Get to the root of it

Healthy, green turfgrass has as much to do with what happens below ground as what happens above it.

At the GCSAA education conference at the Golf Industry Show, Keith Karnok, Ph.D., a professor in the department of crop and soil sciences at the University of Georgia, conducted a seminar about managing root systems in the North and discussed ways to improve root systems. Altering cultural practices, such as aerification, fertility and irrigation, can help improve root zones.

To start with, there’s a lot of genetic potential in a root system, but temperature and cultural practices inhibit that potential. What happens to a plant above ground is significantly affected by the root system. Roots have five main functions:
1. Uptake water
2. Uptake nutrients
3. Anchor the plant
4. Store carbohydrates
5. Produce hormones.

However, not all root systems are the same. The size and extent of a root system varies with turfgrasses, primarily because of genetics. Some turfgrasses are more sensitive to cultural practices – such as pesticide applications, mowing, irrigation, fertility and cultivation – than others.

Temperature
Soil temperature dictates what’s going to happen with the above-ground part of the plant. Soil temperature effects root growth, and it’s important for superintendents to take advantage of temperatures during the year, Karnok says. The optimal growth temperatures for root growth of cool-season grasses is between 50 F and 65 F. For shoot growth, the optimal temperature is between 60 F and 75 F. Root-zone temperatures are higher in the fall than they are in the spring. During these two optimum times (spring and fall), superintendents need aerify and put down phosphorus and potassium, according to Karnok.

Air temperature has a minimal effect on root systems.

Karnok says it’s a good idea for superintendents to take soil temperatures regularly, and the best way to do that is by taking the temperature the same time every day and at a four-inch depth. Taking the temperature closer to the ground’s surface isn’t a good idea because there’s a lot of temperature fluctuation there.

Aeration
Because roots grow from the tips, cutting them stimulates growth. It’s a hormonal effect. The affects of aeration help stimulate that growth because the hollow tines punctured into the ground disrupt root tips. The spring and fall are the two peak times during the year when roots grow the most (because of temperature), hence, those are best times to aerate to optimize root growth, assuming the right fertility practices are in place.

According to the U.S. Golf Association, 15 to 20 percent of the surface area should be impacted each year – for mature greens. That area is determined by tine size, spacing and frequency. For example, using a one-half-inch tine versus a one-quarter-inch tine increases surface area affected four times. Using a five-eights-inch-diameter tine at a 2.5-inch-by-2.5-inch spacing impacts 5.33 percent of the surface area. A one-half-inch-diameter tine at a 1.25-inch-by-1.25-inch spacing impacts 13.64 percent of the surface area.

In addition to aerifying, hydrojecting is a good supplemental practice, but it’s not enough according to Karnok. He says superintendents generally can’t aerify enough, but playability prevents them from doing it as often as they should.

When topdressing after aerifying, Karnok emphasized the need to get sand down into the holes because it breaks up organic matter caused by the plant. That’s why larger holes are better than smaller ones. Enough topdressing should be applied to keep the macropores open. If not enough aerifying is done, a heavy thatch layer develops, keeping oxygen from getting to the root zone. In the top two inches of the soil, the organic matter content should be 3 percent or less. Five percent or more is bad.

Fertility and irrigation
Fertility programs also affects root systems. Too much nitrogen has a negative affect on root growth, but the use of plant growth regulators might have a positive effect in that root-growth decline. The best time to fertilize is before and after the two root-growing peaks in the spring and fall. Also, the fertility programs should include some nitrate, especially in the summer, Karnok says.

Water also is important for root growth. When irrigating, Karnok suggests superintendents measure the amount of water applied more scientifically, such as with a soil probe or calculating the amount of water applied (in millimeters or inches), instead of measuring it by time (minutes).

“You need to make sure you’re not just watering the top of the grass,” Karnok says. “You need to make sure the soil is getting water. You need to go one inch below the majority of the roots.” GCN