Garden Weeds – How Harmful Are They?

Garden Weeds


One definition of a weed is: ‘a plant growing wild in the cultivated garden’. But many would say this is an exact definition of our cultivated plants. Another, perhaps better, definition is: ‘a plant growing where it is not required’. Both descriptions would cover bluebells Scilla non-scripta (syn. Endymion) in some gardens and Peruvian lilies (Alstroemeria) in others.

We would prefer to grow the plants of our own choice in a garden, however beautiful invaders such as bindweed may be; and ground elder and couch grass are certainly persona non grata now, despite the medicinal qualities for which they were originally grown.

groundsel-garden-weeds The ability of weeds to spread is an obvious problem, and their increasing presence means that they will absorb more mineral nutrients and moisture, fill space, and take light that would otherwise be available to the plants we wish to grow. Garden weeds also act as a reservoir of pests and diseases, and nothing looks more neglected than a garden in which the weeds have taken over.


Making Use of Garden Weeds

There are two sides to every story, and garden weeds are not totally harmful. The soil surface beneath a leafy cover of weed growth is more moist than bare soil; it acts as a kind of mulch and more water is retained than where the soil is clear of weeds. The skilful gardener can, therefore, allow the weeds to grow, especially the annual kinds, until just before they begin to flower, and then dig or hoe them in to act as a green manure. If that isn’t convenient they can provide green material for the compost heap.

It is essential, however, to remove weeds before they flower. “One year’s seed is seven years’ weed” is a countryman’s saying that has been proved scientifically. One chickweed plant, allowed to grow unchecked, is capable of being the sponsor of 15,000 million offspring in one season, so eradication is desirable.

Garden weeds also have the useful habit of producing roots that penetrate to varying levels in the soil and tap reserves of nutrients which cultivated plants might not reach, or need. If they are then dug up complete, and removed to the compost heap, the rotting material from this will provide a source of major and minor elements, and cut down further on the need to buy artificial fertilizers. Docks in particular have long tap roots, with an ability to store much food; horsetails absorb silica in such quantities that the top growth was once used for scouring pots and pans.

Do not forget that many garden weeds (native plants) were once used as herbs, and many still have a variety of uses to the gardener or cook, as well as to the doctor, the artist and the perfumer. Nettles can be cooked and eaten like spinach; dandelion roots may be ground up and used as a coffee substitute. Young thistle leaves can be used as a mulch round plants to ward off slugs, and the young fronds of bracken are a very good source of potassium. So don’t be in too much of a hurry to destroy your weeds; you can make them work for you, instead of against you.

There are, of course, places where weeds are simply not wanted at all: paths, paved, tiled and bricked areas, hard tennis courts, drives and so on. Among soft fruit, perennial weeds can strangle the plants and reduce a crop to a few berries, and on lawns the cover, although green, can rapidly become a mixture of daisies, speedwells and moss  very pretty but hardly a lawn.


Types of Garden Weeds

Although garden weeds can most definitely be a nuisance, various methods of removal and prevention are possible, both natural and chemical.

In order to clear the picture a little, however, it helps to remember that weeds can be classified into two groups, each with different methods of reproduction and increase. Once you can recognize how the weed propagates itself, its name does not matter very much, but you will know where it needs to be attacked, and how to do it.


Annual Weeds

Groundsel, chickweed and shepherd’s purse are examples of annual weeds. These germinate in spring, grow large enough to flower, set seed, and die in one growing season. The weed may take the whole of a growing season to do this, from spring to autumn, or it may be an ‘ephemeral’, completing its cycle in a few weeks, and producing three or four generations in a growing season.

With such garden weeds, it is easy to understand why they spread so rapidly, especially as some of the seed may remain viable for several years. So the very least you should do to these is destroy them before they flower.

Biennial weeds are similar, but do not usually flower until the second season; fortunately there are very few of them.


Perennial Weeds

These weeds have herbaceous or woody crowns capable of surviving the winter, and live for several years. They usually produce seed, sometimes in large quantities, but mainly colonize the soil with creeping roots, underground stems, or runners which root at the leaf joints or which produce plantlets.

The roots or underground stems take a great deal of hard work in removal, whether by hand or digging, as anyone will know who has had to dig out a nest of couch grass roots or a tangle of ground elder. They break off easily, and any piece of root, however small, can root of its own accord and establish another centre of infection. Horsetails are particularly irksome as their ‘roots’ (underground stems) may delve down a considerable distance, and breaking them only stimulates the dormant buds lower down on the remaining root to sprout.

A small group within this perennial section has a long and tenacious taproot, which is easily broken, the remaining piece being capable of producing new top growth; dandelions will ensure their survival in this way, and add insult to injury by producing lots of seed.

Some of the common weeds that can plague the gardener are listed here. The heights to which each grows depends largely on the soil and site, those in lawns inevitably becoming prostrate. For instance, yarrow can look quite different growing in cultivated ground, compared with yarrow growing in a lawn. As a guide, approximate heights are:

stinging nettle, 60cm (2ft); chickweed, 5-30cm (2-12in); pearlwort, 2.5-5cm (1-2in); groundsel, 15-23cm (6-9in); black medick, 45cm (11/2 f t), 5cm (2in) in lawns; creeping thistle, 75cm (21/2ft); clover, 45cm (1 1/2ft), 5cm (2in) in lawns; ground elder, 45-90cm (1-½ – 3ft); Oxalis corymbosa, 15cm (6in); couch grass, 30-90cm (1-3ft); Great Bindweed, 1.8m (6ft); horsetail, 30- 75cm (1 -2-1/2ft); creeping buttercup, 30cm (1ft); daisy, 2.5-5cm (1-2in); Japanese knotweed, 1.8m (6ft); yarrow, 30-45cm (1-11/2ft), 5cm (2in) in lawns, broad-leaved dock, 60-90cm (2-3ft); coltsfoot, 30cm (1 ft); creeping cinquefoil, 15-30cm (6-12in); creeping speedwell, 5-10cm (2-4in).


Weed Control

Identifying the weed is the first step to controlling it. Only then is it possible to decide on the most suitable method of eradication.

‘Natural’ Methods

The most obvious non-chemical control is hand removal. A small hand-fork or onion hoe, a cultivator hoe (the kind with three or five prongs which claw through the soil) or Dutch hoe, or a digging fork or spade, all have their uses. It is also possible to buy special tools for removing daisies and dandelions from lawns. Choice depends on whether you are dealing with weed seedlings, rosette-type weeds, creeping weeds or deeply rooted kinds.

If you opt for removing them by hand, it must be thorough to be effective, and the younger and smaller the weeds are when you launch your attack, the easier it will be.

Mulching is an exceedingly good method of control, as it maintains soil moisture and can supply plant food. Materials to use for mulching can be garden compost, leaf-mould, peat or any similar organic material, also plastic sheet (black, or other dark colour), a layer of sand at least 7.5cm (3in ) deep on top of organic matter, or even layers of stones and small pebbles.

Grassing weed-infested areas is another technique. The top growth is cleared off, the soil roughly dug and cleared, a coarse grass seed mixture sown, and the resultant sward mown regularly. In time the weed, whether coltsfoot, horsetails, oxalis or whatever, will die out, though it may take from one to several seasons for this to happen.

Chemical Control

Chemical control is determined to some extent by the area in which the weeds are growing. One group of chemicals consists of those absorbed through the roots of plants and which therefore have to be applied to the soil and must reach the roots to be effective.

Sodium chlorate is one of these, a much-used weedkiller mixed with water and applied at rates varying from 120- 480g per sq m per 4.5 litre (4- 16oz per sq yd per gallon) of water, depending on the degree of weed cover and the strength of the weeds. It lasts for at least six months, but has a tendency to creep sideways in the soil, and can be carried down slopes; it will affect all roots to a greater or lesser extent. There is a fire risk with the dry material (including splashes of the solution that have dried), but some manufacturers now include a fire-depressant with their product.

Simazine is a more modern weedkiller, applied in solution to ground free of weeds; it will then keep it clear for twelve months, provided the soil is not disturbed. It can be used round certain established plants at strengths advised by the makers.

Dichlobenil is also a modern root-absorbed chemical, applied dry, to clean ground. At the strength suggested for uncultivated ground, it will keep it clear for a year; around certain cultivated plants, for three to five months. It will kill many weeds and satisfactorily suppress others, and has the advantage that it does not creep sideways. Being an extremely fine powder, great care has to be taken to ensure that it does not drift on to cultivated plants or the soil around them (it has some effect on the leaves also). Don’t apply it on a windy day.

Hormone weedkillers can be used where soil application is not possible; these can be sprayed onto the top growth, through which they are absorbed. They include those known as 2,4- D, 2,4,5  T, mecoprop, and dalapon. All except dalapon are effective on broad-leaved weeds; dalapon is specific to couch grass and other grasses (so should never be used on a lawn).

These hormone weedkillers stimulate the plant into overgrowth, especially at the growing points, so that the plant cells eventually collapse and die. The spray must be kept off cultivated plant top growth, but the broad-leaved weedkillers can be used on lawns, as they hardly damage the lawn grass.

Other chemicals having their action through the top growth are paraquat and diquat, which destroy the green colouring matter (chlorophyll) in plants, but which are inactivated in the soil. These are most useful on annual and small weeds; they are applied in solution, and work best in sunlight and warm temperatures. Glyphosate is a recent introduction, and also acts through the leaves without affecting the soil. It is claimed to control such difficult weeds as couch grass and ground elder, as well as annuals.

Ioxynil and morfamquat are two more sprayed onto the leaves and stems, and both are recommended for controlling weed seedlings on newly sown lawns. Morfamquat has also been found to be effective against such small-leaved, difficult weeds on lawns as speedwell and black medick (also known as suckling clover).

By careful choice, the cost of material, the amount of time and the degree of labour can all be cut down considerably. Once you know what type of weed you are dealing with, and the methods of control available, you can eliminate weeds and go a long way towards a garden which looks after itself.

Using Herbicides

Herbicides work  but many are not just weed killers. Applied in the wrong place, or at the wrong strength, they will kill desirable plants too. Always apply carefully, avoiding drift of either powders or sprays. Dribble bars are very effective, and can be used in conjunction with a suitable piece of board to shield beds.

Keep a watering-can just for weedkillers, and be careful where cans are rinsed out.

When using any garden chemical, always read the directions before using, and always apply as the manufacturer directs. Always keep them out of the reach of children and pets, and never put them in unlabelled containers.

Chemical names can be confusing, but all the names used in this site are the common chemical names. This is because the same active chemical may be sold by several manufacturers under different trade names. You will always find the common chemical name listed, however, if you read the label carefully. And if you buy from a good garden shop with trained staff, they will be able to sell you the correct chemical if you ask for the names mentioned in this site.


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05. September 2010 by admin
Categories: Garden Management, Weed Control | Tags: , , , , , , , | Comments Off on Garden Weeds – How Harmful Are They?


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