In the Zone: Understand Climate and Hardiness
The climatic conditions that affect plants most are rainfall, extremes of temperature, wind strength and direction, and light levels. While most gardens can be manipulated in one way or another to overcome prevailing conditions, the predominant climate—the macroclimate—will always influence plant selection.
The Effects of Cold
The length and severity of winter often dictate what you can grow. You can water in dry spells, or boost drainage where excess moisture is a problem, but it is harder to keep temperatures above damaging levels. Hardiness is vitally important, and it is worth understanding the different ways cold can affect plants. An air frost—when the air temperature drops to 32°F while the ground remains unfrozen— is most common in early fall.
A ground frost occurs when the soil temperature falls below freezing; it is more frequent in winter, and can occur while the air is above freezing. Long winter freezes largely define which plants are hardy in your climate. Less severe frosts tend to happen under cloudless skies on still nights, and are caused by the ground losing warmth by radiating it out into the night sky. If conditions persist, the ground freezes solid. Harder frosts are caused by a moving mass of cold air, with icy winds driving frost deep into the soil. Evergreen plants keep losing moisture from their leaves in winter, especially in windy weather. With the soil frozen, they cannot take up more water; the foliage and green stems are scorched and killed, often leading to death.
Soil and Moisture
Too much soil moisture in winter or too little in summer is a common theme of gardening life. Walls, fences, and hedges can affect soil moisture by casting “rain shadows.” Dig down to check the depth of the dryness, as this will determine what you plant. Many plants actually prefer good drainage and will thrive in dry soil; incorporate plenty of moisture-retentive organic matter before planting.
It is much harder to improve damp soil. If your garden is soggy, create well-drained spots by building raised beds, or by planting on ridges and mounds. Otherwise, try to improve drainage as much as possible and choose plants that will tolerate wet conditions.
What Is Hardiness?
Hardiness is a plant's ability to withstand the local climate. In an area subject to frost, this means resistance to cold; elsewhere it means resistance to heat or drought. Knowing where a plant comes from does not automatically tell you if it will survive in your garden; lavender and rosemary, for example, grow naturally on well-drained land in a warm climate, but will thrive in moister conditions and cooler climates.
- Hardy plants will survive the levels of frost we can usually expect. Most plants we call hardy make new growth when temperatures exceed 40-43°F; if temperatures stay around this point, there may be little growth.
- Half-hardy plants might not make it through severe weather. In a cool climate, select spots with sheltered microclimates, mulching, providing good drainage and soil structure, and and protecting with horticultural fleece and straw in winter.
- Tender plants may not survive in temperatures below 45°F. These plants have no real chance of surviving periods of severe weather, and need to be brought into shelter, such as a greenhouse, as soon as there is a threat of frost.
Large countries and continents are divided into hardiness zones based on average winter temperatures.