It is bad to water your lawn at night because the water will not be able to evaporate as quickly, and this can lead to fungal growth or mildew. Additionally, watering your lawn at night can cause the soil to become soggy and compacted, which can inhibit root growth. The best time of day to water your lawn is in the morning, when the sun is up and the temperatures are cooler. This will allow the water to evaporate quickly and help your lawn stay healthy.
If you must water your lawn at night, make sure to do it early enough so that the water has a chance to dry before the sun goes down. You should also be using a sprinkler that emits larger drops of water, as this will allow the water to evaporate more quickly.