All seeds require moisture and other elements before they begin to germinate. Once the germination process has begun, if conditions change, the seed or new sprout is vulnerable and can die. Grass seed germination is simply what takes place inside the seed to make it sprout when the seed absorbs enough moisture to start growing. Many factors affect grass seed germination. The success of your lawn is not entirely under your control, but you can do a lot to improve the final result.
The pre-sprout phase is most critical. You must maintain a strong commitment to watering new grass seed. The germination time for grass seed ranges from 10 to 15 days depending on the variety. This is how long it will take to actually see the grass growing. It can be even longer in cooler temperatures. Until this point, the seed, or the soil, must stay moist. It should not be soggy or drenched, or it will rot from too much water. If you give your seeds too little water, they will dry up and die. Grass seeds require oxygen too in this case. If there is too much water in the soil, seeds won't get the amount of oxygen they need. Once the new grass is visible, the roots are also growing down into the soil. This happens quite quickly. As soil moisture below ground is more accessible to the roots, the plant is not as vulnerable. However, don't reduce the amount of watering on new grass seeds yet.
Seeds will not sprout all at the same time. Seeds will be buried at different depths, absorb water differently and this dictates how much sunlight they will get. Many seed mixtures will have different characteristics affecting their development. It is crucial to keep the surface level of soil constantly moist until all seeds have germinated.
Soil temperatures should be in the correct range for the type of grass. Germination requires a minimum temperature range of 50 degrees. Note that soil temperatures are usually cooler than air temperature. Once germination begins, if the environment changes significantly (like being too hot and dry), the seed or sprout will die.
Once the newly seeded lawn is watered, it must remain moist by rainfall, irrigation or hand watering. The first few days and weeks are critical to getting your lawn established. It's important to water daily and for 30 minutes the first time. Water each session for 10 minutes with a spray nozzle that is adjusted for a soft mist gentle spray or an oscillating sprinkler, but not to the point where there are puddles, which may wash some seed away. It is imperative to not saturate the soil, so focus on frequent yet light watering. This may mean sprinkling your lawn lightly as much as 2-3 times per day if the weather is hot and dry or windy. Only the top layer of soil needs to be moistened. Try to avoid disturbing the seed so it can maintain good soil contact. Some species of grass have germination times as long as 21 days, so be patient and continue watering.
Temperature also greatly affects new grass. During hot periods, lawns need more water. Avoid over-watering and saturating the soil. After the seed germinates, continue watering daily for the next week or two. The tiny grass plants have small root systems at this time and may die if the top layer of soil is allowed to dry out. Water makes up 70 to 80% of the weight of our lawn grasses and the clippings alone are nearly 90% water.Other recommendations include:
Soon after your lawn has been aerated and over seeded water the lawn thoroughly to wash the seed into the holes made after the core aerating process. This will give the seeds good contact with the soil for best germination. For most sprinklers this will mean leaving the water running at each location for about 6 minutes.
Good luck with the success of your lawn!