Plants require a complex mix of nutrients, oxygen, water, sunlight and temperature to germinate and grow. If one of these elements changes, the plant reacts in different ways. Most plants can't function in temperatures outside of the range it has adapted to, in order to survive. To protect themselves, many plants stop growth in temperatures at the bottom of their range, and plants will increase growth or reproduce when temperatures get into the top of their temperature zone. Seeds require certain temperatures to sprout to protect young plants from freezing or burning.
The temperature of the air surrounding the plant affects the water inside the plants. When temperatures are hot, water inside plant cells heats up as well, and plants can dry out more quickly than in cooler temperatures. When air temperatures are below freezing, the water inside the plant can turn to ice and break cell walls, killing the plant. Plants like desert succulents are adapted to higher temperatures and have evolved ways to retain water, such as having thick skins around leaves. Other plants like evergreen firs have adapted ways to avoid freezing, such as thick bark or narrow leaves. The country is split into Plant Hardiness Zones to help gardeners understand which plants can grow in certain temperature ranges.
The temperature of the soil affects the way the roots of the plant take in nutrients like nitrogen, which are important for plant growth. When soil temperatures are low, plant roots pull minimal amounts of nutrients from the soil in order to maintain the basic biological functions of the plant and stay alive. When soil temperatures rise, plants begin taking in higher amounts of nutrients to fuel increased growth. For example, studies have shown that the roots of apple trees reduce nitrogen and amino acid uptake from the soil when temperatures are between 4.44 and 18.3 degrees Celsius (40 and 65 degrees Fahrenheit).
Seeds also require very specific conditions in order to germinate and turn into young plants. Just like with plant growth, too hot or too cold temperatures can kill young plants, so seeds have evolved to sprout when temperatures are mild. Most seeds sprout between 23.9 and 29.4 degrees Celsius (75 and 85 degrees Fahrenheit), and some seeds require a long period of cold temperatures before they can sprout. This cold period ensures that seeds will sprout in spring after the dangerous colder temperatures of winter have passed.
Growth and dormancy
The growth cycles of plants are also closely linked to temperatures in the air and soil. To survive extreme temperatures, plants will shut down their biological processes when temperatures become unfavourable for growth. This is called dormancy; plants go dormant usually during winter when temperatures are warmer in the soil than above ground. When they go dormant in winter, plants allow stems and leaves to die to protect root systems in the warmer soil. When temperatures rise, the root systems activate again, fuelling spring growth. During warmer months, plants will grow and produce flowers and seeds. Studies of apple trees showed that increasing temperatures were linked to growth cycles, with periods of colder temperatures preceding budding and growth in the spring, when temperatures warm up.