Some plants can live in the cold. 
Some plants are very tough. They can live in the cold.
Some plants are very tough. We call these hardy plants. They can survive bad weather. This includes heat, wind, or floods. Most people use hardiness to talk about cold.
Hardy plants stay healthy in winter. Some stay still and rest. These are called dormant. Carrots and broccoli are hardy. Some bulbs like tulips need the cold to bloom. Other plants are tender. Tender plants die when it freezes.
How do woody plants survive the ice? They use a way called supercooling. This stops ice from forming in their cells. Some plants also dry out their cells. This lets water freeze between the cell walls instead. This keeps the living parts safe.
People use hardiness zones to help them. These zones show where a plant can live. The USDA uses zones in the United States. They look at the coldest average temperature. In the UK, the Royal Horticultural Society has ratings. They go from H1a to H7. H1a is for tropical plants in heat. H7 is for very cold plants like heather.
Plant hardiness describes how well a plant survives bad weather. Most people use this word to talk about cold weather. It can also mean how a plant handles heat, wind, or floods. A plant's hardiness depends on where it naturally grows. This includes its latitude, longitude, and elevation.
Plants use clever ways to stay alive during a freeze. Some woody plants use a method called supercooling. This helps them stop ice from forming inside their living cells. Other plants survive by dehydrating their cells. This allows water to freeze in the spaces between cell walls. This keeps the important living parts of the plant safe from ice.
People have studied these patterns for a long time. In the United States, the USDA created a hardiness zone system. They made it to cover many different places. This includes everything from hot deserts to frozen tundra. In the United Kingdom, the Royal Horticultural Society uses its own ratings. These ratings go from H1a to H7.
Different plants have different needs to stay healthy. Hardy plants like Milkweed can handle extreme temperatures and poor soil. They are helpful because they need very little care. Some plants are called tender because they die in the frost. There are also half-hardy plants used in gardening. These are often planted after the frost has passed.
Hardiness is a lot like how you dress for the weather. You might wear a coat to stay warm in the snow. Plants use their own internal tools to stay safe instead. Farmers and nursery growers even use a process called hardening off. This helps prepare plants for the real weather they will face.
Plant hardiness describes the ability of a plant to survive adverse growing conditions. While hardiness can include tolerance to heat, drought, flooding, or wind, it usually refers to climatic adversity. In temperate latitudes, the term most often describes cold-hardiness. This is generally measured by the lowest temperature a plant can withstand. A plant's hardiness is defined by the geographic location of its native extent. These attributes include longitude, latitude, and elevation.
Plants use specific biological mechanisms to survive freezing temperatures. Woody plants often use a process called supercooling. Supercooling allows them to suppress the formation of ice within their living cells. This helps them survive temperatures as low as –40 °C (–40 °F). Other woody plants survive by dehydrating their cells. This process allows water to freeze in the spaces between cell walls instead of inside the cells. By keeping ice out of the vital living parts, the cells can survive the freeze.
Botanists and gardeners categorize plants based on their level of cold tolerance. Tender plants are those that are killed by freezing temperatures. Hardy plants are those that can survive freezing at least down to certain temperatures. Some gardeners use the term "half-hardy" for specific bedding plants. These are often sown in heat during winter or early spring. They are only planted outside after the danger of frost has passed. There is also a classification for "fully hardy" plants. This term is used specifically by the Royal Horticultural Society.
Different regions use different systems to rate plant hardiness. In the United States, the USDA uses a system based on average yearly minimum temperatures. This system was developed to handle the diverse conditions of the US. It covers everything from baking deserts to the frozen tundra. Another system used in the US is the Sunset Climate Zone system. This system is more specific to local climates, including precipitation, temperature, and humidity. It relies less on the yearly minimum temperature than the USDA system does.
In the United Kingdom and Western Europe, the climate is oceanic. This climate is tempered by the presence of the Gulf Stream. This causes milder winters and cooler summers. Because of this, western Scotland can grow some subtropical plants. This is possible even though Scotland is located far to the north. The Royal Horticultural Society provides a rating system for the UK. These ratings range from H1a to H7. H1a applies to tropical plants that must stay under glass in heat. H7 applies to very cold-tolerant plants like heather.
Hardiness is not just about a plant's species, but also its current condition. A plant's ability to survive may depend on how much stress it is undergoing. Stressed plants often show less cold tolerance than plants that are well maintained. The speed of weather changes also matters. A plant might die if the weather changes from balmy to exceptionally cold very quickly. To help plants prepare, nursery growers use a process called cold hardening. This is also known as "hardening off." It prepares the plants for the likely conditions they will face in later life.
Understanding hardiness is essential for agriculture and horticulture. Many plants are chosen for being hardy because they require little maintenance. For example, Milkweed and Cranesbill can tolerate extreme temperatures and poor or eroded soils. Winter-hardy plants are also necessary for maintaining winter gardens. This group includes many cultivated plants like cabbage, broccoli, and all kinds of carrots. Some bulbs, like tulips, actually need cold winters to bloom. Others, like freesia, can survive a freezing winter.
Hardiness also connects to other environmental factors like wind. Wind creates mechanical stress and can make a plant feel colder and dryer. Plants may adapt to wind by changing their size. Some become smaller to avoid the wind, while others become bigger to resist it. The mechanical force of wind also causes a cascade of signals within the living plant. While researchers are still learning about wind, they know it is a major factor in how plants survive.
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