I’ve been getting lots of questions about yellowing grass, especially in St. Augustine lawns. I’m not surprised, considering how hot and dry it’s been. It's yellowing that some refer to as chartreuse or a faded-out green.
Unfortunately, there is not a one-size-fits-all solution to yellowing grass in July and August. It could be caused by a number of things, but let’s hope it’s just one of a couple that are easy to fix. It's either:
- Your irrigation is supplying too much chloramine.
- You didn’t do your summer fertilization.
- You have early-onset gray leaf spot.
- It’s early-onset of take all patch.
- You have nutrient lock-up.
- You have iron chlorosis (iron deficiency).
All are fixable, and the last two are pretty simple to take care of. But be warned that it could be the start of something more serious - St. Augustine decline. And that’s unfixable, by any GardenLine measures.
But, most often one of the six. And if your discoloration is under tree canopies, especially those of pines and crape myrtles, the remedies for problems 5 and 6 will fix that, too.
So, here's what I recommend for each issue:
- A water softener company can test your supply and see if that’s the culprit. Or, you can take a sample to a pool supply company and have them test it. If it’s too high in the chloramine, you’ll need to filter that out. Irrigation supply stores and some feed stores sell de-chlorinators.
- Many people haven’t fertilized in months. And, yes, my schedules call for summer fertilizations.
- If you identify the problem as gray leaf spot, that’s fixable by following these measures.
5. Some people who have errantly used all-purpose fertilizers or products not specifically designed for southern lawns, may have applied too much phosphorous (the middle number in the product ratio). That causes a nutrient lock-up, which is easily remedied with lots of soil activator and a compost top-dressing.
6. Iron chlorosis is the need for iron and a soil acidifier. This problem will appear as a yellow "streaking" in an overall necrotic blade, as the picture above shows. Put down any iron/acid combo you can find, and water it in. After two weeks, if you don't see a significant greening up, then assume you have a nutrient lock-up (#5). In this region, just putting down an iron supplement may not work if there’s not enough acid in the soil. So that’s why I always recommend an iron and soil acidifier combination.
A couple of side notes: If cost is no object, it is good to apply the iron/acid treatment and the soil activator together. Or at least one right after the other. Follow that with a compost top-dressing and more soil activator. I prefer this method, because it’s just healthier for the turf, and you don’t have to guess about the application order. I don't want to have to wait around to determine of it’s chlorosis, nutrient lock-up or take all patch. This solves all three, but it can seem costly.
You might also consider the possibility that your lawn mower blades need to be sharpened. Raggedly cut St. Augustine can look yellow or brown from a distance. If it’s been a year or more, sharpen them yourself or get it done at a lawn mower shop. And if you have a mowing service, ask them to sharpen their blades before their next service.
So, what if NONE of these treatments work? It could be the dreaded St. Augustine Decline (below). And frankly there is no cure for that. There are some fungicides that work a little on abating it, but none are completely effective. If you would like to learn more about SAD, see this Texas A&M article.