Many factors determine how successfully grass seed will germinate, not least how temperature effects the type of grass. Cool season grass will grow better in the early part of Spring when the climate is typically cooler, whereas warm season grass favours the warmth of late Spring and the early part of Summer. Different types of grass also grow at different rates, and knowing what rate your grass will grow at is important for determining how long you will need to keep it watered to ensure successful germination.
Specifically speaking, cool season grasses tend to favour temperatures between around 5 to 7 degrees Celsius during Spring, any later and there is a risk that the grass won’t receive adequate moisture as the warm summer sun dries the soil. Warm season grasses will grow best between 18 and 23 degrees Celsius during warmer months.
If you want to accurately measure your soil temperature to see if the time is right to plant grass seed, a soil thermometer can be a good indicator. Dig a hole in your lawn that the thermometer will sit in snugly, and take a reading in both the morning and afternoon to get a reliable average. Remember to take care to keep sunlight off the thermometer as this may interfere and give inaccurate results.
Once the temperature of the soil is confirmed as suitable for the grass to germinate the next task is to make sure the lawn is kept suitably hydrated as it grows to its mature state. This can be anything from 5 to 30 days depending on the species, for example Bermuda grass will take between 5 and 10 days and Bluegrass will take between 15 and 30 days. Always ensure that you keep the lawn hydrated for the full germination time of whichever species you choose. If you struggle to keep on top of watering the lawn manually it might be worth considering having a sprinkler on a timer positioned in the middle of your lawn. This way you can guarentee the lawn will consistently recieve the right amount of water every single day. It goes without saying of course that you should still visually inspect the lawn throughout this process, checking for any areas that aren’t receiving enough water, and topping up by hand if necessary.