Rust disease is another summer problem. Hollyhocks and pelargoniums are among the most common plants infected, with the first signs as pale, raised spots on the undersides of older leaves. Pull off all diseased foliage and take well away for quick control of rust without spraying, but act fast.
Harlequin bugs are now attacking plants, especially fruit and tomatoes. They form large colonies, not only feeding off juices, but destroying and tainting flavour. We see the evidence as pale, yellowish margins just beneath the skins of tomatoes. The black bugs, each with an orange mark behind the head, congregate and breed on mallow weeds, hollyhock, hibiscus and lavatera. Most sprays are
Any roses in too much shade – they all need full sun – are a target for powdery mildew so be prepared to transplant them to a sunny spot next winter
deflected by a water-repellent waxy coat, so special measures need to be taken to destroy harlequin colonies.
They gather together young and old every evening – usually in crevices in wooden fences or around timber beams supporting corrugated iron roofs of garden sheds. They emerge, just after sunrise sometimes in enormous clusters to briefly bask in the warmth on fences, walls and wooden posts before dispersing to continue feeding. This is when they are most easily and safely destroyed by spraying the clusters with an extra-strong solution of household detergent. This penetrates the protective waxy coats, enters their breathing tubes at the sides of their bodies and they die by suffocation.
Some people clean the ash from wood heaters during winter, then mistakenly dump the stuff around ornamental plants. Unfortunately wood ash is highly alkaline and when spread around azaleas, rhododendrons, camellias and ericas – all acid-lovers - it acts like lime, locking up iron in the soil and causing chlorosis. The youngest leaves go pale and the plants remain stunted. The only solution is to apply chelated iron which by-passes the alkalinity created by the ash. The best place for wood ashes is in the vegie patch around lime lovers such as brassicas, beans, onions and celery.
Blood and bone is an ideal organic fertiliser to apply right now – always after watering. It supplies calcium, some phosphorus and slow-release nitrogen. Mineral supplements such as potassium and magnesium can be added to make a complete summer fertiliser for most plants. I add a good handful of sulphate of potash and a heaped dessert spoonful of Epsom’s salts to every 10kg of blood and bone. This mix is easily scattered around all shrubs, citrus trees perennials and leaf vegetables and watered in.
Camellias need protection from persistent winds and long periods of strong sunlight. Bleaching of the most exposed leaves is a common summer disorder. Growth also becomes stunted and late winter flowering can be disappointing. The good news is that badly-sited, over-exposed camellia bushes can be lifted and moved to more sheltered spots at any time of the year – even now. But be sure to water deeply first to keep root-balls sticky and intact during the move and always soak new planting holes first.