How to Overseed a Lawn That Actually Fills In

Cool-season seed does best in late summer because the soil is still warm, so the seed germinates quickly, and the seedlings get two to three months of favorable growing weather before winter stress. Fewer weed seeds germinate in late summer and fall, so the new grass has less competition. Extension in Iowa and in Massachusetts both give mid-August to mid-September, and Pennsylvania's calls late summer the best time for most of the state.

Spring is the second choice, because a spring seeding has to get through more weed competition, a long stretch of open soil while the ground warms, and summer stress on young seedlings. Early summer and summer plantings are best avoided altogether.

Dates move with climate, so use the ones your own state Extension publishes when they exist. The lawn care calendar shows how seeding fits with the rest of the year's lawn work.

Clemson Extension does not generally recommend overseeding a home lawn, and says it belongs only on bermudagrass if it is done at all, because the extra water, fertilizer and shade from the new seedlings can severely retard or damage centipedegrass, zoysiagrass and St. Augustinegrass in spring and early summer. A warm-season lawn therefore follows a different plan from the cool-season one.

On a bermudagrass lawn, ryegrass sown into the dormant grass gives green cover during the winter. In South Carolina, the timing below is generally a good fit. The light rate on an established lawn gives green cover without a thick ryegrass stand that causes transition problems in the spring.

Ryegrass normally dies out in late spring. Seed that did not sprout can come up as big clumps the next year, so a lawn that has been overseeded once usually needs it again, or a fall pre-emergent herbicide to stop the leftover seed.

To thicken the bermudagrass itself with seed, wait for warm soil. Bermudagrass seed will not germinate until the soil reaches about 65°F, which makes May 1 through June 15 the best planting window in Oklahoma.

Seed only helps once you know why the grass thinned, because seedlings meet the same problem the old grass did, and renovation gives only temporary improvement unless the original cause is fixed. Check the lawn against this list of common causes of turf decline.

How much good grass is left decides whether seed is enough. A lawn with more than 50% desirable grass can be renovated by overseeding its bare patches, while one with more than 50% weeds or bare ground needs a complete renovation. That job means killing all the vegetation with a nonselective herbicide and reseeding the whole lawn. The guide on whether overseeding is worth it covers a thin lawn that can still be saved.

Use seed of a species similar to the one already in the lawn, unless a poorly chosen species caused the decline in the first place. Where the choice is open, the mix follows the light.

Perennial ryegrass sprouts in 5 to 7 days, while Kentucky bluegrass can take up to two weeks to emerge, which matters when a bare area has to fill in quickly. Ryegrass is the best species for renovating lawns because it germinates and establishes quickly and its roots can push into slightly compacted soil. Because of its aggressive nature, it is generally not used above 20 percent of a mixture.

On the seed tag, multiply the purity by the germination percentage and divide by 100 to get the pure live seed. A bag that is 85% pure with 60% germination holds only 51% pure live seed, so it takes about 2 pounds to supply 1 pound of pure live seed, while a bag that is 99% pure with 90% germination takes about 1.1 pounds. The cheaper bag can therefore cost more per pound of live seed.

The amount depends on the mix. UMass Amherst's seeding rates differ by mix, with a goal of 15 to 25 seeds per square inch when renovating.

Poor conditions for germination call for about 50% more seed. Otherwise, extra seed does not help, because a stand seeded above the table's rates fills with small immature plants that are slow to mature, and an excessively dense stand raises disease problems and mower scalping.

To apply the rate, multiply length by width for each rectangular piece of the lawn and add the pieces up, which gives the number of thousands of square feet to seed.

Seed that lands on grass blades or thatch never reaches the soil, and seed broadcast onto a lawn without cultivation will not survive. Preparation therefore has one aim, which is bare soil for the seed to touch, and it runs in a set order.

A soil test comes first, taken three to four weeks before the work, so you know what fertilizer and lime the lawn needs. Broadleaf weeds are treated next and well ahead of the seeding date, because a broadleaf herbicide mix used in lawn renovation needs a wait of about four weeks before seed can go down. The label gives the real interval for any product.

Then mow the lawn low for this one cut and take the clippings and debris off. Heights differ by source, from 1½ to 2 inches in Iowa State's overseeding guide, which aims to reduce competition from the established grass, down to about ¾ inch in Penn State's renovation program.

Check the thatch, the layer of partly decomposed stems and roots above the soil. A dethatcher removes it once it is more than ⅓ inch thick, and a layer over ½ inch calls for a full renovation that strips the sod. A lawn with less than that still needs the surface opened, and three tools do it.

On small areas a garden rake can loosen the soil to the depth needed, and the guide to overseeding without aerating covers that route.

A drop spreader suits a small lawn, and seeding in two directions leaves fewer skips and a more uniform application. That is why the steps below split the seed into two halves.

Starter fertilizer is applied just before or just after seeding, at 0.5 to 1 pound of nitrogen per 1,000 square feet. More than 1.5 pounds of nitrogen can burn the young turf, and the starter does not replace the phosphate and potash a soil test calls for.

After seeding, the seed needs a little soil over it and firm contact beneath it, which the last three steps below provide. Straw mulch is for places where little grass remains, since a very light layer holds moisture but too much smothers the seedlings and brings disease problems.

The seedbed has to stay moist from sowing until the seedlings are up. Their roots are delicate and shallow, and the seedlings cannot withstand severe moisture stress, so the soil should stay moist but never soggy. Light, frequent irrigation does this.

Extension pages give different counts, from once a day to three or four times, and the count can drop in rainy, cool or overcast weather or when the seedbed is mulched.

Too much water also washes seed away and encourages disease. The new grass seed watering guide goes further into the schedule. The deep versus frequent watering comparison covers the later switch to deeper watering.

The first cut with a lightweight mower comes before the grass is more than one-third above the height you want, so a lawn kept at 3 inches gets mowed just before it reaches 4. Wisconsin Extension starts when seedlings reach 3 inches, which takes about 3 to 6 weeks.

A quick test is to walk onto the new turf. If the plants spring back upright where you stepped, mowing is probably safe, and if not, you risk pressing the seedlings into the soil.

While the seedlings come up, the established grass keeps being mowed often at 1½ to 2 inches to reduce its competition, and the height then rises gradually toward a final 3 inches. The first mow after overseeding guide covers the first cut in detail.

When the starter fertilizer is not enough to sustain rapid growth, a second feeding thickens the new turf, and the timing differs between sources.

The new grass fertilizing guide sorts out the timing.

New turf is very sensitive to most herbicides, so they are best avoided unless the weed infestation is severe, and many broadleaf herbicides should wait until 4 to 6 weeks after establishment. The label gives the exact interval.

Seedlings are bruised or uprooted when trampled, so foot and vehicle traffic stays off until the grass is well rooted and has been mowed several times. The 4 to 8 weeks after seeding are the critical period for the new lawn. The walking on new grass seed guide gives the details.

Judge the result once the three to four weeks that germination and establishment probably take have passed, with perennial ryegrass fastest and Kentucky bluegrass slowest. A patch that is still thin after that time can be checked against the conditions linked to poor germination and seedling survival.

Once the condition the patch points to is fixed, the patch can be reseeded. On a dead patch about the size of a basketball, loosen the soil, sprinkle the seed by hand and rake it in gently to ¼ to ⅜ inch deep. A bare patch only the size of a baseball may call for lawn fertilizer at 1 pound of nitrogen per 1,000 square feet, to push the surrounding grass to fill the gap.

A shady corner that keeps failing may need a different mix instead of more seed, and a mix that is 50% or more fine fescue suits dry shaded sites.