Water your hydroseeded lawn in the later evening hours (after 6:00 p.m.) through the early-morning hours (before 8:00 a.m.) to cut down on water loss due to evaporation.
Water at least once each day to keep the hydroseed slurry very moist to wet at all times, never allowing it to dry out. On dry, very hot or windy days, more water may be required to fully wet the slurry material and surface soil than on days with less extreme weather conditions.
Apply the water from a sprayer set on light rain or mist, not a strong stream, to prevent the hydroseed slurry form being displaced and bunched up in areas.
Water nightly until the new shoots are more than 2 inches and the roots are knitting into the soil. Scale back the frequency, but increase the amount of water as needed to maintain evenly moist soil throughout the entire grass root zone (4 to 6 inches down).