Leaves on St. Augustine Grass: When to Mulch and When to Rake

Leaves aren't the enemy

Skip Richter, a Texas A&M AgriLife horticulture agent, calls fallen leaves free, slow-release fertilizer, and points out that St. Augustine is one of the easiest lawns to mulch them into. The blades are coarse, so chopped leaf pieces drop down between them and break down instead of sitting on top. If you're already mowing, it's no extra work.

The problem isn't leaves. It's a thick layer that stays put. St. Augustine still needs light and air reaching the runners in late fall and winter, and a wet mat of leaves blocks both.

What matted leaves do to St. Augustine

• They shade it out. St. Augustine handles more shade than most warm-season grasses, but grass under a tree is usually already working with less light. Cover it with leaves and those thin areas get thinner.
• They hold moisture. Cool, wet conditions at the base of the grass are what large patch fungus needs. It's the most common fall and spring problem in Gulf Coast St. Augustine. See large patch fungus.
• They open the door for weeds. Thin, bare spots under a leaf mat are where winter weeds move in. See winter weeds.

How much is too much?

A simple test from University of Minnesota turf researchers: after you mow, you should be able to see at least 80 to 90 percent grass. If the chopped leaves still cover more than that, you've gone past mulching and into smothering. Rake or bag the extra.

Research from Michigan State, summarized by Virginia Tech Extension, found that mowers can mulch a lot of leaves over a season, up to about 6 inches of leaf fall. The key is that it happened over many mowings, a little at a time. That doesn't mean you can wait until there's 6 inches on the ground and mulch it all at once.

How to mulch leaves into St. Augustine

1. Mow when the leaves are dry. Wet leaves clump, clog the deck, and don't chop well.
2. Keep the mower at 3.5 to 4 inches. Don't drop the deck to grab more leaves. Scalping does more damage than the leaves would. See mowing height.
3. Close the side discharge or use the mulching setting so the leaves get cut more than once before they drop.
4. Mow more often during leaf drop. Go by how fast leaves pile up, not how fast the grass grows. A thin layer every week beats a thick layer once a month.
5. Make a second pass if you still see big pieces sitting on top.

A sharp blade makes a big difference here. A dull one tears leaves into ragged pieces and struggles to chop them small. I pull the blade with a blade removal tool, sharpen it, and check it on a balancer before it goes back on. Here's how to sharpen a mower blade.

When to rake or bag instead

• The layer is thick and more than a quick pass can handle.
• Leaves are piling up along fences, beds, and corners where they collect and stay wet.
• The lawn under the tree is already thin. Give those spots every bit of light they can get.
• There's active large patch in that part of the yard. Bagging there keeps you from spreading it around with the mower, and it gets wet material off the patch.
• The lawn is fully dormant and you're not mowing anymore. Leaves on a dormant lawn just sit there until spring unless you move them.

A two-step trick helps with heavy leaf fall: mow once with the bagger off to chop everything up, then mow again with the bagger on. The chopped leaves take up a lot less room in the bag.

For raking and corners, I rake leaves into a Fiskars pop-up yard bag. It stands open on its own, so I'm not holding a bag open with one hand and raking with the other, and it folds flat when I'm done.

Leaf timing in the Houston area

Leaf season here isn't one season. Trees like pecans, red oaks, and sweetgums drop in late fall and early winter, while the grass is slowing down and going dormant. Live oaks are different. They hold their leaves through winter and shed mostly in late winter and early spring, right when St. Augustine is starting to green up. That second round matters, because a leaf mat in February and March sits on a lawn that's trying to wake up during spring large patch weather.

So plan on two leaf cleanups: one in late fall, and one as the live oaks drop. The second one lines up with the light late-winter cleanup in the lawn calendar.

Don't waste the leaves

Leaves you rake up don't have to go to the curb. AgriLife suggests running them over with the mower a few times and using them as mulch around shrubs and trees, or adding them to a compost pile. Chopped leaves are less likely to blow away and break down faster.

For everything else on the fall list (the winterizer, watering cutbacks, and what to expect from dormancy), see fall and winter care for St. Augustine.

Common questions

Will leaves kill St. Augustine grass?

A light layer won't. A thick, wet mat that sits for weeks can. It blocks sunlight, holds moisture against the grass, and creates the cool, damp conditions that large patch fungus likes. Thin areas under trees are the first to suffer.

Is it OK to leave leaves on the lawn over winter?

Not in a thick layer. Even a dormant St. Augustine lawn needs light and air reaching the runners. Mulch light leaf fall with the mower, and rake or bag the rest so it doesn't sit there until spring.

Should I mulch or bag leaves on St. Augustine?

Mulch them when the layer is light enough that you can still see most of the grass after you mow. Bag them when the leaves are thick, when there's active large patch in that area, or when the lawn under the tree is already thin.

When do leaves fall in Houston?

It depends on the tree. Many trees like pecans, red oaks, and sweetgums drop in late fall and early winter. Live oaks hold their leaves through winter and shed them mostly in late winter and early spring, right as St. Augustine starts to wake up.