How To Kill St Augustine Grass In Zoysia

How To Kill St Augustine Grass In Zoysia

Zoysia and St Augustine are both hearty warm season grasses, but they don’t go too well together in the same lawn. Zoysia is a thick and weed resistant grass, but St Augustine is a faster spreader and can outcompete a Zoysia lawn if it finds its way into the yard. 

Grassy weeds can be tricky to deal with since they mix into the desired grass. These two warm season grasses may be similar in some of their habits, but certain differences are key to the strategy for getting rid of St Augustine from a Zoysia lawn.

Zoysia And St Augustine Grasses

Zoysia is a popular grass type due to its dense coverage and drought tolerance. It can also be cut very short, making it a favorite for golf courses. Homeowners can enjoy Zoysia for its soft blades in the sun or shade, since Zoysia has a higher shade tolerance than other warm season grasses.

It comes in various s varieties like Emerald, Palisades, and Empire zoysia, each of which varies in its blade width, green color, and drought tolerance (although, they are all pretty drought tolerant). 

In comparison, St Augustine has stiffer grass blades than Zoysia and is a bit less shade tolerant. Where Zoysia likes to be cut down to one or two inches, St Augustine likes to stay long, around 4 inches. They both spread with stolons (aboveground, vine-like root extensions), but Zoysia also spreads by rhizomes, which are underground root extensions. This extra anchoring and integration into the ground’s turf makes it more robust and a good soil stabilizer, especially for sloping ground.

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They both love full sun, but in drought, St Augustine may suffer long term damage more quickly than Zoysia grass. While they have about the same watering requirements, if these grasses go too long without water, Zoysia will start to brown and go into dormancy after about a week, while St Augustine will remain green longer, for up to two weeks.

Unlike the Zoysia, when St Augustine browns from drought, the plant has died and won’t be able to revive. Zoysia will green back up when it starts to receive water again. 

How Does St Augustine Get Into My Yard?

St Augustine is a top choice for some lawns, but its aboveground stolon system makes it more susceptible to wear and tear, whereas Zoysia’s strong turf system can withstand more foot traffic. 

If you’re active in your yard, you may have chosen to have a Zoysia lawn, but if your neighbor likes to keep their lawn reserved for light use, they may have chosen a St Augustine. Spreading from one property to another is how St Augustine can get into a Zoysia yard: the grass doesn’t reproduce by seeds, only by extension (unlike Zoysia, which is self-seeding).

The stolons of St Augustine are very aggressive, and if they work their way into a Zoysia lawn, they can grow on top of the Zoysia and crowd it out. It also spreads much more quickly than Zoysia, so it can overtake an area before the Zoysia grass is able to compete. Over some time, without giving the problem attention, St Augustine will crowd out a Zoysia lawn. 

What Is The Easiest Way To Remove St. Augustine Grass?

Getting grassy weeds out of your yard can be difficult once they’re established, but there is an effective strategy to get St Augustine out of a Zoysia lawn. Since St Augustine grass doesn’t like to be cut shorter than 4 inches, and Zoysia loves to be cut short, mowing the grass low and often will stress and damage the St Augustine grass. 

Cutting the mixed lawn to one inch for a few weeks, multiple times a week, will keep the grass short and get rid of the St Augustine. A reel mower can be used for mowing this frequently to keep fuel use down.

While the grass is cut low, pulling any St Augustine nodes and stolons that you find will help not only damage the grass but physically remove it from the yard. Although St Augustine can’t be planted by seed, it can be grown from individual stolon plugs, so making sure to identify and remove as many nodes as you see is important for a long-term effect.

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Although you should pull as many as you can by hand, a non-selective herbicide or a selective herbicide that isn’t safe for St Augustine, like 24D, can be used to spot apply where you find St Augustine nodes within the Zoysia. Be careful not to overapply the herbicide: the Zoysia should recover, but it may be slow to do so. Once the St Augustine is out of the way, Zoysia will fill in the space.

How Do You Keep St Augustine Out For Good?

There are a few ways you can keep St Augustine out of your Zoysia yard:

  • Landscape border – Since St Augustine spreads by stolons from adjacent grass, the best way to keep it out of the yard is with a physical border that the grass can’t penetrate. A solid barrier of brick or stone that is four to six inches high along the neighboring St Augustine yard will keep it from reaching into your lawn.
  • Prevent winter weeds – Zoysia is a warm season grass, and it goes dormant when temperatures dip into the 60s and below. Although it is normally a grass that provides tight coverage, weeds may show up in cool seasons that may linger into the summer, like fescue, crabgrass, or clover. Even if you kill these weeds, they will leave the space open that they used to occupy, and if it’s near a source of St Augustine, the grass will gladly fill in those spots.
  • Dense Zoysia coverage – the first line of defense against weeds is a dense lawn in the first place. Since Zoysia is such a tight growing turf grass, encouraging the lawn to remain in its healthiest condition will help it prevent weeds from showing up and resist the spread of any that do poke through.

How Do You Kill Other Grasses In Zoysia?

Spraying Lawn Weeds

Zoysia and Bermuda both like to be cut very low (known as scalping the lawn), but other types of grass like to remain at 2.5 inches or more in height. Aside from cutting low to stress the grass, some herbicide products are safe for Zoysia but not safe for others, in which case, they can be used to get rid of the unwanted growth.

Be sure, however, to not overapply any herbicide so that your Zoysia doesn’t get damaged from eradicating the grassy weeds.