Cool-Season vs. Warm-Season Grass

Almost every timing question in lawn care starts here. When to seed, when to feed, how high to mow and when to put down pre-emergent all depend on which group your grass belongs to. Get that wrong and you end up fertilizing a dormant lawn or seeding at the worst time of year.

I look after my own lawn, and the first thing I’d tell anyone is to find out what grass they have before buying anything. This guide is built on university extension data, mainly from Tennessee, Kentucky, NC State, Clemson, UGA and Illinois.

The two groups at a glance

The temperature ranges come from NC State TurfFiles and University of Tennessee Extension, and UGA’s Turfgrass 101 describes the same split.

Which one do you have?

Look at the calendar. If your lawn is green in January and struggles in August, it’s almost certainly cool-season. If it’s brown from the first hard frost until spring and thick and green all summer, it’s warm-season.

Look at how it spreads. Pull back the grass at the edge of a bed. Warm-season grasses like bermudagrass, zoysia and St. Augustine creep with runners above or below ground. Bermudagrass is the one that crawls into flower beds. Among cool-season grasses, only Kentucky bluegrass spreads by underground rhizomes; tall fescue and perennial ryegrass grow in clumps.

Look at where you live. Roughly, the northern half of the US grows cool-season grass, the Southeast and Gulf Coast grow warm-season grass, and a wide band through the middle, including North Carolina, Tennessee, Kentucky and parts of Missouri and Virginia, is the transition zone. NC State describes North Carolina as sitting right in that zone.

If you’re still unsure, take a sample to your county extension office and ask them to identify it.

Cool-season grasses

These grow in spring and fall, slow down in summer heat and keep some green through winter. UGA notes they have good winter tolerance and adequate shade tolerance, but may become stressed, dormant or injured in hot summers and can need significantly more water than warm-season grasses.

Kentucky bluegrass

The classic northern lawn grass. It spreads by underground rhizomes, so it fills bare spots on its own. Tennessee Extension says one plant can produce 20 to 50 or more feet of rhizomes in five months in good weather.

• Seed: University of Kentucky lists germination at 10 to 21 days, slower than other cool-season grasses.
• Sun: Illinois Extension says it grows best in full sun.
• Summer: it goes dormant in hot, dry weather and greens up when it cools.
• Upkeep: UK rates it high maintenance, with thatch, disease and irrigation needs that rise with heavy fertilizing.

Tall fescue (turf-type)

The most common turfgrass in North Carolina, according to NC State, and the safe choice through most of the transition zone.

• Roots: Illinois says it has a more extensive root system, so it pulls more water from the soil and relies less on irrigation.
• Shade: tolerates some shade but needs at least four hours of direct sun a day.
• Seed: UK lists 7 to 10 days to germinate.
• Habit: a bunch grass. It doesn’t spread to fill bare spots, so you overseed instead.
• Traffic: UK rates it good.

Perennial ryegrass

• Seed: the fastest, 5 to 7 days according to UK.
• Use: often in mixes to fill bare spots quickly. Illinois notes it’s short-lived and Kentucky bluegrass tends to take over in a mix.
• Traffic: UK rates it great.
• Weakness: little heat tolerance; UK says it needs summer irrigation for a thick stand, and is disease-prone.

Fine fescues

A group that includes creeping red, chewings, hard and sheep fescue.

• Shade: Illinois calls it the most shade tolerant of the common lawn grasses, but it still needs at least 2 hours of direct sun.
• Upkeep: low-input; UK notes it’s sensitive to heavy nitrogen.
• Traffic: poor.

Warm-season grasses

These wake up in late spring, grow hardest through summer heat and go dormant after frost. UGA lists poor shade and winter tolerance as their main weaknesses.

Bermudagrass

• Strengths: heat, drought and traffic tolerance, according to UK.
• Shade: very poor.
• Winter: straw-brown from the first killing frost through about mid-April in Kentucky. UK warns of winterkill risk below 15°F when there’s little snow.
• Habit: aggressive. It creeps into beds and neighboring lawns.
• Mowing: short. Clemson lists 1 to 2 inches for common bermudagrass and 0.5 to 1.5 inches for hybrids.

Zoysiagrass

• Strengths: excellent heat, drought and traffic tolerance, according to UK, with decent shade tolerance.
• Weakness: slow to establish. UK says about 3 years from plugs or sprigs in prepared soil.
• Winter: straw-brown from October through mid-April in Kentucky, with less winterkill concern than bermudagrass.
• Mowing: Clemson lists 1 to 2 inches.

St. Augustinegrass

• A coarse, wide-bladed grass for the Deep South and Gulf Coast.
• Mowing: tall. Clemson lists 2.5 to 4 inches.
• Planting: Tennessee Extension says it’s established from sod, plugs or sprigs, not seed.
• UGA notes it grows well in south Georgia but has trouble surviving farther north in the state, which shows how sharply winter limits it.

Centipedegrass

• Low-maintenance, slow-growing. Clemson rates its mowing frequency as low, at 1.5 to 2 inches.
• Can be seeded, according to Tennessee Extension.

How care differs through the year

This is the practical side. Most mistakes come from using cool-season timing on a warm-season lawn or the reverse.

When to plant

Tennessee explains why bluegrass shouldn’t be seeded in spring: it germinates slowly, and the weak seedlings often die in summer heat. My guide on how to overseed a lawn covers the process, and best grass seed covers what to buy.

When to fertilize

Cool-season: mostly fall. Missouri Extension calls September the most important time to fertilize cool-season grass, at 1 to 1.5 lb of nitrogen per 1,000 sq ft. A spring feeding is optional, and Missouri advises against nitrogen after May 1 because it promotes leaf disease.

Warm-season: late spring through summer. UGA says to wait until the soil reaches 65°F. Feeding earlier forces green-up too soon and burns the stored food the grass needs if a late cold snap hits. NC State’s Onslow County office says not to fertilize most warm-season lawns before May.

Always follow the product label for rates. My guide on when to fertilize your lawn goes month by month.

Mowing heights

Whatever the height, Clemson’s rule is to remove no more than one-third of the blade at a time. See how to mow a lawn.

Watering

Both groups need about 1 inch a week in the growing season. Cool-season lawns need it most in summer, when they’re under the most stress; Illinois gives 1 to 1.5 inches a week to keep them green. Warm-season lawns need little in spring and most in summer. Both can go dormant in drought. Details in how often to water your lawn.

Aeration and dethatching

Clemson says to aerate cool-season lawns in fall and warm-season lawns in summer, when each is growing strongly enough to recover. NC State adds that you shouldn’t use a power rake or verticutter on centipede or St. Augustine.

The transition zone

This is the hardest place to grow a lawn. Summers are too hot for cool-season grass to be comfortable, and winters are too cold for warm-season grass to be safe.

What usually works, based on the University of Kentucky guide:

• Tall fescue is adapted statewide in Kentucky and is the default for sun to medium shade.
• Kentucky bluegrass suits the cooler central and eastern parts and does poorly in the hot west.
• Zoysia suits the whole state if you accept a long brown season.
• Bermudagrass is best in the warmest western part.

Tennessee Extension notes that many sod producers blend about 90% tall fescue with 10% Kentucky bluegrass by weight. The bluegrass helps fill gaps that tall fescue can’t fill on its own.

Overseeding warm-season grass with ryegrass

Some Southern homeowners seed perennial ryegrass into a dormant bermudagrass lawn to keep it green in winter. Oklahoma State Extension says this is possible but not highly recommended: it means mowing, feeding and watering through winter.

Pros and cons of each group

Cool-season, the good:

• Green for most of the year, including winter.
• Easy to start from seed.
• Better shade options (fine fescue, tall fescue).

Cool-season, the bad:

• Summer heat and drought stress.
• Bunch types like tall fescue need regular overseeding.
• Ryegrass in particular is disease-prone, according to UK.

Warm-season, the good:

• Thrives in heat and drought.
• Bermudagrass and zoysia handle heavy traffic.
• Spreading types repair themselves.

Warm-season, the bad:

• Brown for months in winter.
• Most handle shade poorly.
• Many types need sod or plugs rather than seed.
• Bermudagrass invades beds.

Which should you plant?

Mowing and mower size also differ. Short bermudagrass looks best with frequent cuts, and UK notes reel mowers are preferred for zoysia and bermudagrass. A robot mower that cuts a little every day suits both groups well; see best robot lawn mowers.