How hot is too hot to plant cool-season grass?

The short answer

There is no single temperature that applies to every cool-season grass, but a useful rule of thumb is this: cool-season grasses grow best in mild weather, roughly 60 to 75°F, and they struggle to establish when it is consistently hot. If daytime highs are regularly in the 80s or 90s °F, it is usually too hot to plant them without extra care. For the best chance of success, wait for spring or early fall.

The answer differs a little for container-grown plants and for seed, as explained below.

Why heat is a problem for cool-season grasses

Cool-season grasses do most of their growing in spring and fall. In summer heat they slow down, and some go semi-dormant. A newly planted grass has a small root system and cannot take up water quickly, so heat and dry weather together put it under stress. Our article on warm-season vs cool-season ornamental grasses explains the growth patterns behind this.

Rules of thumb

These are guidelines, not hard limits. Local conditions matter.

Established or container-grown plants (feather reed grass, blue fescue, tufted hair grass and similar):

• Plant in spring or early fall whenever you can.
• If you are planting in summer, expect to water more often and protect the plants from the hottest part of the day. Nurseries note that keeping summer-planted grasses watered enough can be difficult.
• Early fall planting works best if the plants go in at least 6 to 8 weeks before the first expected frost.

Seed:

• Cool-season grass seed (see where to find ornamental grass seeds) generally germinates best when soil temperatures are roughly 50 to 65°F.
• One seed-germination guide says germination can stop altogether once soil temperatures rise above about 86°F (30°C), and that sustained soil heat above about 95°F (35°C) can kill seed. Treat these figures as approximate.
• Soil temperature at seed depth matters more than air temperature. Dark or bare soil in full sun can run hotter than the air.

If you must plant in summer

Sometimes a plant has to go in during warm weather. These steps reduce the risk:

1. Choose a cooler day. Plant in the morning or evening, and avoid heat waves.
2. Water well. Soak the root ball before planting and water deeply after planting. Keep the soil evenly moist, not soggy, while the plant settles.
3. Add mulch. A layer of mulch moderates soil temperature and holds moisture. Keep it back from the crown to avoid rot.
4. Provide shade. Temporary shade cloth or placing the plant where it gets afternoon shade can help.
5. Plant at the right depth. Set the crown level with the soil surface. Planting too deep can cause the crown to rot.
6. Consider holding the plant in its pot. A container plant kept watered in light shade can wait until fall.

Best planting windows

• Cool-season ornamental grasses: spring, or early fall.
• Warm-season ornamental grasses: late spring to early summer, when the soil is warm. Warm-season grasses planted this way suit the same climates but need the opposite timing. Where summers are long and hot, see ornamental grasses for zones 9 and 10.

See how to care for ornamental grasses for aftercare during the first year.

A note on lawn grass

If you are asking about lawn seed rather than ornamental grasses, the same principle applies. Cool-season lawn grasses (such as tall fescue, Kentucky bluegrass and perennial ryegrass) are generally seeded in late summer to early fall or in spring, when soil temperatures are lower. Local extension services publish regional seeding dates and are the best source for your area.

Related reading

If you are planning a bed that includes both groups, see can I mix cool and warm-season grasses. For an overview of what you can plant, see types of ornamental grasses.