Organic Gardening – Preventing Problems

Preventing problems

Choosing plants

organic gardening - preventing problems Give a plant a good start and it will grow healthily and strongly and be more likely to withstand most problems. Choose plants and cultivars that are suited to your garden conditions. These will thrive where others might fail to prosper: a rose or honeysuckle planted in poor, dry soil up against a house may suffer from mildew; an apple tree next to a pond is be prone to scab; and hostas may suffer from slug damage if planted in a dark, wet corner whereas a fern will thrive.

Fruit types and varieties can be chosen to suit many different locations, and there are even some vegetables that will grow in the shade. Always think about the situation in which a plant is to grow (soil type, shade or sun, exposure and aspect) before buying or planting.

Resistant cultivars

Plants vary in their susceptibility to pests and diseases and those that show some resistance are a valuable means of preventing a specific, recurring problem such as blackspot on roses, or one that has no cure such as cucumber mosaic virus. Use resistant cultivars as part of an overall pest and disease management strategy; they are less susceptible but not altogether immune to attack. Although examples of disease resistant cultivars and varieties are given below, the range changes all the times so consult catalogues for an up-to-date selection.

Plants and planting material Buy the healthiest plants available. Young, vigorous plants will establish more quickly than those that are old and pot-bound. Bare-root plants should have a good fibrous root system. While bedding plants in flower may look more attractive, those not yet in flower are in fact a better buy because the plants will be able to put all their energy into settling in before flowering. Bulbs should be firm and show no sign of mould. Always buy certified virus-free seed potatoes, and fruit trees and bushes where available.

Inspect all new plants for pests and disease, especially those destined for the greenhouse, as this is the way that many greenhouse problems are brought in. Keep plants in quarantine for a while if you are unsure.

Seeds should be as fresh as possible. Old seed may produce plants lacking in vigour; the faster a seedling grows away, the less it is at risk. Older seed may of course be used, but it may need extra care.

Sowing and planting

The care with which plants are sown and plant- ed has a great effect on their future performance.

Timing to avoid pests

Sowing times can be adjusted in order to avoid the periods when certain pests and diseases are most active.

The pea moth lays its eggs on pea flowers in early to mid-summer. Peas sown early (late winter) and late (late spring) should avoid damage from this pest as they will flower outside its period of activity. The carrot root fly lays its eggs around seedling carrots in late spring to early summer. Sow in early to mid-summer to avoid this generation of the pest.

Powdery mildews are worse when the soil is dry. Sow peas early and swedes and turnips late so they are not trying to grow during the height of the summer.

Interplanting for pest control

A monoculture — that is to say, rows and rows of the same plant grown together — is paradise for a pest or disease, because once it has discovered one suitable host plant, every way it moves it will encounter another one equally suitable. Monoculture does not encourage the good mixed population of creatures that is desirable in an organic garden. It is generally a good idea to mix plants up as much as possible as long as individual crops do not suffer as a result of this.

Eliminate some of the division between fruit, flower and vegetable garden. Vegetables can look good in an ornamental border; lettuce, beetroot, carrots, rhubarb, chard and courgettes, for example, all have very attractive foliage. Interspersed among unrelated ornamentals, they are less likely to be discovered by any pests or diseases.

This method of growing can require more attention — making sure that plants do not overwhelm each other and that there are spare plants to fill in the gaps where a vegetable has been harvested, for example — but it can be fun and is a way of growing both flowers and crops in a small garden. Fruit trees and bushes are also attractive, and a few flowers in the vegetable garden can liven up a plot while also attracting beneficial insects that can help with pest control.

Breaking the cycle

A pest such as cabbage mealy aphid or white-fly or a disease such as leek rust can become established where there are suitable host plants all year round. To break the cycle of reinfection, remove all susceptible plants at one time of the year before planting out more.

For example, in early spring, dig up all over-wintered Brussels sprouts, broccoli and other related plants. Bury them in a trench about 30cm (1 ft) deep, chopping up tough stems with a sharp spade. This will get rid of the mealy aphids and whitefly that winter on these plants. Wait a week or two before planting out more brassicas, making sure of course that these are pest-free. This same general principle also applies to fruit growing.


As a general rule, never follow a plant with another of the same type.

Fruit trees and roses are prone to replant disorders, known as rose sickness in the case of the latter. If an apple tree is planted where an old one has recently been removed, it may grow poorly or even die. Try replacing the soil or adding good quantities of organic matter or, better still, plant it on a new site altogether.

A watchful eye

All diseased plant material should be removed as soon as it is noticed. Where appropriate, put it straight into a plastic bag sothat disease spores are not wafted around.

Pests such as aphids, slugs and caterpillars are mostly easy to spot and can be removed by hand. If you keep a close eye on your garden, you should be able to identify and curtail problems before they get out of hand.

Water and watering

Too much or not enough water can damage plant growth, as can an irregular, fluctuating supply. A poorly drained soil can encourage root problems such as Phytophthora and violet root rot and red core in strawberries, but growing on raised beds can help. If the problem is severe, installing drains or planting a bog garden to take advantage of the moisture may be the only answers.

Wilting is an obvious sign of water shortage, but other side-effects can occur before this.

Powdery mildews and potato scab, for example, are much more severe where plants are short of water. The answer is to improve the water-holding capacity of the soil by adding organic matter. You can also time the sowing of particular vegetables to avoid the dry months.

Most established plants in the open garden should not need regular watering. However, if there is a period of very dry weather, they will need extra water to grow well.

Summer plants in a greenhouse are likely to need regular and copious watering, especially those in growing bags. Vigorous fruiting plants such as tomatoes should never be allowed to go short, even for a brief period, otherwise they may get blossom end rot.

Winter digging

While winter digging is not good for soil structure and fertility, it has its uses. For example, winter digging will expose pests such as root fly, millipedes and slugs, which spend the winter in the soil, to predators such as birds.

Soil pH

Altering the pH of a soil can help in the control of certain diseases that thrive in particularly acid or alkaline conditions. For instance, raising the pH of an acid soil can help reduce the level of club root infection. Potato scab, on the other hand, thrives in more alkaline conditions, so increasing the acidity of the soil by adding organic matter such as grass mowings is one means of reducing the effects of this disease.

Plants that have a particular requirement for an acid or alkaline soil may grow very poorly and show severe mineral deficiencies where the pH is not appropriate. It is possible to alter the soil pH if it is close to the required conditions, but if the pH of your soil is very different, select plants that are suited to your soil and grow any acid- or alkaline-loving plants in isolated pots filled with the required growing media.

Good airflow around plants Diseases thrive in the moist air around overcrowded plants. Encourage a good airflow by thinning seedlings, using the correct spacing, and pruning. Good ventilation is particularly important in a greenhouse, even during cold weather.


Companion planting is the growing of specific combinations of plants to the benefit of one or both. While there is evidence to show that plants can have an effect on each other, there is little practical advice as to how to make specific combinations work. Onions can protect carrots from the carrot root fly, but only when there are four rows of onions to one row of carrots. The effect only lasts as long as the onion leaves are growing; when they stop and the bulbs start to form the carrots are again at risk.

One combination that has been studied is that of brassicas interplanted in alternate rows with an unrelated crop such as dwarf beans. The level of mealy aphid and cabbage root fly damage is significantly reduced, the mixture of plants confusing the pests. But for this to work, the plants must be a similar size when young and fill the gaps between plants and rows as quickly as possible. This means raising both crops in pots and planting them out when they are the same size. Sow a fairly compact cultivar of cabbage about three weeks before the beans. Make sure that the beans have grown at least their first true leaves before transplanting both in alternate rows 25cm (10in) apart.

30. January 2011 by Dave Pinkney
Categories: Organic Gardening, Pests and Diseases, Plant Care, Soil Cultivation | Tags: , | Comments Off on Organic Gardening – Preventing Problems


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