Cool-season Grass vs Warm-season Grass

Lawn owners often hear about two types of grass: Warm-season grass and Cool-season grass. But most of them don't know the differences between them, how they grow, which temperature is suitable for them, and when to choose what.

We are here with a detailed comparison of cool-season grass vs. warm-season grass. This comprehensive guide will equip you with everything you should know about these grasses.

Whether you want to select the right grass for your lawn or learn how to ensure optimal lawn care for these grasses, this guide will help you out and resolve any confusion you have.

What are Cool-season and Warm-season Grasses?

By reading the names of cool-season and warm-season grasses, the first idea that comes to mind is that these are grasses meant for different climate regions. That's undoubtedly true, but the question is "Why are they different?"

The primary reason is the difference in their metabolic engine. Both grass types go through different chemical pathways for photosynthesis.

Cool-season grasses are C3 plants that produce the 3-carbon first stable molecule during photosynthesis. They have a special enzyme called Rubisco, which is responsible for capturing carbon dioxide directly from the air and snapping it onto the already present 5-carbon sugar molecule, RuBP, in the grass. Now, the grass can use this solid carbon dioxide to turn it into food (glucose). The process happens anywhere in the middle of the leaf.

Warm-season grasses are C4 plants. The leaf is divided into two parts/rooms because they contain different types of cells. The outer part/ room, which consists of Mesophyll cells, contains PEP Carboxylase that grabs CO2 from the atmosphere. It fixes the CO2 into a 4-carbon acid, which is then moved to the inner room.

The inner room consists of Bundle Sheath Cells. It's where the actual food-making process happens. Firstly, the grass breaks down the acid and releases pure CO2 in the room. Then, the enzyme Rubisco grabs the CO2, which is available in abundance. Then, this CO2 is used to create glucose.

Cool and warm-season grass types have different carbon fixation efficiency. In C3, the Rubisco enzyme can't always differentiate between oxygen and CO2. When it grabs oxygen instead of CO2, it wastes the plant's energy. In contrast, C4 is extremely efficient. The CO2 fixation happens inside, where only CO2 is available. So, the Rubisco enzyme only grabs CO2.

Thus, both types of grasses differ in capturing CO2 and processing it for food production. And this is what affects their habitat, growth cycle, temperature tolerance, etc.

Cool-season Grass vs Warm-season Grass

We have explained the primary difference between warm and cool-season grasses. Now, let's understand how it affects different aspects.

Temperature

Cool-season grasses are meant for cool regions. They grow actively and thrive in low temperatures. The temperature should be between 15⁰C (60⁰F) and 24⁰C (75⁰F) for them to survive.

We have discussed how cool-season grasses grab CO2 from the atmosphere. They have to keep their pores wide open. But the adverse effect of opening pores is that the absorbed water evaporates quickly, which is why these grasses cannot survive in hot temperatures.

Cool-season grasses cannot handle heat stress. After 32⁰C, the C3 pathway begins to fail. Then, the plant starts to burn its stored energy to survive, which is called photorespiration. When there is too much heat, the grass enters a summer-dormant state, turning brown.

In winter, cool-season grasses stay green longer than warm-season grasses. They stay green even if there is frost or heavy snow. Even if they stop growing when the ground freezes, they remain green by maintaining their chlorophyll content.

Warm-season grasses require 27⁰C (80⁰F) to 35⁰C (95⁰F) for photosynthesis. We already know that these grasses go through an extra step to capture CO2 and fix it onto the 4-carbon acid. This extra step requires immense heat, which is why it cannot happen when the temperature is low.

When the temperature reaches 30⁰C, these grasses start to grow actively. They can handle high heat. And as they can keep their pores (stomata) closed because of the extra step in the CO2 fixation, they can prevent energy wastage even in high heat. That's why they remain healthy and green even when the temperature is high.

However, warm-season grasses are very sensitive to cold. When the temperature goes below 13⁰C, the grass stops producing chlorophyll. Which is why these grasses change their color from green to bleached tan or straw color near winter.

It doesn't mean the grass is dead. It enters a dormant state. All its nutrients are pulled downward into the roots, so when the soil warms up again, it wakes up from sleep and starts to grow.

Simply put, both grass types require a specific temperature range to survive. Without that, they cannot undergo photosynthesis.

Growth Habit

Most cool-season grasses grow in bunches. Which means they cannot grow outside the bunch and spread throughout the whole lawn. You can grow most of the cool-grasses from seeds. They germinate quickly and grow faster in bunches.

One downside of such grasses is that they cannot repair themselves. If any bunch dies, it creates a patch in the lawn, and it cannot be filled by the bunches around. You need to do manual overseeding. If you have any patches in your lawn, check out how to overseed lawn in spring.

Warm-season grasses are mostly creepers. They have above-ground and below-ground runners. The above-ground runners (stolons) allow them to go over the soil and grow roots downward in new locations. While the below-ground runners (Rhizomes) run underground and pop up from new spots in the soil.

Simply put, warm-season grasses can grow on their own and spread in the lawn. Even if there is a patch, these runners will help grasses recover on their own. You will see the patch green in a few weeks.

For growing warm-season grasses, you need to install them via sods or plugs. If you try to grow them from seeds, it will be a slow process, and often, it fails. Mostly, you have to install sods 12 inches apart to cover the whole lawn.

Growth Cycle

Both grasses behave differently as the temperature changes, which is why their growth cycles are also different.

Let's discuss cool-season grasses first.

In cool-season grasses, you will witness rapid vertical growth and root development in the spring season. From March to May, these grasses grow actively.

Mostly, lawn owners start with cleaning weeds from their lawn and applying a pre-emergent. Then, they add fertilizer to the lawn to boost the lawn's growth. Check out when to fertilize your lawn in spring.

You will notice excellent growth during the season, but May is the month for heavy growth. You need to mow more frequently.

When the temperature rises in June, these grasses start to enter summer dormancy. They do not grow in high temperatures. However, you still need to take care of your lawn through mowing higher, giving more water, and dethatching. By the end of August, you can start overseeding your lawn to prepare the grass for the next phase of growth.

Fall is the season for immense growth for cool-season grasses. It's a mini-growth season, but the grass grows rapidly as the temperature cools down. You can do lawn aeration in September if your soil is compact. Along with that, you can plant new seeds for better growth.

From October, these grasses prepare for winter. They store the energy they need before going dormant. You mow for the last time in November, and then you have to wait for spring to see the lush, green lawn again.

Now, let's see the growth cycle of warm-season grasses.

Warm-season grasses wake from their winter dormancy in spring. But it's not the time for their aggressive growth. They just turn green, grow slowly, and start preparing for the growth phase in the next season. Lawn owners get rid of thatch, apply pre-emergent, and give fertilizer to their lawns in spring.

Summer is the time when warm-season grasses grow aggressively. During the whole summer season, these grasses grow rapidly, so you need to mow more often. Along with that, deep watering is required.

Fall is the transition phase for warm-season grasses. Growth slows, and they begin preparing for winter dormancy.

For more details on the growth cycle and how to take care of your lawn year-round, check out our lawn care calendar.

Energy Efficiency

Both grasses follow a different photosynthesis process, so they also differ in energy consumption.

Cool-season grasses or C3 plants are more energy efficient in mild climates. They require less ATP as they do not involve an extra step to fix CO2. They take CO2 through the stomata and fix it in the acid.

On the other hand, warm-season grasses need to execute an extra step, which involves moving carbon from the first room to the second. It requires plant energy.

But in the summer, C3 becomes inefficient. The temperature rises, and it also increases the evaporation. Thus, some of the energy is wasted, and in this case, C4 consumes less energy.

Popular Varieties

Here are some cool-season grass varieties:

Kentucky Bluegrass: It's a grass known for its deep green color and soft texture. It feels extremely soft and cushiony. Unlike most cool-season grasses, it repairs bare spots itself as it has underground runners (rhizomes). However, it needs high maintenance, which includes fertilizer and frequent watering.

Tall Fescue: It is a common cool-season grass that can handle high foot traffic and heat. It's also known for its drought-tolerance because its roots go too deep in the ground and find water. Thus, you don't need to do excessive watering.

Perennial Ryegrass: People opt for this variety as it germinates faster than other grasses. If there is a patch, it can be repaired quickly, which is why it is often used with other grass types. It looks incredible when mowed in stripes.

Fine Fescue: It's the best cool-season grass for shade. It can survive even in 3-4 hours of dappled light. It has very thin and fine, needle-like blades. Once it covers your lawn, it looks like a fine carpet. It also requires less watering and fertilizer. However, it cannot bear high foot traffic.

Here are some Warm-season grass varieties:

Bermuda: It's often used for golf courses and sports fields. Common lawn owners can also plant these as they can bear high-foot traffic of children and pets. It grows through stolons and rhizomes, which is why it can quickly recover. If there is a bare spot, it will fill up quickly. Keep in mind that this grass goes dormant in winter for a long period (November to April).

Zoysia: This grass is known for the luxurious feel it offers. It's soft, thick, and comfy, which is why many residential lawn owners prefer to have it. It feels like an expensive rug and also bears foot traffic. It grows in high density, which prevents weed growth. The best part is that it also survives in winter and does not die. Moreover, it's also a moderate shade-tolerant grass.

St. Augustine Grass: It is the best option for lawns with shade. It requires only 4 hours of sunlight. It can also grow in salty air near coastal regions. It has thick and coarse blades that look distinct. It is very dense and can quickly fill up your lawn, which also reduces weed growth.

Centipede Grass: It's also a moderate shade-tolerant grass. It grows slowly, so you have to mow it less frequently. Plus, it requires less fertilizer and watering. It can grow in soils with a pH of 5.0 to 6.0. It has a distinct yellowish-green color, like apple green. It does not get dark green even if you add more fertilizer. Moreover, it cannot handle high foot traffic.

Location and Climate

For the North, cool-season grasses are ideal. The reason is the climate. Usually, the temperature remains low. Even in the peak summers, the temperature remains in the range of 27⁰C-30⁰C (80⁰F-86⁰F).

This temperature is ideal for cool-season grasses. They can thrive well and save themselves from hot days that pose a risk of heat death.

Cool-season grasses can handle the cold of the North. They stay green longer in winter by storing energy.

But during the heatwave in the North, when the temperature goes over 30⁰C (86⁰F), these grasses struggle and stop growing.

If you live in the South, warm-season grasses are perfect. The temperature usually stays above 32⁰C (90⁰F), so cool-season grasses cannot survive here. Secondly, high temperature is best for C4 photosynthesis. It's efficient and quicker on hot days.

These grasses can handle high heat and stay green under scorching sun. However, these grasses change their color and go dormant early in winter. They cannot bear cold. That's the reason you cannot plant warm-season grasses in the North.

Now, the challenging part is the Transition zone. It's a special belt of around 200-300 miles where both grasses have to struggle.

In winter, it's too cold for warm-season grasses, and in summer, it's too hot for cool-season grasses. Thus, you have to opt for a specific grass that could work here, and you have limited options.

The first option is the Tall Fescue. It's a cool-season grass that can handle the summer of Transition zones. The second option is Zoysia or Bermuda. They are warm-season grasses that can bear slight cold.

Another option is mixing both grass types. But it doesn't work with every mix and every region. Usually, people opt for Bermuda to bear high temperatures, and they overseed with Ryegrass in the fall season. Thus, when Bermuda goes dormant in low temperatures, Ryegrass stays green. It ensures your lawn remains green for a longer period of the year.

Shade Tolerance

Generally, cool-season grasses are more shade-tolerant. The reason is their C3 biology, so they don't need high heat or immense solar energy to carry on the CO2 fixing and produce their food.

Some of the cool-season grass types, such as Fescue, need only 3-4 hours of sunlight to survive. They are capable of producing food under shade and low-light conditions. Even if there is no direct sunlight, they can thrive in dappled light.

Warm-season grasses, on the other hand, require more sunlight. The reason is that their C4 pathway needs more energy. That's the reason intense sunlight for a longer period is required. However, there are a few grass varieties, such as St. Augustine, which survive in shade.

But keep in mind, no matter what the grass type is, it still needs sunlight to survive. Some need less while some need more. Sunlight is essential for photosynthesis.

Check out our detailed guide on Best Grass for Shade.

Mowing

Cool-season grasses are usually mowed taller. They have a shallower root system, and they can't bear extreme heat. When they are tall, they reduce water evaporation and prevent the soil from getting too hot.

In contrast, warm-season grasses are kept shorter. They can bear heat, so you don't have to worry about soil heat. When you keep these grasses short, they spread quickly.

Now, coming to the frequency. You need to mow cool-season grasses twice a week in spring and fall. They grow actively in these seasons. However, in the summer, you need to mow once every two weeks.

Warm-season grasses need frequent mowing in the summer. You will notice significant growth every 48 hours. You will have to mow less frequently in spring and fall.

This isn't a hard-and-fast rule. Each grass variety has a specific height to maintain for optimal growth. You can refer to our grass cutting height chart. Secondly, you should never mow more than one-third of the grass. Otherwise, you will put extra stress on it.

If you want to keep your lawn healthy and green year-round, you need to pay attention to mowing. Mowing once a week on weekends doesn't work. You have to mow when it is needed and also follow the one-third rule.

The best way to maintain your lawn is to get a robotic lawn mower. It will help you mow your lawn at the right time. Once you set the schedule and desired mowing height, it does everything on its own. It's like putting mowing on automation. Thus, you never have to worry about overgrown grass, paying someone to mow your lawn, or wasting your weekends.

Watering

You might think that warm-season grasses might need more water on hot days. But in reality, they need less. The reason is that during C4 photosynthesis, the stomata or pores on the grass blade remain closed. Therefore, water cannot escape due to evaporation. The water you provide is absorbed and used in the food-making process. There is minimal or no wastage.

That's why warm-season grasses need low to moderate watering. You can water deeply once, and they utilize water more efficiently. The C4 photosynthesis requires 20 to 30% less water compared to cool-season grasses in high temperatures.

Warm-season grasses can also tolerate drought. Even if you don't water them for a long time, they might go dormant. But they will recover quickly when you water again.

In contrast, cool-season grasses need more water in high temperatures. To capture CO2 in the atmosphere, they keep their stomata open, which results in water loss due to evaporation. That's the reason it's advised to water them more in the summer. And if you don't do that, the grass will suffer water scarcity and might die instead of going dormant.

For more details, check out our detailed guide on how long to water grass.

Fertilization

Both grasses need fertilizer, but the timing differs.

Cool-season grasses are fertilized mainly in the fall. They grow aggressively, and they also need to prepare for the winter dormancy. The secondary fertilization phase is early spring. It helps your lawn recover after winter and become green again.

Warm-season grasses need fertilizer in summer. They grow actively in the season, so they need more nutrients. The secondary feeding time is late spring to support growth in the summer.

The difference is in the Nitrogen efficiency. Warm-season grasses are C3 plants, and they have an inefficient Rubisco process. They need more nitrogen to prepare new Rubisco enzymes and carry on photosynthesis. As a result, you need to add more Nitrogen-based fertilizer.

In contrast, warm-season grasses have an efficient mechanism. They capture the desired CO2 with less Rubisco, which is the reason they need less Nitrogen fertilizer.

FAQs

What does it mean to be a warm-season grass?

A warm-season grass is suitable for warm regions. It grows actively in high temperatures, which means the summer season. It's a C4 plant that works great in temperatures between 27⁰C (80⁰F) and 35⁰C (95⁰F).

Can you mix cool-season grass with warm-season grass?

It is generally not recommended to mix cool-season and warm-season grass. It will fill your lawn with patches and different colored grass. However, you can mix both if you live in the Transition zone, where none of the grass survives, and you need a green lawn year-round. Keep in mind that only a few mixes work well, such as mixing Perennial Ryegrass into Bermuda.

What is the best grass for hot summers and cold winters?

Tall Fescue is the best grass for hot summers and cold winters. It's a cool-season grass that handles heat and cold well. It's an excellent option for Transition Zones.

Which grass is best for full sun?

Bermuda and Zoysia are excellent for full sun. Bermuda is called the Sun Addict as it has intense direct sunlight. Zoysia also works, and it needs less watering.

What temperature is too hot for cool-season grass?

After 27⁰C (75⁰F), the growth begins to slow down. At 32⁰C (90⁰F), the grass burns more energy than it creates due to photorespiration. And 35⁰C (95⁰F) and above poses a risk of heat death.