It will keep your garden green all winter and will die off when it will get warm in spring. Depending on the dimensions of your lawn (and the way strong you might be!) you may want to rent an electric scarifier. It is best to topdress the lawn in spring or autumn, as a result of grass seed germinates when the temperature is approximately 15ºC - 20ºC. The perfect time to prime seed your garden is in the fall.