When is the best time for that Pre-Emergent application?

Published 4:32 pm Thursday, January 19, 2023

I was in Raleigh last Friday for a meeting. My motto is if you are on time, you’re late, so I was there about an hour early so I could get some work done before the meeting started. The place was on lock-down! Every office was locked and the conference room where we were meeting was even locked. I thought maybe it was a nice time to go for a walk. It was around 60° that day and the sun was high and bright.

I walked the greenway along Centennial Campus. It was a beautiful walk, there are ponds, a stream, and some beautiful older oaks along the way. I noticed that nearly all of the maples were in bloom and the pears were looking like the buds had swelled and were ready to open as well. I guess even though we have had some unusually cold temperatures this season, all in all we have had a fairly warm winter so far. I am writing this on my deck while sitting in the sun wearing shorts and it is 73°.

This tells me that we probably need to be looking for our winter annuals to go to seed a little early and for our summer annual weeds to most likely be germinating early. The lawn burweed is a typical winter annual that becomes a problem when it goes to seed in late winter/early spring. The crazy thing here is that it is usually very difficult to find until late February/early March. Due to its prostrate growth habit, it stays along the ground and hides somewhat from sight. Bermuda and/or other warm season grasses may not have hit good dormancy yet. One of the most common practices on Bermudagrass is to spray Round-Up in January to clean up any leftover winter annuals. This is a very safe practice when the grass is dormant but there is a good chance it isn’t. It most likely won’t kill Bermuda but it could certainly delay green-up in the spring by spraying Round-Up when it isn’t dormant. This can happen if you spray it too late as well.

Our prescription is to get a pre-emergent herbicide (PRE) application out in the first two weeks of February. This will prevent our warm season annuals from getting a foothold before green-up. Look for something that is safe for your grass and be weary of using DNA herbicides in the spring (prodiamine, pendimethalin, trifluralin, oryzalin, and so on). These are root inhibitors that can really hurt you in the long run on stoloniferous grasses (basically all warm-season turf). This is the reason we often see clubroot conditions on warm-season turf in the spring into early summer. Warm-season grasses reproduce using stolons (stems that grow across the top of the soil) that root at the nodes. If you are using a root inhibiting PRE, these stolons can’t root at the nodes, the burnt roots and long stems become wasted energy.

If you have winter weeds that have emerged, it is best to get those cleaned up now, however you need to choose your application time wisely. It does no good to put post-emergent herbicides (POST) out when the weeds are not actively growing. It is best to apply herbicides in the winter during times when we get those above average temperatures for a couple of days in a row. So, be watching the weather to look for three or four days in a row when temps get into the high 50’s to low 60’s and make sure to spray your POST then. Otherwise, it is a waste of money.

Also, do not think that you can spray a PRE and it will take care of those weeds that have already emerged. This doesn’t work in most cases unless you are using something that has both PRE and POST activity such as mesotrione (Tenacity), atrazine (Atrazine), or simazine (Princep). By the way, you will notice that the new atrazine/simazine labels are going to begin making their way to the shelves now. Atrazine will only be used for spot-spraying (less than 30% of the whole lawn), applied by backpack sprayer only and simazine will have to be watered in immediately or used at a lesser rate. Be careful with mesotrione too if you are getting close to green-up, it can cause bleaching of the grass for a few weeks.

Next, we want to apply a POST sometime after green-up to clean up any weeds that our PRE application missed or that germinated after it wore off. One of the safest on warm-season turf is Celsius. This is an expensive product but it gets the job done. There are also several three-way and four-way products that will do just fine that you can find at many of the local hardware stores. These are comprised of three to four chemicals that work together to take out your emerged weeds. Always keep in mind that typically the smaller the weed is when you pray it, the better chance you have of killing it. Once weeds get larger, we can often only hurt them, not kill them. Lastly, read the label! There is a good bit of information here that will both keep you safe and ensure that your herbicide is effective.

If you are having trouble with growing in your home landscape, call the Extension office at (252)946-0111 or email me at gene_fox@ncsu.edu. I will have a commercial ornamentals and turf class coming up in February. If you need pesticide or landscape contractors credits, please keep an eye out for this class, it will be February 27th. Until then, Happy Gardening!