Previous articles have covered researching weeds and putting things in place to prevent weed infestation, this article covers weed control strategies that may be applicable to your situation. While advertisers would have us believe that the first and best control should be chemical, there are other options, and this article discusses physical rather than chemical options.
Heat
Sufficient heat or heat applied over a longer term can have a negative effect on plant cells, disrupting them. Heat may be applied in a number of ways to control weeds –
1. Solarisation – This is the process of covering an area in black plastic over summer and allowing the summer heat to kill weeds and weed seeds. Before applying the plastic, mow the weeds or cut them back with a whipper snipper or scythe. Also, ensure the area is damp, this will encourage weed seed germination, which the solar heat and lack of light will then kill. Irrigating the area first will also allow the solarisation to be more effective because damp soil will be more conductive to heat than dry soil. Leave the plastic in place for four to six weeks in the middle of summer to get the most out of the solar heat. It is also important to dig in the edges of the plastic sheet or weigh them down to exclude light as much as possible.
The soil may reach as high as 60C at a depth of 50mm, which will also impact pest insects and soil pathogens. The goal is to achieve a temperature of 45c to 50c in top 150mm of the soil. This is a comparatively cost effective way of dealing with a large area of annual weeds.
The downsides of solarisation are that the soil biota will also suffer as a result of the heating, but this should return to normal after the plastic is removed. Solarisation may also be less effective in controlling perennial weeds that have deeply buried underground vegetative structures such as rhizomes, corms or tubers, which are below the hotter levels in the soil and that may re-sprout once the solarisation is complete.
2. Boiling water – In this weed control method the heat is applied directly by heating water to boiling and then pouring it over the weeds to be controlled. The boiling water acts as a contact herbicide in that it will kill what it comes in contact with, so it will be easier to kill plants with fibrous root systems rather than a long tap root, because the taproot will require more water, going deeper into the soil. This method is ideal for small area surgical strikes on weeds.
Day 0 (prior to boiling water application)
The classic teapot or kettle is ideal for this manoeuvre, fill it with boiling water and put on gloves (just in case, keeping in mind the possibility of burns and scalds)) then transport it to and pour the water on the soil around and over the weeds to be taken out. Keep an eye on the weeds and re-treat after seven to ten days if required.
Day 9 (after application)
This a comparatively cheap method requiring no large investment in equipment, it works well for small patches of weeds. It will require some energy to heat the water in the first place (using a rocket stove will reduce costs) and also the process will also have a deleterious effect on soil biota, but due to its nature of only being used in small areas, the soil biota should recover quickly.
Initially
Two days after application of 3 kettle-loads of boiling water
3. Flame – The easiest and most ergonomic way (you don’t need to get down on hands and knees) to control weeds with fire is to use a flame wand. Unfortunately a flame wand will be powered by a fossil fuel, in the old days it was kerosene, I still have an old kero powered heat wand I inherited from my father (see pic below), but these days they are powered by butane or propane. They will also require a small to moderate sized investment in equipment.
Cheaper flame wands (<$40 AUD in 2023) are lighter and have a small attached disposable butane gas bottle and have limited life and limited ability to get to the root of the problem (ie the weeds!). For a small area, small weeds without deep roots and occasional use these may do the trick, but even so you will need to replace the gas bottle, hopefully by recycling rather than landfill.
Cheaper flame wand
Larger flame wands cost more ($250+ AUD in 2023) but have a number of advantages, mainly that they get hot enough to actually give weeds a hard time and a refillable gas bottle is easier, more cost effective and reduces waste, particularly if you have a large area to treat. If possible, get a flame wand that can change from pilot flame to full flame manually (saves gas between weedy areas), that has a tube long enough so you don’t need to bend and has the correct connection for your gas tank. Strapping the gas bottle to a trolley will help make the setup more mobile so you don’t have to lug the bottle around by hand. A larger wand will despatch newly emerged and shallow rooted weeds with one treatment but deep rooted perennial weeds may still need multiple (3 to 4) treatments to compete the job.
Large Flame Wand
Weeds need less than a second of exposure to the flame to kill them by disrupting their cells, they do not need to be incinerated.
Barriers
Another way to limit the impact of weeds in your growing spaces is to set up a barrier between areas where weeds such as grasses like couch or kikuyu move from the lawn (where they are not weeds) into your flower or veggie beds (where they are!). These barriers can be put together in a number of ways.
1. Plants – Often referred to as competition barriers, these plants can be planted around beneficial species such as fruit trees or vegetable beds to prevent ingress from grass, weeds or other invasive plants. Examples of competition barriers include daffodils, comfrey and perennial alliums (garlic chives, wild leek or Egyptian onions) which are planted around the area requiring protection.
Comfrey - makes a great barrier plant
2. Intentional Soil Compaction – Wait! What? Yup, because soil compaction is bad for plants, the soil and soil biota in general it is something that we as gardeners strenuously avoid, resorting to practices like double digging and raised beds to actually prevent soil compaction. In this case though, the idea is to create an area of compacted soil which becomes a barrier to the infiltration of weeds that spread vegetatively by stolons, runners or the like.
The degree of effectiveness will depend to a certain amount on the level of clay in the soil, the more clay, them more effective the compaction, and around here on good old Sydney clay, it can be very effective. The idea is to create a 30cm to 60cm barrier either around a weedy area to contain them, or around the area to be protected to keep them out. The tool of choice to achieve this is the imaginatively named ‘tamper’ which is a flat plate screwed onto a vertical handle of various lengths.
Mow the area to be compacted as low as possible, wet it down and then use the tamper to compact the soil, as well as walking on it whenever you get the chance. When any plants try to reclaim the area, mow and re-tamp. You will need to keep an eye on the barrier, but once the tamping has become effective weed regrowth will be much reduced.
3. Mulch – this is a different type of barrier than the previous two, being more a barrier between weeds and the sun, which will settle them down. An organic mulch laid down 75cm to 100mm thick will discourage all but the most invasive and persistent weeds, but if your bed is already a home to an invasive weed like oxalis, a thick layer of newspaper or cardboard under the organic mulch will help in preventing the weeds getting access to the sun and they will die off.
Mechanical removal
1. Manual removal (whole plant) – that is just fancy language for ‘pulling ‘em out’ by hand. This is comparatively easy and quick for small areas of annual weeds, but it is always advisable to wear gloves, especially if you have previously identified weeds that can irritate the skin like stinging nettle or petty spurge. It is more problematic for perennial weeds with tap roots, although deeply irrigating the area can soften the ground to the point where the plant can be removed taproot and all. Also, pulling them early in their life cycle before they have a chance to develop extensive root systems or set seed will ensure a better outcome.
2. Hoeing – If you are of a more mature persuasion or have mobility issues, you may need to remove your weeds while standing and to do this you need a hoe of some description. Generically speaking, a hoe is a piece of metal on the end of a stick used to dig out or cut off weeds, and we have a number of designs which we use. I find the ‘Heritage Multi-use Hoe’ to be the most useful for weeding although our standard hoe and small hoe also work well, and we have a ‘Raymond Wonder Weeder’, but I prefer the others. There is also a ‘Dutch’ hoe or push hoe that is pushed through the soil to cut off weeds just under the surface, I have not tried one but have been told they work well. Hoes are used to cut off annual weeds and dig out the more persistent perennial weeds, but in doing so they may stimulate weed seeds in the soil to germinate. Using a hoe is easier in dry soil rather than wet puggy soil and dry conditions will also cause the weeds to wither once hoed as well.
Raymonds Wonder Weeder and Heritage Multi-use Hoe
Small hoe and Standard Hoe
3. Grow, then mow – while this tends to work better early in the growing season and more quickly with annual rather than perennial weeds, if persevered with it will give just about any weed a really hard time! Also pulling out weeds totally (as in ‘manual removal’ above) can actual act as ‘tilling’ the soil and result in dormant weed seeds germinating, so it can be better to remove the foliage and allow the roots to die and break down rather than disturbing the soil.
Cut the weed foliage down close to the soil level and keep an eye on the area. While annual, young and shallow rooted weeds will expire quickly, as the more resilient weeds start to re-sprout, hit them again. Eventually even the most committed weed will give up.
My Scythe*
A number of tools can be used to cut the weeds down and keep them down, and in an open area the first that comes to mind is a lawn mower (hopefully hand or battery powered) on its lowest setting, or a scythe if you want to go for a more ‘hands on’ approach. There are downsides to these tools in more confined areas like veggie patches and flower beds, so a whipper snipper may be used carefully to keep their heads down. If a whipper snipper is not for you and you want to get more up close and personal with your weeds you might want to consider a sickle, a Japanese Kama, or for smaller areas a Korean Ho Mi.
I bought this from a second hand shop years ago, not sure what it is called, perhaps a sickle scythe?
Conclusion
There are a number of physical means of controlling weeds, but they will need to be tailored to the particular weeds in your circumstances so it is important to identify your weeds and do some research first to ensure the most effective technique is used to control each weed.
