How do turf managers keep bermudagrass fields green all year long? Is there something in the water that allows the fields to stay green? Is it magic? Well, maybe, but in reality the majority of turf managers are overseeding their bermudagrass fields with a cool-season grass to keep them green when the bermudagrass goes dormant.
After the first couple of frosts bermudagrass turns brown or goes dormant. Brown grass is an unsightly appearance for users, so each fall season the decision is made to overseed sports fields. In the Transition Zone we overseed from September through November and sometimes continue to reseed in December/January. In other zones seeding is done at different times depending on temperatures and schedules. Overseeding can be simple or complex depending on the level of maintenance required.
The simplest method to overseeding warm-season grasses is to mow low, spread the seed, drag in, and water. Mowing the grass a little lower than you typically do will allow the cool-season grass to grow higher than the warm-season grass. Essentially the warm-season grass is slowing down in growth to harden up and prepare for the winter. The cool-season grass will grow higher than the dormant short cut grass giving you green instead of brown dormant grass showing. You will want to be careful not to scalp the bermudagrass too low when you lower the mowing height. Scalping the grass will shock the plants. The seed can be dragged in with a metal drag or cocoa mat. This helps to work the seed down through the canopy to reach the soil. The last step is watering in the seed. Short frequent cycles of irrigation will help the seed to germinate more quickly.
A more complex method to overseeding is to vertically mow, then horizontally mow lower, followed by spraying fields with a plant growth regulator, spreading the seed, topdressing, dragging in, applying a starter fertilizer, and watering. This method requires more time/effort and resources to accomplish.
First vertically mow your field. For some bermudagrasses that are more aggressive, like Patriot, this may require going over the field twice or more in different directions. Vertically mowing will cut down through the thatch and thick grass canopy getting to the soil. The height setting on your vertical mower will depend on how aggressive you want to be cutting into your field. In some cases where the field is prone to overuse, like soccer goalmouth areas, you may want to be less aggressive by setting the verticutter to a shallower depth. Being less aggressive will help to protect these areas from wearing out from the high use.
This is followed by mowing the grass a little lower than what you are currently mowing, which will allow the cool season grass to grow higher than the bermudagrass. After mowing the grass lower, spray the field using a plant growth regulator (PGR) at proper chemical label rates. Spraying the field with a PGR will assure that the warm-season grass will not flush back growth that will result in a dormant/brown showing through the winter. In the Transition Zone, we receive some warm days in October that allow the bermudagrass to continue to grow. PGRs help to slow this growth down.
The next step is seeding, which is followed by topdressing with sand. The sand helps to cover the seed and fill in the vertical mowing cuts. After topdressing with sand you can drag the sand in with a mat or brush. Dragging will help to work the sand down in the bermudagrass.
Next you put out a starter fertilizer. This will be a fertilizer with a high percentage of phosphorus, such as 18-24-12 or a complete fertilizer such as 17-17-17. Phosphorus helps new plant growth. Some turf managers put this out soon after topdressing thinking that the fertilizer will be broken down and taken up by the plants as soon as the seed starts to germinate. Others wait to fertilize after the seed has germinated. Waiting until the seed has germinated makes certain that the new seedlings, not just the bermudagrass are able to take up the fertilizer. This process, just as before, is completed with watering. Short frequent cycles help to keep the soil moist to encourage the seed establishment process.
Either one of the methods, simple or complex, will work well when it comes to overseeding bermudagrass with a cool-season grass. You can choose to use some of the processes or all of the processes when overseeding. For example if you are in the middle of a soccer season and there is not much time between practices/games on the field you may just put the seed out and have the players cleat the seed into the ground. Do not aerate or aerate your field before overseeding. The seed will germinate in the aeration holes rather than having uniform coverage across the field. The goal is to get good seed to soil contact and plenty of water on the seed afterwards to successfully overseed a field. One guarantee for failure is never actually putting the seed out.
The most common type of cool-season grass seed used in the overseeding process is ryegrass, both annual and perennial. Ryegrass has the ability to germinate within 5-7 days. Annual ryegrass will germinate in 3-5 days, if conditions are right; it has a lime green color, which is lighter green than most other grasses. Perennial ryegrass will germinate in 5-7 days with correct conditions and has a darker green color, and is most often used for overseeding fields.
In recent years, some managers have been experimenting with overseeding both annual ryegrass and perennial ryegrass on the same field. For example on a football field, alternating grasses every 5 yards to give a light green (annual ryegrass) versus dark green (perennial ryegrass) appearance on the fields. The drawback to this method is that using both types of grasses on one field makes managing more complex, because there are 3 different types of grasses on the field.
Many managers use different seeding rates depending on a number of variables including expected usage and budget. If high use of the field is expected and the budget is large, then a higher rate may be used. Normal rates for overseeding are between 8-10 lbs. per 1000 sq ft. For example, for a football field 57,600 sq ft would require between 460 lbs. and 576 lbs. of seed. To help offset usage and costs, rates can be higher in more heavily trafficked areas rather than seeding the entire field at a higher rate. On a football field this can be the area in between the hashes, in soccer the goal box areas, and for baseball the infield would require more seed than the outfield. Design your plan to fit your fields and meet the expectations of users and administrators.
Using higher than normal overseed rates can create immature overseeded grass. The maximum rate is between 30-40 lbs. and sometimes even up to 50 lbs. per 1000 sq ft. For a football field this is between 1728 lbs.-2304 lbs. and 2880 lbs. Usually when higher than normal rates are used, managers are seeding multiple times throughout the season. This helps to create a seed bank where seed will germinate throughout the season. The biggest concerns with higher rates are the extra cost and creating competition for bermudagrass come spring green up.
Using lower rates of overseeded grass can still provide good winter green color. The minimum rate for a field should be around 5 lbs. per 1000 sq ft. For a football field you will need 288 lbs. of seed. Anything less than 5 lbs. per 1000 sq ft. creates a thin stand of overseeded grass that is too thin to properly cover the dormant bermudagrass. Using a minimum rate is cost effective and creates less competition come spring green up for the bermudagrass. Just be sure that your rate is not so minimal that it does not meet the expectations of users.
When seeding, there are two types of spreaders most commonly used to get the seed out: a rotary spreader and a drop spreader. A rotary spreader is quicker and can cover more square footage. A drop spreader is used for going along edges or trimming out a field and/or can also be used for seeding an entire field. The user just needs to be careful to get full cover with both types by using a checkerboard pattern, which is created by going one direction, for example east to west on a field, and then spreading perpendicular to this direction, going north to south. The applicator should split the seed application rate for each direction. Using a checkerboard pattern will ensure good uniform coverage of the field.
Alternatives to keep green
There are a couple of alternatives to overseeding a bermudagrass field: painting the fields green or using grow covers to help protect the turf. Not overseeding will make spring green up of the bermudagrass and transitioning of the field a lot easier.
Painting the field green can be time-consuming and a costly process. There are a number of turf colorant products on the market that can be used depending on the required green color needed. The painting process requires multiple applications that should begin before the bermudagrass goes dormant. Each time painting the manager should go in multiple directions to cover both sides of the grass blades. Depending on the weather painting the field could take 4 to 5 or more applications per winter. Challenges include having to repaint field markings, upsetting equipment managers (green paint tends to stick to white uniforms and is difficult to remove), and the wearing out of seals on your spray machine’s centrifugal pumps.
Grow covers can be used to keep the bermudagrass from going completely dormant. The bermudagrass will retain its green color, but will not have normal plant growth like it does during the growing season. The area covered will not be able to recover from damage, but will stay green. Over extended periods of time the crown of the plant will start to separate from the soil, acting as if the grow cover is the top layer of the soil. The field will become puffy, like walking on a sponge. Using covers requires a lot of work to pull them on and off the field. Make sure the covers have complete coverage of the field or there will be dormant spots anywhere not covered.
When it comes to overseeding there are just as many downsides as there are upsides. Choose what works for you and your fields. Work within your budget to meet the expectations of users. If you choose to overseed get ready to mow all year long. Be creative with your mowing patterns to make your field look great!
Scott Stevens, CSFM, is the sports turf manager at Elon University, Elon, NC. He is currently a member of the STMA Editorial Committee.
Essentials for all year green
Timing: choose a time in the schedule when there is at least a week’s time of little to no use on the field to give the seed time to germinate
Seed to soil contact: mow lower, drag in, verticut, and topdress to get the seed down into the soil
Strategic spreading: go a little heavier with seeding rates in high wear areas on the field
Light, frequent irrigation: keep the soil moist
Germination: proper temperatures, oxygen, water, and sometimes light are required for successful germination
Get the seed out: seed sitting in a bag of your shelf will not help keep your field green