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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.

The amount of time it takes for grass to germinate varies depending on the grass species. Even after germination, it’s going to take a while to see grass blades start emerging from soil. Some grasses germinate quickly and emerge after several days of growth (and healthy watering). Others may take up to three weeks to show themselves. Cool-season grasses tend to take the longest to germinate at one month.

While hydroseeding is designed for bare soil, it is possible to apply it over an existing lawn with certain adjustments. For the new seed to be successful, it needs good seed-to-soil contact to germinate and take root. Without proper preparation, most of the new seed will fail to establish and will die. To hydroseed over existing grass, first mow the lawn as low as possible, then loosen and clear any exposed soil. It is also recommended to use a finer mulch mixture in the hydroseed spray, which helps the seeds penetrate the existing turf and reach the soil without suffocating the established grass.

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.