What are the worst weeds in my garden and what do I do about them?

The simplest definition of a weed is that it’€™s a plant that you don’€™t want there. That means a plant can be a weed in one place and highly valuable in another. The key to keeping weeds under control lies in understanding how they propagate themselves and then taking steps to stop them doing that.

Types of weeds -€“ perennial and annual

Annual weeds are those that complete their life cycle within a single year and usually propagate themselves by seed. Included are plants such as bindii and chickweed. Preventing these plants from seeding by either pulling them out or poisoning them when small and before they have flowered is the best way to get rid of them. If you let them drop seeds, you’€™ll have more weeds to deal with next year.

Perennial weeds are those that live for more than one season and they may be either woody (eg privet and lantana) or herbaceous (eg kikuyu and wandering jew). Perennial weeds may spread by seed but often also by some vegetative means such as runners, bulbs or root suckers. This often makes them doubly hard to control as we have to not only stop them seeding, but also either kill or remove the vegetative structures as well which we need to look at case by case below.

The list of weeds below covers the whole range of weed types and gives you strategies that can be applied to other weeds in your garden.

The worst types of garden weeds and what to do about them

1. Weeds that grow from bulbs and bulb-like structures

Onion weed– This plant belongs to the lily family and the reason why it is such a problem is that it grows from a bulb that rapidly multiplies into many small bulbs. The leaves have a grass-like appearance and it produces slender heads of small, white flowers. Trying to pull it out by the leaves usually leaves the bulbs intact underground and they simply sprout again. Trying to dig out the bulbs can work if you are very patient but it can often worsen the problem as it disturbs and spreads masses of tiny bulblets that form around the original bulb. Probably the best option is to apply concentrated glyphosate (the active ingredient of Zero, Roundup and several other popular herbicides) from a weeding wand, or spraying a larger area if there is no danger of hitting your desirable plants. It is very important to follow up the initial treatment as poisoning will kill bulbs that have leaves present but this in turn causes the small bulblets attached to them to sprout. If you don’t want to use herbicide, then you can exclude light until the plant dies by covering with plastic or similar. This will still leave the bulblets which will grow once you uncover the area, so you will need great patience with either herbicide or light exclusion to rid your area of onion weed.
Oxalis– which can be recognised by its clover-like leaves and small lily-like flowers, poses the same kind of problem as onion weed as it also spreads from bulbs and bulblets. The bulbs tend to be closer to the soil surface which makes it an easier proposition to dig out successfully if you have the time, otherwise resort to the glyphosate treatment.
Other plants that fall into this category include Montbretia, which has sword-shaped leaves and orange flowers that are attractive but this plant can be very invasive. Hand-pulling this weed is an easier proposition, as it is shallow-rooted.
Other bulbous weeds that can be given the same treatment include;
Watsonia, nutgrass (Cyperus rotundus) and onion grass (Romulea rosea)

2. Annual weeds that spread from seed

Chickweed– This is a very fast growing annual weed that has small mid-green leaves and tiny white flowers that produce copious amounts of seed within weeks of the weed appearing. They can be easily removed by hand but this will usually result in many seedlings coming up within a few days. If you either dig them out with a garden fork or pour boiling water on them while they are small these seedlings can be effectively controlled.
Bindii– This wretched plant produces spiny seeds that infest lawns. It is an annual that germinates in autumn and forms a prostrate mat that forms the mature seeds in late spring and early summer. By this time it is too late to prevent the seeds being a hazard (as they are still spiny even if they are dead) unless you dig each plant out individually, something that is only practical in very small lawns. The easiest solution is to use a selective herbicide such as ‘Bindii & Clover Killer’€™, however it is critical that you apply it when the plant is young to prevent the plant going to seed. Once the seed head starts setting, even if you kill the plant, the seeds can mature and fall to start the whole prickly cycle again.
Other major weeds that fall into this category and can be treated the same way-
Capeweed (Arctotheca calendula), catsear (Hypochoeris radicata), common sowthistle (Sonchus oleraceus), dandelion (Taraxacum officinale), fat hen (Chenopodium album), lamb’€™s tongue (Plantago lanceolata), pigweed (Portulaca oleracea) and shepherd’€™s purse (Capsella bursapastoris)

3. Woody weeds

Privet is a shrub with a very tenacious root system that is very hard to pull out. It has small spoon-shaped glossy leaves and forms masses of black berries that are often eaten by birds that in turn spread the seed which can come up anywhere where birds deposit these ‘€˜greeting cards’€™. Seedlings should be pulled out as soon as they appear but for larger plants, which can sucker from the root system, the best method is to chop the top off and immediately paint the freshly exposed stump with concentrated glyphosate solution. The treatment may need to be repeated more than once before the weed is completely dead and is most effective when the plant is actively growing during spring and summer. Other woody weeds that can be treated in this way include blackberry, gorse, lantana, sweet briar (Rosa rubiginosa), tree of heaven, willow, wisteria.

4. Perennial weeds that spread from runners

Wandering Jew- The fast growing, rapidly rooting stems are the problem with this shade-loving weed. The waxy leaves seem to make them resistant to most herbicides. You can easily pull it out by hand, however you must be particularly careful not to leave behind any little pieces of stem and roots, as they will take off again and choke out other plants again. If you have a particularly rampant infestation the best solution is to use a garden rake to collect up the bulk of the material. You can then meticulously remove the stems that are remaining or cover it up with several layers of newspaper followed by a very thick layer of coarse mulch such as woodchip or pine bark.
Other weeds, which have similar habits, include the lawn grasses, kikuyu, couch and buffalo, and while these respond to the same treatment they are more susceptible to herbicides such as glyphosate.

These are a few examples of the worst weeds you will encounter. There are many others such as Madeira vine, Crofton weed that can be found in reference books. It should also be said that perennial weeds that spread from vegetative parts should be disposed of thoughtfully. They should not be thrown into the bush or on reserves, where they can regrow to become a nightmare to get rid of. Most councils now provide green waste collection services that usually result in weeds being thoroughly composted to the point where they are no longer a threat, or altenatively bag and bin them into the general rubbish collection.