Understanding Weeds

Weeds are native plants in direct competition with cultivated vegetation for the same basic essentials of light, water and nutrients. Native plants growing in the fields and hedgerows are admired, protected and called wild flowers. A rather different attitude exists in gardens and with some justification for if you don’t exercise some form of weed control, your garden will inevitably suffer. With a few exceptions, native plants are better adapted to the local climatic and other conditions than any alien that you may grow in your garden. By virtue of their greater hardiness, faster reproductive rate or other reasons, they are quite simply more efficient competitors. Much as you may like and admire them in their wild habitat, native plants must be controlled if your garden is to realise its full potential.

Perhaps 50 or 60 different plant species are of major importance in Britain as garden weeds. Their relative effects on your gardening activities will vary depending on the area in which you live, the type of soil in your garden and the types of plant that you cultivate. Nonetheless, weed species can be grouped into two major categories on the basis of the way they grow and reproduce, and these groupings largely dictate their efficiency as competitors and the control measures that are likely to be effective against them.

Types of weed

dandelion spreads rapidly by seed and also has persistent rootsSuccessful weeds have individual attributes that set them apart, not just from garden plants, but also from many other native species. An understanding of these attributes is important in each instance to help you select an appropriate control method.

Most numerous in terms of species and individuals are annual weeds, which produce flowers and set seed within a single year. The seeds of some annual weed species require no period of cold or enforced dormancy before they can germinate and if, as with groundsel (Senecio vulgaris), this is combined with the ability to produce flowers and seeds very early in life, more than one generation may occur within the course of a single season. A few species, like the annual meadow grass (Poa annua) can germinate in every month of the year. A short generation time is combined in most annual weeds with the ability to produce vast numbers of seeds. This can vary from less than 100 seeds per plant in cleavers (Galium aparine) to over 18,000 for some sow thistles (Sonchus spp.). Once in the soil, the seeds of most annual weeds have the potential to survive for at least 10 years; although some, such as black nightshade (Solanum nigrum) can survive for four times this long, while field poppy seed (Papaver rhoeas) famously persists in a viable state for at least 80 years. It is evident, therefore, that the key to keeping control of annual weeds lies in using a method that eliminates the plants before they set seed.

There are a few shallowly rooted perennial weeds, such as daisies (Bellis perennis), that behave in beds and borders in much the same way as annuals, although in lawns and paths, they assume a rather different status that I shall come to shortly. For the present, however, my concern is a much more important group that, for simplicity, I call persistent perennials.

method of spread of some important British perennial garden weeds

All perennials differ from annuals, of course, in that each individual lives for longer than one season (although not in a literal sense, forever; perennial doesn’t equate with immortal). While they too produce seeds, sometimes in large numbers, their importance as garden weeds derives much more from vegetative methods of spread, such as creeping roots, spreading rhizomes or bulbils. Some plants can even regenerate from small parts of stem or roots left behind in the soil. Different weed species vary in this ability; for example, dandelion (Taraxacum officinale) can produce new plants from any part of its tap root, but docks (Rumex spp.) can only regenerate from the top 7-10cm (2-3/4 – 4in). Plants like ground elder (Aegopodium podagraria), couch grass (Agropyron repens), or horsetail (Equisetum arvense) have creeping rhizomes with frequent nodes bearing buds. If the rhizome is severed or otherwise disturbed, these buds are stimulated to grow.

The physical eradication of perennial weeds can be exceedingly difficult for, not only is finding the roots and removing them from the soil hard work, but if a portion can break off and regenerate the problem can be almost insurmountable. Perhaps in a league of their own are weeds that produce bulbils (small bulbs that break off from their parent) for these cannot be controlled by physical means and, in the case of the pink-flowered Oxalis spp., this attribute is combined with almost total resistance to available weedkillers.

Non-chemical weed control

The most straightforward way of controlling annual or shallow-rooted perennial weeds is by digging or pulling them up by hand or with a hand fork or by cutting them down with a hoe. Hoeing is best done on a warm, dry day so the cut plants will shrivel quickly, although groundsel and annual meadow grass can continue to mature their seeds if they are hoed down during flowering so they should be raked up for disposal. If the weather and soil are moist when weeds are hoed, however, some weeds may produce new roots in response to wounding and end up, in effect, merely having been transplanted. When you are clearing new ground, regular digging with a fork has a role even for perennial weeds such as couch grass, nettles and thistles, but I advise against using a spade or rotary cultivator which will chop up and disperse them further.

On lawns, it is often possible to remove small numbers of daisies, dandelions and other rosette-forming perennials by means of a small, two-pronged digging tool called a daisy grubber. However, care must be taken to remove all the root as simply snapping off a portion may encourage re-growth. Long-handled ‘weed keys’ are also available for removing weeds from lawns, while a path weeder, a small hook-like tool, can be used for cutting and hooking out weeds from between paving slabs.

Hoeing and digging are, at best, curative methods of weed control; how much better to prevent weeds from emerging. By adding material over the soil surface we may not prevent weed seeds from germinating but, starved of light, the seedlings will be unable to survive. The simplest way to do this is to apply a loose mulch of garden compost, leaf mould, well rotted manure, bark chips or similar organic matter. A 5cm (2in) thick layer is needed to prevent annual weed seedlings from reaching the surface and this will need to be topped up at least once a year. In the vegetable and fruit garden, black plastic sheeting can be used to suppress weeds, although this is not an attractive option in more ornamental parts of the garden unless the sheeting is covered with a layer of loose mulch. It is almost impossible to eradicate perennial weeds by mulching, however, although plastic sheet mulching will achieve a significant reduction if left in place for one season. It is, however, unsightly.

Most annual weeds have a high demand for light, so garden plants with large leaves that grow rapidly can soon create a canopy over the soil and prevent the weeds from emerging and growing. These so-called ground-cover plants are often widely promoted as ideal for ‘low-maintenance’ gardening. If you choose the right plants and plant at realistic spacings, this can work well in large gardens, but the ground must first be clear of perennial weeds.

Chemical weed control

It is apparent that physical methods of control have serious limitations with persistent perennial weeds in beds and borders. On lawns and paths, too, it is virtually impossible to control all weed growth phvsically. But there will also be occasions when it is difficult to keep on top of an annual weed problem in this way.

It is not always practicable to mulch all parts of the garden. Soil being prepared for seed sowing, for example, and in the spring especially, with weed seedlings emerging at a prodigious rate and the land too wet for hoeing, even annual weed growth can soon become out of hand. This is why I doubt if many gardeners could manage totally without resorting to a weedkiller. However, choosing an appropriate weedkiller is critically important. Using the wrong weedkiller can have disastrous consequences, worse than with any other type of garden chemical.

We need to consider five principle types of product although, as will be apparent, these categories are not all mutually exclusive. It is important to remember first that only you know the difference between a weed and a garden plant; in other words, it is you as a gardener who must play the major part in deciding how effective and safe a weedkiller is to be by using it appropriately.

Total weedkillers kill all vegetation with which they come into contact. By contrast, selective weedkillers only kill certain types of plant; the only type presently available to gardeners are those that kill only broad-leaved weeds and can therefore be used safely on lawns. Some other weedkillers are selective in the sense that they can only kill seedlings and are sale to use, therefore, among established plants, but would be devastating in a seedbed. Some weedkillers work by contact, killing green tissues more or less through surface action; others are absorbed and then translocated within the plant, and these are obviously of special value for deep-rooted persistent perennial weeds. Residual weedkillers (which are also usually total weedkillers) persist in the soil for some weeks or months to kill seeds or freshly germinated seedlings. Clearly, a residual total weedkiller will render an area bereft of plant life for some time, a season or more, so such products should only be used on paths or other unplanted areas. Non-residual total weedkillers kill all existing vegetation but are rendered inactive in the soil so replanting or sowing can, therefore, proceed very soon afterwards.

I have listed below the weedkillers that are currently available to gardeners and indicated their modes of action and garden uses.

chemical weedkillers available to gardeners

Most are sold in proprietary mixtures for particular purposes, but it is very important that you don’t attempt to make up mixtures yourself; two incompatible chemicals could produce a very harmful cocktail. Weedkillers are usually applied as a diluted liquid, made up from a powder or liquid concentrate, using a watering can (especially on lawns or paths) or a sprayer. Never spray in winch weather and take care to ensure that the chemical is applied only to the target plants. Always wash out watering cans and sprayers thoroughly after use and as an added precaution, never use weed-killer containers for applying any other type of chemical.

Sometimes, weedkillers can be applied as granules or as powders — some powdered lawn fertilizers have a selective weedkiller incorporated with them — and at least one translocated weedkiller is available as a gel formulation for painting on to individual plants. I have never found this gel to be effective, however, and prefer to obtain localised activity by using a liquid formulation in a small sprayer and shielding nearby plants with a piece of card.

When spraying, remember that, in general, the finer the droplets, the more effective the coverage. And remember, too. That more than with any other type of garden chemical, weedkiller effectiveness is closely allied to climatic conditions. Warm dry weather, combined with a moist soil, is ideal, although the special conditions necessary for each product should be checked carefully on the label; the very useful translocated weedkiller glyphosate, for example, requires six hours without rain after application if it is to be fully absorbed into the plants’ tissues.

And finally, remember that you may need to apply a chemical weed treatment more than once during the season to keep the weeds under control.

New active weedkiller ingredients haven’t yet been developed for amateur use, and the costs are prohibitive, but there are new developments in the way weedkillers are applied. Ready-to-use products which do not need to be diluted have become more popular, often sold either as hand-held sprays, as gels or solid sticks that are painted on. The advantage with these methods is that small areas of weed can be dealt with quickly. However, they have their drawbacks – they are expensive for heating large areas and nozzles may become blocked on ready-to-use sprayers.


Moss is one of the major weeds affecting lawns and, for many gardeners, applying a moss killer is an annual ritual. The growth of moss on lawns is an indication that the lawn needs a change in its management so the grass can compete more effectively; simply applying a moss killer won’t change that fact. In particular, moss will become a serious problem if the grass is mown too short, it it is shaded, underfed or the soil is water-logged and compacted.

Disposal of weeds

When weeds are controlled with a weedkiller. The dead plants tend to shrivel away. When weeds are controlled physically, however, by hoeing or digging, they may be disposed of safely in a properly functioning compost bin. The temperature in the bin should attain at least 70°C (158°F) and this will be adequate to kill almost all seeds. In practice, I can say that I have never known weed seeds of any type to survive my own compost heap; no seedlings have ever emerged from compost subsequently spread as a soil mulch, at least. My only reservation is with rhizomes of couch grass. I don’t know the temperature necessary nor the period over which it must be maintained to kill these rhizomes, and as the consequences of spreading couch further are so serious, I feel that the rhizomes are better bagged up and removed to a public refuse tip.

31. July 2013 by Dave Pinkney
Categories: Garden Management, Weed Control | Tags: , | Comments Off on Understanding Weeds


Get every new post delivered to your Inbox

Join other followers: