In the world of turfgrass, there are only two main grass types, cool-season grasses, and warm-season grasses. Both require photosynthesis to create their own food. Both perform differently in different heat situations, with cool-season grasses performing better in cool weather and warm-season grasses performing better in warm or hot weather.

But it isn’t enough to simply recognize this simple difference. To best care for each species of grass, it is important to know why each one is different and how they grow and produce carbohydrates through photosynthesis due to those differences. It’s far more important than most people realize.

 How Things Appear on the Surface

Cool-season grasses include species such as Kentucky Bluegrass, Tall Fescue, Creeping Bentgrass, and Ryegrasses to name a few. These emerge from dormancy quickly in the spring, grow rapidly, and slow down in late spring and summer. Growth is slowed or stopped by mid to late summer, especially if no irrigation is given. In the cooler weather of fall, growth picks up again but at a more moderate pace. Cool-season grasses can stay somewhat green all winter even though all growth has ceased.

Warm-season grass like Bermudagrass, zoysiagrass, centipedegrass, St. Augustinegrass, and others emerge from dormancy slowly as soil temps rise above 50 degrees, but full growth rate is not experienced until mid-summer. Growth rate slows in the fall and dormancy begins as temps fall below 50 or with the first heavy frost. Warm season grasses lose all of their chlorophyll in dormancy and will not green up again until it breaks dormancy in spring. In the deep south, some species of tropical grass will remain green all winter if the temps do not drop low enough to send the grass into dormancy.

Each one has advantages and disadvantages. But below are the primary factors that shape why they grow this way and how to care for them. It is all centered around how they carry out photosynthesis.

The process of photosynthesis is carried out through a process of photosynthetic pathways. It sounds confusing, but it will become clear in a bit. The pathways have to do with the carbon compounds around which photosynthesis takes place. These two primary pathways are called the C3 and C4 carbon compound pathways.

C3 will always refer to the cool-season grasses and C4 always represent warm season grasses. There is no crossover. Therefore, scientists, when speaking of these grasses to other scientists or those educated in turf science will sometimes refer to these grasses as C3 and C4 grasses.

 Understanding C3 Grasses

Cool Season Grasses, or C3 grasses, are great at fixing CO2 at cooler temperatures. These are the temps they grow best in. However, at higher temperatures, above 90 degrees they are not as efficient. C3 grasses at higher temperatures have a hard time distinguishing between CO2 and O2. Remarkably, C3 grass can also catalyze the fixation of O2 which it does on equal or greater portions as CO2 in hot weather. When O2 is fixed it is called Photorespiration, and the result is lowered carbohydrate production.

When Photorespiration occurs the grass slows in growth and vigor and without water, the grass will often go dormant. It simply doesn’t have the energy to keep going.

A lot of people just don’t understand what is happening to their grass. They will assume the grass just needs more fertilizer and water. They will often over-fertilize with a high nitrogen mix in hopes of greening up the grass and restarting growth as in the spring. The result will often damage or kill the grass. This is because the grass simply cannot produce enough carbohydrates to match the nitrogen uptake and maintain growth. The fertilization is often accompanied by a lot of water or irrigation and for a while, the grass looks good. However, if the hot weather remains the victory is often short-lived. By adding too much nitrogen fertilizer, the nitrogen pushes the grass to grow at a time when it can’t handle it, thereby using stored nutrients in the roots since it cannot make enough carbohydrates to match the growth. As summer stress sets back in, it doesn’t have the reserves to draw on, and thinning or dieback often occurs. (Learn more about Organic Nitrogen in the Soil)

Therefore, it is good to remember that no matter how much nitrogen you apply in summer, C3’s will never overcome the grasses’ inability to produce the carbohydrates needed to grow in high heat. This is where experience comes in. It is possible to have a cool-season grass that is green all year, but the professional turf manager understands the balance between how much fertilizer to apply and how much irrigation and when not to push the grass. It takes a lot more irrigation for C3 to stay green in hot weather.

For general maintenance, C3 grasses can be fertilized by using low nitrogen or organic fertilizer including Turf Formula. The low nitrogen, slow release fertilizer will feed the plants what it needs, along with sufficient moisture, without pushing excessive growth that will damage the grass.

 Understanding C4 Grasses

Warm Season Grasses, or C4 grasses, are more efficient at CO2 fixation in high temperatures. This is because C4 grasses use a different enzyme than C3 and attach the CO2 to a different compound making up the 4 carbon compound. While it takes more energy for C4 to produce carbohydrates than C3, due to the extra steps involved, it does it without photorespiration and the end result is far greater carbohydrate production. 

As a result of the more efficient carbohydrate production, C4 grasses, depending on the grass variety and nitrogen needs, can be fertilized to maintain maximum growth in mid-summer or high summer heat. This will keep warm-season grasses growing through the summer months as long as water is available. If it is a low nitrogen grass, be careful that you don’t over-fertilize. Some grasses, such as Zoysia, are low N grasses and do not need more than 2 lbs of N per 1,000 sq ft/year.

 Conclusions

C3 grasses are best grown in cooler environments. They are more efficient at fixing CO2 in cooler weather than C4 grasses. In addition, C3 grasses have a higher photosynthetic rate in shady conditions. This makes them more suitable for highly shaded, low light sites compared to their C4 relatives. (Learn more about the Science of Shade Grass Management Here) 

C4 grasses have a specialized photosynthetic process that focuses on CO2 in high heat. When C3 grasses are struggling to stay green, C4 grass is in its fastest growth of the year. C4 grasses in hot conditions have an increased Nitrogen Use Efficiency (NUE) and Water Use Efficiency (WUE). They are able to increase root mass and biomass in hot conditions when their cousins are starting to shut down.

 Special Situations

If you have turf-type tall fescue growing together with bermudagrass, you will have to make a decision on which you will maintain in summer. Bermudagrass requires up to 6 lbs N per 1,000 sq ft/year, double what fescue requires with much of it applied in the warmer months. This may damage or invite diseases in the fescue. If you hold back and do not fertilize in summer the bermudagrass can likely go dormant, especially with low moisture, due to not having the nitrogen to keep growing.

How to decide? Do you prefer the bermudagrass and overseed with fescue for wintergreen? Then fertilize through the summer. You can always add seed in the fall.  If you prefer the turf-type tall fescue, then do not fertilize in summer. It may take several years for the bermudagrass to die back and the fescue to dominate. 

To help the fescue dominate quickly, overseed in fall so it grows in thick by spring. Mow the grass at 2 -3 inches in spring before fescue green-up. The bermudagrass remains low since it won’t start growing until later in the year. Then set the blade at the mower’s highest height to shade the bermudagrass and weaken the grass as it is trying to emerge. Bermudagrass cannot grow in shade.

In contrast, if you prefer the bermudagrass, mow the grass low just before bermudagrass emergence from dormancy so the bermudagrass has plenty of sunlight. Fertilize and irrigate as needed to ensure maximum growth.

 How Can We Help?

Turf Formula® is a fantastic product that helps keep the grass functioning even in extreme or adverse conditions. University studies confirm Turf Formula®’s ability to increase fluid and nutrient uptake, provide more efficient photosynthesis, and lower sodium saturation in soils. Sodium competes with Potassium uptake. Potassium is essential for many plant functions, but stress relief in plants is what it is best known for.

Turf Formula® increases the plant available nutrients naturally. In studies at the University of Missouri/Columbia, naturally occurring beneficial soil microorganisms were increased by 3400% in 24 hours and by 5000% in 72 hours. In these trials, the samples included Super-Cal Calcium in with the Turf Formula®. These beneficial microbes perform a wide range of functions, including soil elements’ break down into nutrients the plants can use and to reduce disease pathogens.

Also, Mycorrhizae Fungi are significantly increased in the soil. Mycorrhizae are tiny organisms that colonize the plant roots forming a symbiotic relationship. These fungi draw nutrients and moisture back to the plant in exchange for small amounts of plant sugar. They can form long strands that act to extend the root’s reach. Plants in some parts of the country survive only because the Mycorrhizae are doing most of the work. All outdoor plants need Mycorrhizae to live and are naturally part of healthy soils. With healthy Mycorrhizae populations on root structures, plants can reach more nutrients than bare roots can achieve.

Russ James
AgriGro Turf Specialist

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Sources:
Lawn Care Academy, Russ James, Photosynthesis of Turf Grasses
Fundamentals of Turfgrass Maintenance, Photosynthetic Pathways Page 11, Dr. Nick Christians