Reseeding is the practice of spreading new grass seed over an existing lawn to improve density and health. Achieving a successful, thick lawn in Colorado requires precise timing due to the state’s semi-arid environment, high altitude, and intense temperature fluctuations. Following generic lawn care calendars often leads to poor germination, as the unique climate demands that seeding be perfectly synchronized with soil and air temperatures. Understanding the grass type and its specific growth cycles is the first step in determining the ideal window for successful establishment.
Selecting Turfgrass Varieties for Colorado
The vast majority of turfgrass grown in Colorado consists of cool-season varieties that thrive in the state’s climate. These grasses, which include Kentucky Bluegrass, Tall Fescue, Fine Fescues, and Perennial Ryegrass, are active during the cooler parts of the year, specifically spring and fall. Kentucky Bluegrass is the most common choice, prized for its deep color and ability to repair itself using underground stems called rhizomes. Tall Fescue and Fine Fescues are valuable for their deeper root systems and better drought tolerance, making them suitable for the arid conditions and varying elevations across the state.
Warm-season grasses, like Buffalograss and Blue Grama, require much hotter soil temperatures and are typically only suitable for the lower elevations of the eastern plains. These native grasses are extremely drought-tolerant but have a shorter growing season, often going dormant and turning brown during the cooler months. The reseeding calendar for most homeowners in Colorado is therefore dictated by the needs of cool-season turf.
The Primary Reseeding Window: Late Summer and Early Fall
The single best time to reseed a cool-season lawn in Colorado is during the late summer to early fall period, typically from mid-August through mid-September. This timing is optimal because the soil remains sufficiently warm from the summer heat, which encourages rapid seed germination and root development. The target soil temperature for cool-season grass seed germination is between 50°F and 65°F, a condition consistently met during this window.
During this period, air temperatures begin to cool down, reducing the heat stress on fragile young seedlings once they emerge. Additionally, the threat from aggressive summer weeds, such as crabgrass, has significantly diminished, allowing the new grass to grow without intense competition. The goal is to allow the seedlings at least six to eight weeks of growth to develop a strong, deep root system before the ground freezes and winter dormancy sets in.
Secondary Timing: Spring Reseeding Considerations
While late summer is the ideal time, spring reseeding is sometimes attempted, generally between mid-April and early June. However, seeding in the spring presents significant challenges that increase the risk of failure. The primary obstacle is the intense competition from annual weeds, including the aggressive crabgrass, which germinate readily as the soil warms.
Weeds are often faster-growing than the new grass seedlings, quickly outcompeting them for sunlight, water, and nutrients. Furthermore, the application of pre-emergent herbicides to control summer weeds is not possible when seeding in the spring, as the product prevents all seeds, including the new grass, from germinating. Any new grass that manages to establish itself then faces the intense, dry heat of the Colorado summer before it has developed a resilient, deep root system, leading to high stress and potential die-off.
Essential Soil Preparation and Initial Care
Proper preparation of the soil is as important as timing and directly impacts the success of the reseeding effort. Before spreading the seed, the existing lawn should be mowed shorter than usual to ensure the new seeds make contact with the soil, not just sit on top of the grass blades. Core aeration is highly recommended before seeding, as it alleviates soil compaction and creates small holes where the seeds can settle, protecting them and promoting better germination.
After the seed is spread, a starter fertilizer can be applied to provide the necessary nutrients for the emerging seedlings. The most critical factor in the initial care phase is the watering regimen; the seeded area must be kept consistently moist, but not saturated, for the first two to three weeks. This usually requires multiple light waterings—two to four times per day—to prevent the newly exposed seed from drying out, which would immediately halt germination.
Once the new grass reaches a height of about two inches, the watering frequency should be reduced, and the duration increased to encourage the roots to grow deeper into the soil. The first mowing should be delayed until the seedlings are tall enough, generally around three inches, and the mower blades should be sharp to prevent tearing the fragile new grass.