It depends. Most of the time, rain will help the Hydroseed to germinate and establish itself. However, if there is a lot of rainfall in a short period of time, it could wash away the seed mixture. You can minimize this risk by watering your lawn regularly after you seed it (every other day for about 10-15 minutes).

It's unlikely that rain will wash away hydroseed. However, if there is an excessive amount of rain, it could cause the seed to be washed away or move around. If this happens, you may need to reseed the area.