Cool-season grasses, which include popular varieties like Kentucky Bluegrass, Tall Fescue, and Perennial Ryegrass, are adapted to thrive in temperate climates, showing their most vigorous growth during the cooler temperatures of spring and fall. These grasses possess a natural survival mechanism known as dormancy, which allows them to endure periods of environmental stress. Understanding when this process occurs and what triggers it is the first step toward maintaining a healthy lawn throughout the year. This natural cycle of rest ensures that the grass plant can conserve its internal resources until favorable growing conditions return.
What is Dormancy and Why It Occurs
Dormancy is a state of suspended growth that turfgrass initiates to protect itself from extreme conditions like intense heat, drought, or severe cold. When a grass plant enters this phase, it is not dead; rather, it is strategically conserving energy and moisture to ensure the survival of its most vital parts. The plant essentially sacrifices the leaf blades, which turn a straw-like brown or tan color, to protect the crown and root system.
Physiologically, the plant’s metabolism slows dramatically, and the production of chlorophyll, the pigment responsible for the green color, ceases. Energy reserves, primarily carbohydrates, are drawn down and stored in the crown, the base of the plant located at the soil line, and in the roots. This conservation effort ensures that the plant has the necessary fuel to resume active growth quickly once moisture returns or temperatures moderate.
Triggers and Timing of Summer Dormancy
Summer dormancy is primarily triggered by a combination of high temperatures and moisture deficit. The onset typically begins in mid-to-late summer across most temperate zones when environmental conditions become persistently harsh. The grass begins to shut down when air temperatures consistently exceed 85°F to 90°F, especially when coupled with soil temperatures that rise above 85°F.
The concurrent lack of soil moisture, or drought stress, is the reinforcing factor that forces the plant into survival mode. Without regular rainfall or irrigation, the grass cannot cool itself through evapotranspiration and cannot sustain its metabolic processes. Lawns often show initial signs of distress by developing a gray-green tint or losing their turgidity, a visual symptom known as “footprinting” where grass blades remain compressed after being walked upon.
The speed at which cool-season grasses enter this state varies by species; Kentucky Bluegrass and Fine Fescues tend to go dormant faster than the more heat-tolerant Tall Fescue or Perennial Ryegrass. Once a sustained drought has lasted for approximately three to four weeks, most established cool-season lawns will have fully entered summer dormancy, appearing uniformly brown. This protective state can usually be maintained for about three to four weeks without permanent damage, though survival time is shorter in extremely high temperatures.
Triggers and Timing of Winter Dormancy
Winter dormancy is triggered by the natural progression of the seasons into winter. This process is driven by two main environmental cues: sustained low temperatures and the photoperiod.
The physical shutdown of growth occurs when air temperatures consistently drop below 50°F, and more specifically, when the soil temperature falls below approximately 45°F to 50°F. This typically happens after the first hard freeze in late fall or early winter, depending on the region. Unlike summer dormancy, which is largely dependent on moisture levels, winter dormancy is an expected, temperature-driven response that prepares the grass to withstand freezing conditions. The grass will remain in this dormant, brown state until soil temperatures warm back up, usually in early spring.
Management and Recovery Strategies
Managing Summer Dormancy
During summer dormancy, the primary management goal is to keep the plant’s crown hydrated enough to remain alive without forcing the grass out of dormancy prematurely. For established lawns in a drought, this means providing light, infrequent watering—about a half-inch of water every two to four weeks. This small amount of moisture is sufficient to prevent the crown from desiccating but not enough to encourage active growth.
It is advisable to cease mowing entirely once the lawn is fully dormant, as the leaf blades are not actively growing and further cutting adds unnecessary stress. If mowing is absolutely necessary, the deck should be raised to the highest possible setting, removing no more than one-third of the blade length. Foot traffic should be limited as much as possible on a dormant lawn because the brittle, dry blades and fragile crowns are susceptible to mechanical damage that can create bare spots.
Recovery
Recovery from dormancy is triggered when the environmental stresses are relieved, meaning cooler temperatures arrive and moisture returns. For summer dormancy, the grass will begin to green up and resume growth when sustained rainfall or irrigation is provided and air temperatures fall back into the 60°F to 75°F range, typically occurring in late summer or early fall. Full recovery involves the plant drawing on its stored carbohydrates to push out new leaf blades, a process that can take a week or more of favorable conditions. For recovery from winter dormancy, the grass will naturally begin to green up as soil temperatures rise in the spring.