Your potato fields should be sprayed with a broad-spectrum herbicide 6 weeks before ploughing and tilling. The planted area should be sprayed again with a broad-spectrum herbicide before the potato plants come through the ground -this is known as a pre-emergence spray.
The spray should have residual activity – meaning it continues to work for a couple of weeks after spraying. This gives the potato plants the best possible chance of growing big and strong and will smother any weeds which may return.
How to grow potatoes
If you would like more information about growing your own potatoes, please read my article grow your own potatoes. This covers everything you need to know from planting, preparing seed, growing, harvesting and storage and more about this great vegetable.
Weeds in potatoes
Weeds – the archenemy of horticulturalists everywhere! Seeming to multiply overnight, and stealing precious resources from the more desirable garden inhabitants, weeds are a serious nuisance in any garden – particularly a vegetable crop.
And weeds don’t just compete with garden vegetables for light, moisture, and nutrients; they can also be a refuge for the garden pests and diseases that can attack your potato patch. Weeds can decrease the yield of a potato crop, and – when present at harvest – can reduce yields further by slowing down the harvesting process or increasing mechanical damage to the tubers.
The success of your potato crop is dependent on controlling weeds throughout the growing season. Early season weeds can reduce crop yields if they’re not controlled within four to six weeks of the potato plants sprouting and until the foliage canopy is closed.
What to spray potatoes with to control weeds
In order to develop an effective herbicide program, it’s important to consider not only what the type of weeds are – or will be – in the field, but also the soil characteristics, tillage and irrigation practices and, if you practice crop rotation, you should also consider the previous and future crop plans.
Though biological and mechanical methods are used for controlling weeds, growers typically rely on herbicides to manage weeds most effectively. Herbicide performance will depend on the weather, as well as the method and timing of the application. What types of herbicides are available?
Soil-applied herbicides
Soil applied herbicides will generally not be effective on emerged weeds, as the herbicides need to interact with the weed seeds for adequate control.
Soil-applied herbicides are typically applied in emulsion or granular form but can also be sprayed after hilling the potatoes – usually two to three weeks after planting.
One downside of this type of weed control is that it relies on rainfall or irrigation to soak into the soil, where they can prevent weed seeds from germinating. Sometimes you may not get the desired rainfall within the time frame and the application will be of no use.
Broad spectrum spray herbicides
Many potato growers use broad applications of pre- and post-emergence broad-spectrum herbicides for effective weed control in their crops.
High concentrations of herbicides can be harmful to the environment. In rare instances using a broad-spectrum spray can increase the risk of the weeds developing a resistance to the herbicides. Although normally this isn’t really a major issue.
Rotating the herbicides can help to reduce this problem; so can scouting the fields to identify which weeds are present and you can then make a note if they seem to be becoming a problem. In which case you could switch to another herbicide and try it for a while.
Crop rotation is a very good way of tackling this issue so that the soil becomes inhospitable for the troublesome weed species.
Organic Herbicides
Growers who choose to avoid chemical compounds were traditionally limited to mechanical means of controlling weeds in their potato fields – they either hand-weeded or hoed the rows to kill any emerging weeds.
But in recent years, with the push toward organic farming, a lot of research has been focussed on organic alternatives to chemical herbicides.
Acetic acid to control weeds in potatoes
Research shows that one of the most effective organic methods of controlling weeds in potato crops is with acetic acid – the active ingredient in vinegar. While household vinegar using contains a 5% concentration of the acid, a stronger solution is required to eliminate weeds effectively.
Research yields results
Canadian researchers at Dalhousie University, led by Jerry Ivany, experimented with three concentrations applied at different stages of plant growth to determine the most effective concentration that would control the weeds without causing significant damage to the potato crop. A 30-centimetre band of 20% acetic acid spraying over the potato row in the early stages of weed growth – before the emergence of the potato leaves – resulted in control similar to metribuzin, a popular synthetic herbicide.
Synthetic Herbicides
The most popular method of weed control is still through the use of synthetic herbicides, whether applied directly to the soil or sprayed on the emergent weeds. In fact, most of the ‘failures’ of synthetic herbicides are the result of a lack of the grower’s understanding of how to best use the product.
Many growers establish a routine and disregard specific package instructions, either for the water volume or the product coverage; the grower might also neglect to spray the sides of the hilled areas, where weeds can grow, and focus instead on the flat surfaces between the rows.
Several different chemical herbicides can be effective when incorporated before planting or on either pre-emergent or existing weeds; spraying chemical herbicides means that a large area of soil can be treated quickly. It’s even possible to mix herbicides for a combined effect.
Clomazone
The active ingredient in Gamit 36CS will be most effective on pre-emergent, broad-leaved weeds.
Linuron
Used in several commercial herbicides, linuron should be used before the potato leaves emerge; the dosage will depend on the soil type and planting date. It can also be used if up to 10% of an early crop has emerged, or up to 20% of the maincrop when mixed with paraquat (either with or without diquat) or glufosinate-ammonium.
Metribuzin
The active ingredient in Sencorex, Artist, and several other herbicides can be used as part of a comprehensive weed management program. It should, however, only be used on certain varieties of potato and it’s best to verify the product label to ensure that it will be compatible with your crop.
Glufosinate-ammonium
While it’s preferable to apply before crop emergence, glufosinate-ammonium can be used up to 10% emergence of earlier or seed crops and 40% emergence of the maincrop.
Paraquat and Diquat
Used in various synthetic compounds, these two chemicals can be applied before crop shoots are 150mm tall, or before 10% crop emergence of earlies and 40% of maincrop. If using with seed crops, it’s preferable to apply pre-emergence.
Bentazone
This herbicide compound acts on emerged weeds, but it can be finicky; it’s recommended that you read the label carefully regarding the weather conditions and consult the manufacturer before use should you have any questions. It will control broad-leaved weeds on selected varieties of second earlies or maincrops if it’s applied before the crop is 150mm tall, though it shouldn’t be used on seed crops or earlies.
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