Growing fresh herbs at home is rewarding, but successful planting in Colorado requires a precise understanding of the state’s diverse and often unpredictable climate. High altitude, microclimates, and sudden temperature shifts present unique challenges. Gardeners must move beyond general spring planting ideas, as the window for safely establishing herbs depends entirely on the plant’s specific cold tolerance and the garden’s exact location.
Understanding Colorado’s Climate Framework
The foundation for any planting decision in Colorado is the Last Average Frost Date (LAFD), which serves as the central benchmark for the start of the growing season. The LAFD represents the average day after which the probability of a 32°F freeze drops to an acceptable level, typically around 30%. This date varies widely across the state, ranging from mid-April in warmer pockets to as late as the end of July in the highest elevations.
Colorado’s vast geographic diversity is reflected in its USDA Plant Hardiness Zones, spanning from Zone 3b in mountain regions to Zone 7b in sheltered valleys. The Front Range, where most of the population resides, generally falls into Zones 5b to 6a, with LAFDs typically between May 1st and May 20th. Gardeners must also contend with significant microclimate variations, meaning the general zone map is only a starting point. For instance, a sheltered urban yard may warm up faster than an exposed rural location, creating a warmer pocket that affects the LAFD.
Timing for Cold-Tolerant Herbs
Hardy herbs withstand cooler conditions and can be planted weeks before the LAFD, allowing them to establish a strong root system early in the season. These plants tolerate light frost and thrive in cool, workable soil. Direct sowing of seeds for herbs like parsley and dill can begin four to six weeks before the local LAFD, typically in late March or early April in Front Range areas.
Perennial herbs such as mint, chives, thyme, and sage can be transplanted or divided as soon as the soil is thawed enough to work. Planting these varieties in cool spring soil stimulates root growth before the heat of summer arrives. To protect early plantings from unexpected late freezes, gardeners can utilize season extension techniques, such as covering young plants with floating row covers or a cold frame when temperatures dip below 32°F. Chives, sage, and thyme are particularly frost-resistant, sometimes enduring temperatures as low as 10°F once established.
Timing for Warm-Weather Herbs
Tender, warm-weather herbs require consistently warm soil and no risk of frost for survival. These Mediterranean-origin plants, including basil, cilantro, rosemary, and oregano, are sensitive to cold damage. Planting them prematurely in cold soil can lead to stunted growth, yellowing leaves, or death, even without a hard freeze.
Planting must be delayed until well after the LAFD, often pushing the safe window into late May or early June in many parts of Colorado. A reliable indicator is the soil temperature, which should consistently measure above 60°F, especially for basil. For cilantro and basil, transplanting started seedlings outdoors is common, providing a necessary head start in Colorado’s shorter growing season. Rosemary, which is not reliably winter-hardy, and tender oregano varieties are best planted as transplants once nighttime temperatures stay reliably above 50°F.
Adjusting Planting Times for High Altitude Gardens
Gardening at elevations above 7,000 feet presents a significantly compressed growing season, demanding a delay in all planting schedules. In these mountain environments, the general LAFD of the plains is often irrelevant, with the last freeze sometimes occurring in mid-June or early July. The average frost-free period can be fewer than 90 days, which constrains plant maturity.
Mountain gardeners must push back the planting of all herbs, even cold-tolerant ones, sometimes by several weeks past the time recommended for the Front Range. While hardy herbs can still be planted first, tender herbs like basil must be held indoors until true summer warmth is established. Utilizing season extension tools, such as hoop houses, cold frames, or greenhouses, becomes necessary to capture enough heat units before the early return of fall frosts in September. The reduced atmospheric pressure and cooler air temperatures mean that every day of the short season is precious, making meticulous timing and environmental protection essential for a successful harvest.