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By removing the plant for at least two years, the life cycle can be interrupted, thus controlling the pest. Both organic and synthetic insect-killing materials, called insecticides, also are available to control insect pests (see Pest Control). Diseases caused by fungi, bacteria, or viruses also can damage plants (see Diseases of Plants). In most cases, once a plant has a disease it cannot be saved, though some fungal diseases can be controlled with a fungicide. The best approach to disease prevention is to provide plants with optimum soil, nutrients, light, and water so they can fight off disease, and to grow plants that have been bred for disease resistance or have natural resistance. Gardeners harvest plants at different stages, depending on how the harvested plants or plant parts are used. Crops grown for their fruit, such as tomatoes and eggplant, are harvested when the fruit is ripe. Some plants are harvested before they flower—lettuce and spinach, for example, are grown for their tender leaves and develop a bitter flavor if allowed to flower. Plants grown for their roots, such as carrots and radishes, are harvested when the root is large enough, but before it gets tough or woody and loses its sweetness. In flower gardens, the sign of maturity is the formation of seeds. Many plants stop flowering once they set seed, so to make plants produce flowers longer, gardeners can remove the faded flowers before they produce seed, a technique called deadheading. Gardeners may attempt to control the shape of woody plants, such as trees and shrubs, by removing, or pruning, branches growing in the wrong direction. They also prune to removed damaged, disease, or dead branches. Some shrubs, such as lilacs, bear the most flowers in young wood, so gardeners remove the oldest branches. Gardeners prune plants at different times of the year, depending on how they hope to affect the plant’s growth. In regions where winter is characterized by freezing temperatures, the gardening season ends in autumn. In these areas, autumn tasks for vegetable gardeners include removing old plants from the garden. This helps eliminate insects and disease organisms that may survive the winter in dead leaves or flowers. To prepare the soil for the following spring, gardeners may work manure or other fertilizer into the soil, or they may add compost, partially decayed organic material that improves water drainage in the soil. Flower gardeners also may remove annual flowers from the garden to control insects and disease. To preserve perennials, gardeners cover the ground with a deep layer of mulch to prevent the soil from thawing too early. Soils that repeatedly freeze and thaw during the winter can push the roots of perennial plants out of the soil, a process called heaving. Some gardeners try to extend the growing season by protecting vegetables and annual flowers through the first mild frosts of autumn. One popular method is to cover the plants with plastic or bed sheets when frost threatens, which traps heat that the soil gives off at night and keeps frost from settling on the plants. Another technique is to surround the plants with milk jugs or other containers filled with water. As the water freezes it gives off heat, and the slight increase in temperature is enough to protect the plants. In late winter or early spring, before new growth begins, gardeners may cut back the brown leaves of ornamental grasses and the dead branches of perennials so the plants will look tidy as the growth begins again. It also binds the mineral nutrients so they remain in the soil and are not washed away. The spaces, or pores, between the tiny particles of minerals and organic matter are occupied by either water or air. Water holds vital dissolved nutrients, while air provides the roots with oxygen. Most plants do best in a soil in which half the pore space is filled with air and half with water. Plants use nutrients obtained from soil to build the cells and tissues needed for growth. Nutrients that plants need in large amounts, called macronutrients, include oxygen, hydrogen, carbon, and an array of minerals. They also need micronutrients, or trace nutrients, which consist of cobalt, chlorine, boron, iron, zinc, molybdenum, nickel, manganese, and copper. Soil texture, the size of the individual soil particles, affects how fast water drains and how well plants absorb nutrients. The largest soil particles are grains of sand. Sand grains fit loosely together with large gaps between them, resembling marbles in a jar. The large pores let water (and the nutrients dissolved in it) drain out too quickly for most plants to absorb it. Clay particles, on the other hand, are very tiny, and they pack closely together, resembling tiny beads in a jar. The pores between clay particles are so small that water drains very slowly. Slow drainage can lead to oxygen deprivation because the water takes the place of air in the pores. Another disadvantage of clay is that it binds water and some nutrients so tightly that most plants cannot absorb them. A third soil particle is silt, which is larger than clay but smaller than sand. Most plants thrive in a soil type known as loam, which contains roughly 50 percent sand, 25 percent clay, and 25 percent silt. A loam soil drains water well, but not too quickly, and as a result, the plant can absorb nutrients more readily. Exceptions include desert plants, such as cacti, which do best in a sandy soil, and cottonwoods, which flourish in silty soils. Plant nutrient absorption is also dependent on a soil's pH, a measure of acidity or alkalinity. The minerals from which a soil formed and the amount of rain that passes through the soil determine its acidity or alkalinity. Pure water has a pH of 7.0, which is neutral—neither alkaline or acidic. Soils with a pH of less than 7.0 are acidic, those with a pH greater than 7.0 are alkaline. Most plants absorb nutrients best in a soil with a pH between 6.5 and 7.5; however, plants such as rhododendrons require an acidic soil, while others, such as lilacs, grow better in an alkaline soil. Acidic soils are more common in the eastern half of the United States, where rainfall is plentiful, while alkaline soils are more common in the drier west. Not all soils have enough nutrients or the right balance of nutrients. In addition, plants remove nutrients from the soil as they grow, so these nutrients must be replaced in order for the soil to remain productive. For these reasons, gardeners enhance soil by adding fertilizer, a material that contains one or more of the nutrients plants need.
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