They are stressed by summer heat and drought and by winter cold. In the spring grass plants are recovering from winter dormancy and they draw on reserves of energy stored in the root system the previous fall. Those reserves are restored with spring feeding. The plant needs these nutrients for spring growth and to replenish roots which are storing more energy for summer.
Summer brings insects, heat and dry winds which put lawns under heat and drought stress.
Grass plants may become dormant as a way to survive. Dormancy protects the lawn for four to six weeks but if water is not supplied many plants will die. High phosphorus, slow release fertilizer at this time strengthens the root system and replaces energy used to survive summer stress and recover from dormancy. Grass thrives in the fall and develops many new roots and shoots which cause the lawn to thicken. Fall feeding supports this active growth.