Sow grass seeds in New Jersey outside of times of temperature extremes. The roots need time to strengthen before they are exposed to a cold, snowy winter. If the planting is pushed too close to summer or winter, the chance of success diminishes.
April is the best time to grow grass in New Jersey. By then, the cold weather and frost are gone. Take the current weather conditions into consideration, though. If you have an unusually early warm-up, you can sow grass seeds in March, but if it's a cool spring, you can sow as late as May. If you miss that window altogether, cool season grasses can be sown in the fall--between Aug. 15 and Sept. 15.
Take the temperature of the ground. You can plant warm or cool season grasses in New Jersey. If planting cool-season grass, sow it in a soil temperature of 50 to 65 degrees Fahrenheit. Warm-season grass seed needs warmer temperatures of 65 and 70 degrees Fahrenheit.
If you decide to plant in the fall, you must disperse the seed at least 90 days before the first frost is expected. If you wait much longer, the seed won't have time to become established before winter sets in, and it will die.