Remember that all herbicides are different and the exact time you must wait to apply weed killers to newly planted grass will vary from one product to another. Also, some herbicides cannot be applied to certain species of turfgrass. For best results, always refer to the herbicide bottle’s label.
Before you plant grass seed, you should always prepare the area by removing any weeds that may be growing in the location. Even with careful preparation of the planting site, weeds can still develop among the newly planted grass seed. Weed killers, however, can harm grass seeds and seedlings if applied too early or improperly.
Some pre-emergent herbicides can safely be used during seeding and usually come mixed with a seed starter. These products have the active ingredient Siduron – also known as Tupersan – that works by suppressing weed seeds while improving root development of the new grass. The fertilizer and pre-emergent herbicide mix is applied with a drop or rotary spreader using a rate of 2 1/2 pounds per 1,000 square feet. The spreader setting and actual application depends on the brand of starter fertilizer plus weed control you use, and you should always follow the instructions found on the label.
Weed Control after Seeding
You can control weeds in newly planted grass seed and seedlings without the use of herbicides. Manually pulling the weeds by hand when they first appear keeps them from producing seeds and prevents the problematic plants from spreading, according to the University of California Statewide Integrated Pest Management Program website. They suggest keeping the newly planted grass weed free with proper mowing, irrigation and fertilization. Since newly planted turfgrass has short roots, keep the root zone moist by watering the soil lightly. However, avoid over saturating the soil. After the turfgrass has become established, promote deep and healthy root growth by watering infrequently but deeply.
Weed management should be completed before seeding the lawn with a non-selective herbicide seven to 14 days before you till the soil. A second application of the herbicide may be required to kill any weeds you missed during the first treatment. Wait another seven days until tilling the soil if a second application is used.
A general rule of thumb is to wait at least until you have mowed the new grass four times before using any standard postemergent broadleaf herbicide. A standard pre-emergent herbicide should not be applied until at least three to four months after seeding the area.
Marylee Gowans has written about gardening for both online and print publications. She attended the University of Akron, graduating with a Master of Fine Arts in creative writing. In 2009, she received master gardener certification from the Master Gardeners of Summit County, Ohio.
One of my favourite times for complete lawn weed control is in late August or September whilst there is still some warmth around. I would recommend this if you just need the one application in the year. The reason for this is:
A well fertilised lawn will help you get the most from your weed killer. In spring apply fertiliser in April and weed kill end of May or early June. In autumn feed 7-10 days before weed spraying.
Our recommended weed killers Resolva and Weedol (Verdone) suggest only one treatment per year but if you use each one once that allows you two full lawn treatments in a year. This only applies to weed killer for the coverage of the whole lawn NOT ready to use for spot treatments.
Conditions for using lawn weed killer
You will only kill weeds when they are actively growing. This means commencing sometime in May and finishing sometime in late September or early October.
I would not rush to apply lawn weed killer to the whole lawn in the early spring as there will be a lot of weeds that won’t surface until later on. In early spring just spot or hand weed. For a good weed kill over a broad range of weeds in your lawn try starting in May. If you need to repeat the treatment you still have plenty of time before cold weather makes it impractical. Read the label of your weed killer to make sure repeat treatments are allowed.
This all depends on the weedkiller being used. Each manufacturer is different so do read the label.
My advice would be a maximum of 2 treatments per annum using two different weed killers with different active ingredients. Further, you are only likely to treat this often in the first and possibly second year to get rid of established weeds. Once you get in to subsequent years a treatment in September will be the most you’ll need if you do some weed control with a spot weed killer during spring and summer. If you have good lawn care practices you’ll find you won’t need a complete lawn treatment for several years. I give my lawn a full treatment about every 3 years!