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While hydroseeding is typically meant for bare soil, it is possible to hydroseed over an existing lawn with some adjustments. For the new seed to germinate and thrive, it needs good seed-to-soil contact. Without it, most of the seed may not take root and will die. To improve your chances of success when repairing a patchy area, mow any existing grass as low as possible and loosen any exposed soil before application. It is also recommended to use a finer mulch mixture when spraying, as this will help the seeds reach the soil and root without suffocating the existing grass.

Provided you're using an appropriate starter fertilizer for grass seed, you can put down seeds and fertilizer at the same time. However, putting them down separately, one after the other, on the same day will help to avoid over- or under-applying the lawn food. Avoid using a fertilizer with weed control elements, as these will likely keep the grass seeds from starting.  

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. 

In the fall, you should aerate the soil, overseed the bare spots, kill weeds, mow the lawn low, and rake your leaves. By boosting your lawn’s health in the fall, your grass will be stronger throughout the winter and come back healthier in the spring. You should also trim your perennials so that they withstand the winter and are ready for new growth in the spring.