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Whether or not grass will fill in bare spots itself depends on the type of grass you have. For example, grass with rhizomes (e.g., Bermuda grass) and stolons (e.g., Saint Augustine grass) will naturally fill in bald spots as it spreads laterally. However, clump-forming grasses will not spread. 

When stored under ideal conditions, grass seed lasts anywhere from 10 to 18 months from the testing date without any real decrease in the germination rate. You can still use the seeds after this timeframe, but the germination rates drop 10% to 20% each year. 

If you’re just embarking on an overseeding project, using seeds that have been in storage for a few years isn’t too big of a deal. However, if you’re planning on redoing your entire landscape, your best bet for a healthy, luscious lawn is to use new seeds.

If your grass is stressed from being over-trimmed, try watering it heavily and then allowing it to recover for at least two weeks before mowing again. If you notice brown patches where the grass has died off because it’s too short, you should remove the dead grass and overseed the area. Leaving dead patches can invite weeds and other problems, so if you want a totally green lawn, getting rid of dead patches should include removing the dead sections before reseeding the area.

Adding seed to existing grass is often known as overseeding, as opposed to reseeding. Reseeding refers to the process of filling in bare or dying patches of lawn—or in some cases, full lawns—whereas reseeding supports existing lawns to prevent thinning. Overseeding is an ideal annual lawn care practice for grass fighting drought, disease, weeds, or pest issues.