By: Chip Osborne, Dr. Thomas Green, Jodi Schmitz and Caitlin Seifert

Overseeding, or adding grass seed to established turf, can improve the health and attractiveness of lawns and the playability of athletic fields. Overseeding also promotes thick turf that can outcompete most perennial broadleaf weeds and eliminates bare spots where weeds can gain a foothold, reducing reliance on herbicides.  

Assess your turf first

The two most important variables to evaluate before overseeding are the number of grass plants growing in a square foot (density) and the level of soil compaction. No bare ground should be visible in turfgrass areas, although natural density will vary by grass type. Turf varieties with a lower density and coarser leaf texture can require a higher mowing height and more regular overseeding to produce better quality turf. Additionally, some varieties such as fescue do not produce runners, making periodic overseeding more necessary.  

Soil compaction is an ongoing challenge in athletic fields. Compaction occurs when air around soil particles, called pore space, is squeezed out. Overseeding a field that is compacted will have little or no long-term benefit; there is simply no room for new turf roots to grow. Compaction can be alleviated mechanically by core aeration, solid tine aeration or slicing to create air space. Additionally, compost top dressing, organic slow-release fertilizers and aggressive overseeding will over time build soil organic matter and biomass including roots and beneficial organisms such as earthworms, improving resistance to compaction.

Other problems that should be addressed prior to overseeding to ensure seedlings germinate and remain healthy are poor drainage, insufficient or too much water, poor soil fertility and excess thatch. Be sure to note where turf is doing poorly and consider sun and shade. Grasses that need full sun may not thrive in shade and shade-tolerant varieties may wilt in too much sun. In a non-athletic field situation, keep in mind that tree roots compete with grass roots for water and nutrients. Grass growing around trees may have a harder time becoming established. The right plant in the right place is the rule for turf and other plants.

Timing

In northern regions of the US, the fall is the best time to grow grass. The genetics of cool-season turf grasses are such that shorter, cooler days in fall are ideal for seeding and establishment. If an herbicide is to be used, delay treatment for four to six weeks after the new grass seed germinates. Herbicides can be harsh on seedlings and can be a major cause of poor seedling establishment. In southern regions, the best time to overseed is when the warm-season grass is slowing down, or about two weeks prior to the first frost. Deep southern regions can overseed into November and December.

Choosing the right variety

Seed choice is largely determined by the area's climate. Western, northern and eastern areas of the US generally need cool season grasses such as Kentucky bluegrass, perennial rye grass and fine and tall fescues. In these regions, bentgrass is preferable for golf courses and tennis and croquet courts. Warm season grasses such as Bermuda grass, centipedegrass and Bahiagrass are better suited to the Southern regions of the US.

Purchase good quality seed native to your area whenever possible. Mixtures of two or more species of grass can help reduce losses in the event of disease. Avoid buying seed with a high percentage of weed seeds. Check the label; any less expensive mixtures contain a high percentage of weed seeds. Consider looking at national and local turf trial results, which should be available for most seed species and cultivars.   

Proper watering for good seed-soil contact

The most important factor governing seed germination is seed to soil contact. A heavy watering immediately after seeding will help achieve this contact. Water lightly on a daily basis until the grass seed germinates, which is generally about ten to twenty days. After germination, water less frequently but soak deeply to encourage deeper root growth. After the seedlings become established, water at the recommended level for the type of grass used.  

Overseeding technique

Before seeding, rake and dethatch the turf if necessary. Then, simply broadcast the seed by hand or with a spreader, or use a "slit seeder" which opens a small furrow in established turf by cutting through the thatch layer. Slit seeding offers better seed to soil contact, so less seed may be needed than with broadcast seeding. To avoid a striped appearance when the new grass grows, consider making two passes, each at 50 percent of the recommended rate, and at 45 degree angles to each other. This diamond pattern will more evenly distribute the new seed over the turf area.   

If you broadcast the seed, aerate the turf several times before seeding to expose more soil. Core aerating tines that remove soil plugs are best. After seeding, water heavily right away. Seeds can get stuck in the thatch layer or grow only in the aeration holes, giving the turf a spotted look. Grasses such as Kentucky bluegrass have a creeping growing method that can deter clumping.   

Regardless of how you overseed, keep records of your pre-assessment, the methods you use and your results. Those records will be invaluable to help you learn what works best for your climate and sites.