Know your weeds

Learn to recognize some of the more persistent and troublesome weeds in your garden and you re halfway to eradicating them.

The charming rock garden plant, snow-in-summer (Cerastium tomentosum), will quickly smother its neighbours if uncontrolled.

Weeds become a problem when they compete with other plants, reducing the available nutrients, light, water and space. They can also look unattractive, and may harbour pests and diseases.

A weed is simply any plant growing where it’s not wanted. Native wild flowers and grasses are generally re-garded as weeds whenever they appear among cultivated plants, though some gardeners may actively encourage them in a ‘wild garden’. Blackberries are welcome in the fruit garden but are enemies in the flower border. Equally, some cultivated plants become weeds when they grow too vigorously for their site – for example, local environment. They often germinate and establish faster than introduced species, so you must get rid of them quickly.

A predominance of certain weeds is a good indicator of soil type. Groundsel and fat hen, for example, grow freely on loamy soil; bindweed prefers an alkaline soil; dandelions and docks indicate mineral deficiency; sowthistles point to heavy, badly drained soil. Annual weeds live for just one year. They produce seeds that lie dormant over winter then germinate in spring or summer. Some, such as fat hen, produce tens of thousands of seeds on each plant. A chickweed seed may remain alive in the soil for more than a quarter of a century before germinating. The seeds may be spread around in mud on your shoes, by wind, on animals’ fur, in bird droppings or in compost. Perennial weeds are often the most difficult to get rid of. Many of them multiply by creeping stems, either above ground or below, as well as by seed. New plants can also be produced from tiny pieces of roots or underground stems which become severed during digging or hoeing.

Docks and dandelions are able to withstand long periods of drought since their long tap roots penetrate the soil deeply to obtain water. Some species, such as bindweed, may send down roots as deep as 3m (10ft), but others creep along just under the surface with shallow rooting systems.

Controlling weeds

Whenever weeds first appear in your garden, take prompt action to avoid having to wage a long-term war on them.

Eeds are unsightly, especially among flowerbeds and borders, and they also threaten the health and vigour of cultivated plants. During hot spells they compete for water, and throughout the year they tap nutrients and shade out sunlight. Many weeds also harbour pests and diseases.

The old saying ‘one year seeding means seven years weeding’ is very apt. Both annual and peren- nial weeds, if left uncontrolled, shed seeds which germinate when conditions suit. Never allow weeds to flower and produce seed, even though some may be attractive in their own right.

Learn to distinguish between weed seedlings and cultivated seedlings – Know Your Weeds, Know-How 25-30 will help you. Sowing in orderly rows in seed beds makes it easier to identify the weed seedlings.

Manual weeding

Individual weeds can be pulled up directly by hand, especially where a few large weeds are growing between ornamental or crop plants, and where weedkillers are unsuitable.

Pull the weeds firmly, holding the main stem as close to the soil as possible, to remove the whole root system intact. Many perennial weeds can regrow from severed root sections, rhizomes or bulbils left in the soil. A border fork can be used to loosen deep-rooted perennial weeds around the base of a cultivated plant.

For dealing with more widespread annual weeds, use a draw hoe or a Dutch hoe and a small border fork. A hand fork, trowel and onion hoe may also be useful. Use an onion hoe, a short-handled version of the draw hoe, where cultivated plants are growing close together. Drawing the hoe towards you, cut off the weeds with a chopping action.

A draw hoe can be used to chop down larger annual weeds. Use a Dutch hoe for general surface hoeing or between crop rows, skimming back and forth through the soil to sever weeds and remove their roots.

Sometimes the roots of perennial weeds such as bindweed and ground elder become entangled with the roots of ornamental plants. The only manual solution is to dig up both, separate out the weed roots and replant the ornamentals. With all perennial weeds, check the site about a month after the initial weeding to remove any remaining rooted fragments.

Collect weeds to make sure they don’t re-root. They can be put on the compost heap with the exception of any seedheads or perennial roots.

Choosing weedkillers

Chemical weedkillers – herbicides – take much of the physical effort out of weeding, but they need careful handling and application. There are several types of weedkiller, classified by their mode of action.

Chemicals referred to below are the active ingredient, not the trade name – read the contents label on weedkillers if you are unsure. Selective foliage-applied types kill only certain plants, leaving others unharmed. The most commonly used lawn weedkillers contain one or more herbicides such as 2,4-D, dicamba and mecoprop. They kill broad-leaved weeds without harming the grass. There are also selective weedkillers that kill grass while leaving non-grassy ornamental plants unharmed. Non-selective foliage-applied types kill most plants they touch. Some have a contact action, killing only the parts above ground directly touched by the chemical, for instance paraquat and diquat mixtures. Others are translocated -absorbed into the plant’s system and kill the entire plant, including roots and bulbs. These include glyphosate which takes several weeks to act thoroughly but is very effective, especially for perennial weeds.

Non-selective types are most effective when applied uniformly through a sprayer, dribble bar or brushed on the weed leaf surface. Soil-applied residual types remain active in the soil for some time and are taken up by the roots. They are effective against existing weeds and subsequent germinating seedlings – they act as weed preventers – so are useful for clearing land which won’t be used for several months.

Some soil-applied weedkillers have a selective action when used in low concentrations, or a nonselective action at higher doses, so follow manufacturer’s instructions carefully.

Total weedkillers, for example sodium chlorate, are used for clearing scrubland and on paths and drives. Since they remain very active in the soil for six to twelve months, according to the chemical used, do not use them among or-namentals or crops, or where you want to replant in the near future.

Applying weedkillers

Weedkiller chemicals may be sold as liquids which need dilution, as dry granules, powders or gels for direct application, or as wettable powders or soluble granules. According to the type, they are applied directly on to the weeds, or by watering can or sprayer, or sprinkled over the soil.

Foliage-applied types are most effective when applied uniformly over the weed leaf by sprayer, dribble bar or brush. When spraying weedkillers don’t pump up the sprayer as you would for an insecticide or fungicide – you should be able to get very close to the weeds. Choose a calm day when the spray won’t drift on to wanted plants.

Liquid weedkillers can be applied to paved areas or bare or open ground with a watering can. For even distribution replace the standard rose with a dribble bar. Walk across the area at a constant speed and apply systematically.

Spot weeding applicators include weeding sticks, liquid gun and gel brush types. The pre-diluted chemicals can also be attached to or incorporated into a long handle. The applicator head is simply wiped over any part of the weed plant. Glyphosate is the active ingredient – eventually it kills the whole plant, including roots.

Granular types need careful application, again on a calm, windless day. First calculate the total area to be treated and weigh out the appropriate total quantity of the chemical. Then square off the area into small sections and take enough chemical to treat one section at a time.

Troublesome weed spots Weeds in paths, drives and patios – to weed by hand, slice annuals off with an old knife or trowel. Carefully pull out perennial weeds with deep tap roots.

Chemical weedkillers provide the longest lasting cure. Choose non-selective or total foliage applied weedkillers, such as sodium chlorate and atrazine formulations, aminotriazole and simazine, paraquat and diquat, or dichlobe-nil. Proprietary mixtures for use on paths and paving usually contain a foliage-acting chemical to kill existing weeds, and a residual soil-acting one for preventing germination. Most mixtures are effective for several months, or even a year or more.

Aerosols containing aminotriazole are also available for spot control of individual weeds. Weeds in flowerbeds are more of a problem. Hand weeding is usually the most effective method, but be careful not to disturb any cultivated plants.

There are no chemicals which can select weeds from wanted plants. Non-selective weedkillers can be applied selectively by painting them on with a brush or special spot-treatment applicator.

Glyphosate gels are ideal for dabbing on problem weeds. Or dip the growing tips of trailing or climbing weeds into a container of weedkiller solution – a lawn weedkiller can be used for this. . For troublesome perennial weeds such as bindweed, train the weed to grow up a wigwam of canes so that you have plenty of foliage for the weedkiller to work on.

If the cultivated plants are well spaced you can use a sprayer, but keep the nozzle very close to the ground and the spray pressure to a minimum. Soluble chemicals such as paraquat and diquat mixtures, and propachlor, have a residual action. Dalapon is effective against couch grass.

Weeds among shrubs and in rosebeds can be treated with paraquat and diquat mixtures, dichlobenil or simazine.

Hoeing is quite easy since the ornamental plants have sturdy stems and they usually stand clear of the soil surface. Weeds among fruit crops – treat as for weeds among shrubs. Paraquat and diquat mixtures can be applied to unwanted suckers among cane fruits such as raspberries. Don’t use weedkillers among strawberries, which are sensitive to chemicals. Hand weeding is the only safe solution. Weeds among vegetable crops compete for nutrients, water and light, which significantly reduces the vegetable yield. Glyphosate, and paraquat and diquat mixtures, give good control between rows, but follow the instructions regarding the safety period before harvesting.

Hoe regularly between rows to get rid of as many germinating annual weeds as possible. Brushwood and coarse weeds on neglected areas can be treated with liquid weedkillers containing mecoprop, dicamba and 2,4-D, or with sodium chlorate.

These weedkillers must be handled with special care. They remain active in the soil for some time, so are unsuitable for clearing a site immediately before planting. Areas that have been treated with sodium chlorate must not be planted for six months.

Paraquat and diquat kill the top growth of coarse weeds and are harmless to the soil. Weed re-growth is possible but replanting can be carried out immediately.

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