Selective herbicides or weed killers will control the majority of broad-leaved weeds in turf. When used correctly (in suitable conditions and correct application rate) they control the weeds without harming the grass.
Most, if not all, weed killers will contain at least two active ingredients. This is because each ingredient will target different weeds. There fore by mixing ingredients together, the spectrum of control is much broader, controlling a wider range of lawn weeds.
Active ingredients used to control weeds include:
At least one ingredient will be translocated (moves through the weed), with growth-regulating properties, such as 2,4-D, mecoprop-p or MCPA. These chemicals are absorbed into the weed and upset the normal growth pattern.
Following application, a twisting and curling effect can be witnessed on the plant leaves, as growth is distorted. These chemical work quite slowly and it may take several weeks for the weed to finally die and disappear from the lawn.
Some weeds are moderately resistant to selective herbicides, and a second or even third application may be required for complete control. Difficult weeds to control include yarrow, speedwell and field woodrush. To obtain effective results from herbicide application visit our weed killer application page. There are lots of tips and advice on applying selective herbicides.
Liquid vs granular selective herbicides
Selective herbicides are available in both granular and liquid formulations for weed control. Both have their advantages and disadvantages.
Liquid weed killers are formulated to be applied through a sprayer or watering can. Sprayers fitted with a nozzle that produces a fine spray will yield the best results.
To achieve optimum results with a liquid herbicide, the solution / mixture needs to cover as much of the weeds leaves as possible. A fine spray is the best way of doing this.
For that reason, we prefer to use a sprayer instead of a watering can when applying a selective weed killer. Watering cans produce big droplets, causing a certain amount of the solution to run off the leaves and be wasted.
These type of selective herbicides come either as a concentrate (needs to be mixed with water before they are applied) or pre-mixed ready to apply out the bottle, so to speak.
The majority need to be mixed with water before being applied to the lawn. The applicator, be it a knapsack sprayer or watering can, should be half filled with water.
Then the correct amount of weed killer should then be added, before topping up the remainder with water. Once it has been mixed up, it should be applied without delay to keep the mixture agitated (mixed properly).
Like all weed killers of this nature they work best when the weeds are actively growing. This is usually between April and September. I often find that May – June are good months to treat weeds, as the weeds are fully developed, easily identified and growth is strong ensuring optimum results.
To encourage good growth, many turf professionals apply a feed to the grass, prior to weed control. This ensures optimum results and further applications are not required.
Liquid weed killers should only be applied when it is dry with no rain forecast, rainfall will wash the chemical off the leaf making it somewhat ineffective. Always spray when the grass/weeds are dry, again spraying onto a wet surface will cause the chemical to run off.
Never spray during drought, excessive heat, windy conditions or periods of frost.
Most granular weed killers are available as weed, feed & moss control products, this can be a big plus as it means they cover a number of tasks in a single treatment. E.g. feeds the lawn, controls the moss and kills the weeds. These type of products are formulated to be applied through a push garden spreader of hand held spreader. They are great for
As with liquids, the optimum time for application is during April to September during periods of good growth. However, it is worth noting that these products contain a fertiliser to encourage growth.
Bear in mind they will only encourage growth if the ground is warm, fertiliser will not make the grass grow if it is very cold.
These products should be applied when rain is not imminent, but for best results the surface of the lawn should be wet. This helps the chemical stick to the leaves of the weed. If no rain falls within 48 hours it will need watering in.
Do not walk on the lawn until it has had some form of irrigation. Note: These figures may vary between different manufactures, so always read the instructions before applying the product.
The main drawback with granular weed, feed and moss killer products, is that moss control would normally be undertaken during early spring, late March being a common time.
Weed killers are better being applied several weeks later. For this reason we prefer to use liquid weed killers. They are also easier to apply, especially over larger areas and are great for spot treating weeds.