Overseeding helps maintain lawn’s lush, green color

: My lawn has not been overseeded for winter. I assume that your recent advice about overseeding with fescues only relates to the lawn in summer. In my case, I assume I should overseed in the fall with Palmer or Prelude as you mentioned.

What is a power rake? Or, does that mean just vigorously raking to disturb the top of the lawn? Also, what time in fall is ideal for overseeding with winter rye? How should I coordinate fertilizing with the planting? When do I need to aerate the lawn?

A: I hope I did not cause some confusion. Overseeding is applying seed to an existing lawn. Never overseed any lawn in the summer. I seldom say never but in this case I cannot think of a good reason you would ever apply grass seed to an existing lawn when it is hot outside.

Overseeding is done to help fill in areas in tall fescue lawns and other cool-season grasses that were damaged during the summer months. Or, it is applied to Bermuda grass lawns to maintain a green winter lawn. Otherwise, the warm-season Bermuda grass will turn brown in the late fall and stay brown through the winter months.

Overseeding in midfall establishes a temporary winter lawn, nearly always some sort of rye grass, into an existing warm-season lawn like Bermuda grass. In the spring, Bermuda grass grows again and begins to “choke out” the overseeded grasses or they die due to the heat. The overseeded grasses usually persist until about May.

All cool-season grasses, like fescues and ryes, have to be seeded or used in overseeding during cool weather or the seed will not germinate. Overseeding is usually done from mid to late September to mid-October. I would not use a tall fescue for overseeding Bermuda grass. I would use a rye grass, and perennial rye grasses are better looking than annual rye grasses.

Fertilizing is done at the time of overseeding.

I am assuming you are overseeding a Bermuda grass lawn but you did not tell me.

There are two major concepts you need to understand for a successful overseeding. First is the concept that you need to reduce competition from the existing grass before seeding. You do this by mowing shorter (reducing the shade on the ground by existing grass) and thinning out some of the existing lawn (this is by power raking, also called dethatching).

Dethatching can be done with a machine or manually with a dethatching rake or a lawn rake if a dethatching rake isn’t available. Dethatching a lawn in any form is quite damaging since it removes living grass by ripping plants out. The lawn needs time to recover from this type of damage but it does open up the lawn for better seed germination.

Of course it also opens the lawn up to weed seed germination as well. There are many more weed seeds to worry about that germinate in the spring rather than the fall so try to avoid power raking in the spring. Most equipment rental places have gasoline-powered power rakes for rent. They also may call them dethatchers or even vertical mowers.

The second major concept to understand is that good seed germination requires that the seed be in contact with the soil. The seed must rest on the soil surface, not on top of a bunch of dead grass lying on top of soil. So by power raking or dethatching, you remove this layer of dead grass, called thatch, and improve seed contact with the soil. When you are done dethatching, you should be able to see the soil surface through the grass.

Aeration has little to do with overseeding. Aeration, or punching holes into the ground, doesn’t disturb the grass very much so it can be done any time of the year. But it is best done at least a month before hot weather arrives. Aeration helps water and air movement into the soil and allows for deeper rooting of turf grass.

Deeper rooting of most plants means better resistance to heat and drought. As water and air are allowed to move deeper, plant roots will follow.

To stimulate rooting, I like to suggest an application of a high-phosphorus fertilizer right after aeration. Phosphorus stimulates root growth.

If your lawn needs some nitrogen too, then apply something like a fertilizer with the numbers 16-20-0. If the lawn is a nice deep green color and does not need more nitrogen, then just apply something like triple super phosphate or 0-46-0 and avoid excessively fertilizing your lawn with nitrogen. Excessive nitrogen can weaken lawns.

Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at morrisr@unce.unr.edu.