Overseeding Zoysia with Kentucky Bluegrass is a poor choice. Zoysia is an aggressive grass that will choke out bluegrass, so your overseeding efforts will typically fail. Plus, most Zoysia requires a lower mowing height than Kentucky Bluegrass, so it is hard to keep both types of grass healthy at once. Bluegrass often struggles in high summer temperatures when Zoysia has its fastest growth. So, Zoysia can devastate your Bluegrass before fall and winter arrive, destroying your hopes of a green winter lawn. Instead, consider overseeding Zoysia with annual ryegrass in the fall. This will allow you to have a green lawn in winter while your Zoysia is dormant.
4 Reasons to Avoid Overseeding Zoysia with Kentucky Bluegrass
Although some lawns are established with a mix of Zoysia and Kentucky Bluegrass, these two grass types rarely coexist for long. Before you spend money on Kentucky Bluegrass seed, consider these risks:
Zoysia Will Crowd Out Your Bluegrass
If you spread Kentucky Bluegrass seed on a Zoysia lawn, the Zoysia will probably smother the Bluegrass quickly. Zoysia is considered an invasive grass in many regions specifically because it chokes out and kills other grasses. Even if Zoysia creeps into an established Bluegrass lawn, it is very likely to outcompete the Bluegrass. Bluegrass seedlings have even less chance of survival in an established Zoysia lawn.
- Zoysia is known for choking out other grasses.
- Bluegrass seedlings will struggle to compete with Zoysia.
- The Bluegrass you sprout from overseeding will usually be choked out within a few seasons.
In areas where mixed Bluegrass/Zoysia lawns are established, the most common outcome is that the Zoysia kills off the Bluegrass in a few years. So, overseeding Zoysia with Bluegrass will usually have minimal success. It may be a waste of money to try to mix these two types of grass.
They Thrive in Different Regions
There are few regions where Kentucky Bluegrass and Zoysia will both grow well. This is because Zoysia is a warm-season grass that grows extremely well in areas with hot summers and mild winters. Kentucky Bluegrass is the opposite. It is a cool-season grass that grows best in regions with mild summers and cold winters. If you live in an area that is good for growing Zoysia, it’s probably not good for Kentucky Bluegrass.
- Zoysia grows well in regions where Kentucky Bluegrass struggles.
- Kentucky Bluegrass grows best in regions where Zoysia performs poorly.
- If your area supports thick Zoysia growth, there is a low chance that overseeding Kentucky Bluegrass will be successful.
The regions where both Zoysia and Bluegrass can thrive are located in the transition zone in the United States. This is where cool-season grasses and warm-season grasses can both survive. However, even in this zone, there are differences in the local climate. If your established Zoysia lawn is growing well in your lawn, it’s unlikely that the Kentucky Bluegrass seed you spread will grow very well there.
Incompatible Mowing Height
Most varieties of Zoysia grow at their best when they are mowed to a height of 1.5 to 2 inches (4–5 cm). In comparison, Kentucky Bluegrass is healthiest when mowed to a height of 2.5 to 3.5 inches (6–9 cm). Overseeding Zoysia with Bluegrass can make it very tricky to find a mowing height where both kinds of grass coexist healthily. If you mow for one grass, you risk causing the other to have a poor appearance or ill health.
- Most Zoysia cultivars grow best when mowed at a height of 1.5–2 inches (4–5 cm).
- Kentucky Bluegrass should be mowed at a height of 2.5–3.5 inches (6–9 cm).
- Mowing Kentucky Bluegrass too short will cause it to struggle.
- Letting Zoysia grow too long can ruin its appearance.
- There are a few Zoysia varieties that grow well when mowed at taller heights.
There are some varieties of Zoysia that grow well when mowed as tall as 2.5 inches (6 cm), but these are not planted in every Zoysia lawn. Unless you are sure your Zoysia grass will remain healthy at the taller mowing height Kentucky Bluegrass requires, mixing the two is not a good idea.
Bluegrass Will Struggle in Summer
Like other cool-season grasses, Kentucky Bluegrass growth slows in the summer heat. In these same conditions, Zoysia grass experiences its fastest growth. This makes overseeding Zoysia lawns with Kentucky Bluegrass frustrating. If your Bluegrass grass seeds sprout, Zoysia can make up ground and choke them out in summer. Even if you overseed your Kentucky Bluegrass in the fall, it may not be able to establish itself well enough to resist the Zoysia summer growth spurt.
- Zoysia grass grows and spreads aggressively during the summer heat.
- Bluegrass growth slows significantly in the summer months.
- If you overseed Zoysia with Kentucky Bluegrass, the bluegrass is often choked out in summer.
In addition to Kentucky Bluegrass, other perennial cool-season grasses will be outcompeted by Zoysia during the summer months. This is one of the reasons why overseeding Zoysia with Fescue is so challenging. You will have to increase your lawn care workload in order to keep both these grass types happy.
Annual Ryegrass is a Better Choice
Instead of battling year-round to keep Kentucky Bluegrass from being choked out by Zoysia, it’s much easier and more effective to overseed your lawn with Annual Ryegrass each fall. As temperatures drop and Zoysia growth slows, spread Annual Ryegrass seed on your lawn. The seeds will sprout and green up your yard while the Zoysia goes dormant. This makes it easy to maintain a green lawn year-round.
- Instead of Kentucky Bluegrass, overseed with this Annual Ryegrass in the fall.
- Annual Ryegrass will sprout as the Zoysia goes dormant in cold weather.
- Ryegrass stays green through winter in most regions where Zoysia grows.
- Annual Ryegrass is lower-maintenance than Kentucky Bluegrass.
- Using Annual Ryegrass gets better results with less work.
The benefit of Annual Ryegrass is that it only survives for one season. It will sprout in fall, remain green through the winter in mild regions, and die off naturally as your Zoysia greens up in spring. Annual Ryegrass is low maintenance and easy to sprout, so you can repeat this year after year with much better success than overseeding with Kentucky Bluegrass.
Can You Mix Kentucky Bluegrass with Zoysia?
Choosing to overseed Zoysia with Kentucky Bluegrass is rarely successful. Here are the reasons why:
- Zoysia easily chokes out Kentucky Bluegrass.
- Kentucky Bluegrass and Zoysia grow best in different climates.
- Zoysia requires a shorter mowing height than Kentucky Bluegrass.
- The summer growth spurt of Zoysia usually kills off new Kentucky Bluegrass sprouts.
- Annual Ryegrass is an easier alternative for keeping Zoysia lawns green year-round.
It is very challenging to keep both Zoysia and Kentucky Bluegrass healthy on the same lawn for more than a few seasons. Rather than set yourself up for this battle, it’s best to choose a different option.