Weed control in your garden and lawn doesn’t always have to be a nightmare.
Tilling can effectively reduce weeds if it is deep enough that goes at least 8 to 10 inches (20 to 25cm) below the surface. That way weed seeds can be turned from the topsoil to a lower layer and can be prevented from germinating. Tilling only the topsoil is not effective and may even encourage them to grow compared to no tilling.
In this article, we’ll take an in-depth look at how tilling can or cannot control weeds.
1. How you till matters
Whether tilling can kill weeds depends on how deep and intense the tilling is done. In general, there are four types of tilling that farmers and gardeners most commonly use:
Conventional tillage is a deep tilling technique and typically involves turning the soil two times.
The soil is first tilled after harvest when it is still moist using a moldboard plow or chisel plow at depths between 8 to 12 inches (20 to 30 cm). The aim is to break and aerate the soil and bury weeds and crop residue.
The second time the soil is turned again to incorporate fertilizers, control weeds, and level the soil surface.
Reduced tillage is characterized by shallow and less intense turning of the topsoil (the top 4 to 6 inches or 10 to 15 cm of the soil) using cultivators, harrows, chisel plows. This technique leaves at least 15% of the crop residue on the soil.
Not tilling the soil is in fact also a technique by leaving the soil undisturbed in its natural state, with crop residue left on the surface. It may be combined with the use of herbicides.
Rototilling is frequently used in gardens and lawns to prepare the soil before planting. It uses a machine with rotating blades (called tines) that break up the soil, distribute fertilizer, and remove weeds as it moves across the field.
Rototilling is typically done to depths between 4 and 6 inches (10 to 15 cm).
2. Impact on the number of weeds
Deep tilling techniques can reduce weed infestation.
This is demonstrated by a study where the conventional tillage technique is found to result in the least amount of weeds (Woziak, 2018). Conventional tilling involves turning the soil two times deeply at a depth of 8 to 10 inches (20 to 25cm) after harvest and before sowing. This technique resulted in between 18 and 23% fewer weeds compared to the other techniques.
Tilling the soil less intensely (or the “reduced tilling” technique) involves plowing before and after harvest both to a shallow depth of between 4 to 5 inches (10 to 12cm). Even though the soil was plowed twice after harvest and before sowing like the conventional tilling technique, it still resulted in a higher amount of weeds than the conventional tilling technique and the use of herbicide with no tilling.
|Number of weeds/m2|
|Herbicide application with no tilling||46.2|
(Source: Wozniak, 2018)
The reason is that deep plowing can effectively transfer weed seeds from the topsoil to the deeper layers, preventing them from germinating.
Shallow tilling, in contrast, only shovels the topsoil where weed seeds accumulate and they can germinate easily when the topsoil warms up and gets moist. Moreover, shallow tilling does not uproot perennials with deep root systems, leaving roots in the soil will eventually send out new growths.
The use of herbicide (Glyphosate or Roundup herbicide) is also not effective in reducing the number of weeds and can only reduce the diversity of weed species, where only the herbicide-tolerant species can survive and eventually dominate.
Tilling the soil using a machine with rotating blades, or rototilling, can be effective against annual weeds as it cuts up the roots and stems of weeds in its path. But, it can carry cut-up roots to new areas as it moves across the field and can bring buried weed seeds to the surface, thus promoting them to grow into new plants (Oregon State University, 2020).
3. Impact on weed diversity
The tilling methods also affect the type of weeds, that shallow tilling and no tilling methods result in more perennial weeds while deep tilling produces more annual ones.
This is proven by a study in Slovakia where fields that are tilled deeply have twice as many perennial weeds as annual weeds as fields that are tilled only at the topsoil (Demjanova et al, 2009).
This is because deep tiling actions disrupt, uproot, and bury perennial roots that extend deep in the soil.
On the other hand, shallow tilling cannot remove perennial weeds but can remove the annuals, leaving 1.3 to 2 times more perennial weeds than annuals according to the study.
No tilling favors perennials over annual weeds because the reproductive structures underground are not affected and the herbicides used for annual weeds are not completely effective on established perennials. Although the weeds on the surface may die off, new growths will emerge from the underground roots.
Rototilling also results in more perennial weeds than annual weeds. This is because the machine is effective in cutting up and destroying annual weeds in topsoil, but not the perennials with deeper root systems. In fact, the machine may even spread the perennial weeds, resulting in more perennial species and fewer annuals in comparison (Oregon State University, n.d).
4. Should I kill weeds before tilling?
Killing weeds before tilling can help reduce the number of weeds that will germinate and compete with your plants later on.
The best approach is to kill the weed seeds in the topsoil, not only the existing weeds. This is to ensure that no viable seeds that can germinate later are tilled into the soil.
Weeds and weed seeds can be killed using heat by putting a plastic cover over the ground to generate temperatures up to 140 F (60 C).
Mulching with bark chips, and hay can suppress weed growth and also attract crickets and beetles that feed on the weed seeds.
Herbicides can also kill weed seeds by attacking them just as soon as they start germinating.
Tilling can be an effective way to reduce weeds. However, you should till at least 8 to 10 inches (20 to 25cm) deep into the soil so as to turn the majority of the weed seeds from the topsoil to a deeper level to prevent them from germinating. That way you would also be able to remove a big portion of the hardy perennial weeds.
Apart from tilling, another effective way is to put mulch over the soil.
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Woźniak, A. (2019). Effect of tillage system on the structure of weed infestation of winter wheat. Spanish Journal of Agricultural Research, 16(4), e1009.
Martens, J., Irvine, B., Entz, M., & Derksen, D. (n.d.). Weed Management Options: Tillage System. University of Manitoba.
Demjanova, E., Macak, M., & Majernick, F. (2009). Effects of tillage systems and crop rotation on weed density, weed species composition and weed biomass in maize.
Oregon State University. (2020). Pull weeds as they pop or desired plants will suffer. Life at OSU.