The USDA Plant Hardiness Zone Map is a standardized resource for gardening and landscaping across the United States. This system determines which perennial plants, shrubs, and trees are likely to survive the winter cold in a specific geographic location. The zones predict survival based on the coldest temperatures a plant can typically endure. Consulting this map helps gardeners avoid selecting species that cannot withstand the region’s extreme minimum temperatures, ensuring a higher success rate for permanent plantings.
Identifying Lincoln, Nebraska’s Hardiness Zone
The city of Lincoln, Nebraska, is officially designated as straddling a transition between two zones on the most recent USDA Plant Hardiness Zone Map. The majority of the city now falls within Zone 6a, reflecting a slight warming trend. This designation means the average annual extreme minimum temperature is between -10°F and -5°F (-23.3°C and -20.6°C). Some outlying and northwestern areas, however, remain in the slightly colder Zone 5b, which corresponds to a range of -15°F to -10°F (-26.1°C to -23.3°C). Gardeners should use the Zone 6a designation as the primary guide for plant selection while remaining aware of the colder pocket areas.
Understanding the USDA Plant Hardiness System
The USDA Plant Hardiness Zone Map is built upon a single climatic metric: the average annual extreme minimum temperature. This is not the lowest temperature ever recorded, but the average of the lowest winter temperature recorded over a 30-year period. The latest map version utilizes temperature data collected between 1991 and 2020 to establish these averages.
The system is divided into 13 major zones, with each zone representing a 10-degree Fahrenheit temperature band. To achieve greater precision, each main zone is split into two subzones, labeled ‘a’ and ‘b’, which represent a difference of 5 degrees Fahrenheit. For instance, Zone 6a is the colder half of Zone 6, while 6b is the warmer half.
The hardiness zones are solely focused on winter survival and do not account for other factors that affect plant growth. Metrics such as summer heat, humidity levels, soil type, or rainfall are not considered in the USDA’s calculations. Therefore, a plant designated for a particular zone might still struggle in the summer heat, emphasizing the need for supplementary research.
Practical Planting Applications Based on the Zone
For gardeners operating within Lincoln’s primary Zone 6a, the selection of perennial plants should prioritize species rated for Zone 6 or colder, such as Zone 5. Perennials like Daylilies (Hemerocallis), Coneflowers (Echinacea), and certain varieties of Boxwood (Buxus) are reliably hardy in this temperature range. Selecting plants rated for Zone 7 would be a risk, as they are not guaranteed to survive the lowest temperatures typical of a Zone 6a winter.
While the hardiness zone dictates what can survive the winter, the average last frost date is a separate but equally important consideration for spring planting. In Lincoln, the average date of the last spring freeze is typically around April 25 to May 1. This dictates the safe timing for setting out tender annuals and vegetable seedlings, as planting before this date risks losing vulnerable plants to an unexpected cold snap.
Local microclimates within Lincoln can further refine the practical application of the Hardiness Zone. Areas close to large, south-facing brick walls or paved surfaces, known as urban heat islands, may be slightly warmer, sometimes effectively acting as a Zone 6b. Conversely, low-lying areas where cold air tends to settle, or spots exposed to strong winter winds, may experience temperatures closer to Zone 5b, necessitating the selection of hardier plants. Gardeners should observe their specific location to determine if they need to plant more conservatively.