Aiken has two growing seasons for turfgrass. We grow warm season grasses like centipede, St. Augustine, Bermuda and zoysia from late April until the first frost. A first frost can come as early as the first part of October or it may not show up until mid-December. Our current temperatures point toward a later than average first frost. Cool season grasses like bluegrass, fescue or rye grow between late October until mid to late April. They add a nice green color to a dull landscape but are they worth planting? Do you like to mow, irrigate, and fertilize during the winter? Is it appropriate to overseed your lawn for the winter? These are just a few questions that you need to ask yourself before you overseed your turf.
Overseeding is defined as seeding onto an existing turf, usually with a temporary cool-season turfgrass (i.e. annual or perennial ryegrass), to provide green active grass growth during dormancy of the warm season turfgrass. It is used extensively on sports fields and golf courses, and to some extent, on commercial sites and home lawns. Sports field managers and golf course superintendents overseed their turfgrasses primarily to offset the excessive traffic during winter play as well as to have a green, quality turf. While this works, there are negative effects to overseeding. Competition between the cool and warm season grasses can be great, especially in the early spring when the warm season turf is trying to re-grow after winter dormancy-often referred to as ‘spring transition’. If the spring is cool and wet it will favor the persistence of the over-seeded grass at the expense of the re-growth of the warm season grass. Improved turf-type annual ryegrasses typically have a better spring transition than do the over seeded perennial ryegrasses. In years that favor continued persistence of the overseeding, there can be significant damage to the Bermuda turf. Another big negative with overseeding is if the existing turfgrass should be “scalped down” to provide a seedbed to favor a quick fall transition to the overseeding turfgrass. This scalping, along with the fall competition from the cool season grass prevents the warm season turfgrass from being able to store the necessary carbohydrates in the fall months. This means the turfgrass is going into winter dormancy in a weaker condition, with less stored reserves to recover well the following spring. If you have a great deal of traffic during the winter period at your site, then overseeding may be appropriate.
Warm season grasses such as Bermuda and zoysia go dormant during the winter months and can be over seeded with a cool season grass variety to maintain green color and adequate quality. This does not include St. Augustine or centipede grass which does not go completely dormant and may suffer significant damage when over-seeded! Bermuda and zoysia grasses are rhizomatus, which means that their roots are mainly underground and spread horizontally. Being underground, the roots are protected from cold temperatures as low as 15-30 degrees below zero. Centipede and St. Augustine lawns are stoloniferous. This means that stolons grow above ground and are in constant contact with ambient temperatures. The stolons are similar to very thin stems, branches or shoots. A stolon has nodes that produce the roots that start above ground and find their way into a receptive soil. Because of this configuration, centipede and St. Augustine respond to our warm days in December, January and February and try to put out new roots. As is normal during these months, warm days can be followed quickly by cold snaps and those new roots can be easily killed. 2009 and 2010 had some severe weather that caused a lot of winterkill to centipede and St. Augustine lawns.
OK, you have heard my arguments against overseeding but you are going to do it anyway. I’ll help you as long as it’s just for Bermuda or zoysia. The best time to overseed the home lawn is mid to late October and early November, but more accurately after the first frost. Annual ryegrass is the fastest germinating variety and probably the cheapest. It looks very similar to perennial ryegrass with a dark green color and shiny leaves. Annual ryegrass grows quickly and requires frequent mowing (around 2 to 2.5 inch height) especially during late fall and early spring. Water requirements are moderate and fertility requirements are low – maybe one to two pounds of nitrogen over the winter months. For a dense stand of rye grass, overseed at a rate of about 10-12 pounds of seed per thousand square feet. And keep the lawn lightly irrigated for several weeks to ensure germination.
Clemson recommends core aerating the lawn 30 days prior to overseeding if possible. For some homeowners, this is not possible or too hard to do. If so, right before you overseed, scalp the lawn down as low as your mower will go. This will help get the seed down in contact with the soil, which is very important. Fertilize over-seeded sites with a complete fertilizer such as 16-4-8. Fertilizer can be applied prior to or right after seeding but to avoid competition between the warm and cool season grasses, wait until after the first frost. After seedlings emerge, light applications of nitrogen will help produce a dense, healthy stand of grass. As soon as the seed is planted, start watering. Water lightly a couple of times per day until the seed start to germinate and grow. This watering process is not the same as planting a new Bermuda lawn where you water two or three times daily until the seed sprouts. If you water rye seed too much, it will rot.
One final yet important argument against over-seeding. You cannot put down a pre-emergence herbicide to attack winter weeds when needed and then over-seed. The pre-emergence herbicide will treat the grass seed the same as a weed seed and kill the new lawn before it starts. So now you have a new lawn coming in along with poa annua, and wild garlic and onion to compete with it. I love brown lawns in the winter. It gives me so much more to look forward to in the spring.
Now is the time to put down a pre-emergence herbicide for winter weeds. This will be a dry granular product such as prodiamine, pendimethalin, dithiopyr, isoxaben or several others that must be watered in. A second application for late blooming weeds may be applied toward the end of December.
The “Meet A Master Gardener” team will be at the Aiken Farmers Market on Saturday, September 3, 2011 from 8am to noon to answer your gardening questions. The team will also present a special demonstration on dividing perennials. You will learn to divide a variety of perennials, including day lilies, bearded iris, bee balm, liriope and salvia. Master Gardeners are busy planning an educational event to be held at the Aiken Farmers Market on October 1, 2011. The morning will feature 3 speakers about fall gardening and lawn care, a variety of timely gardening displays and demonstrations, and door prizes. Look for more information in next month’s article.