A plant's life cycle begins with germination, usually recognized by the above-ground appearance of a growing shoot. Mated to this shoot are two small, round leaves known as cotyledons (see photo at right). As these leaves begin manufacturing food, the plant enters its seedling stage of growth. During this time, the plant develops its first set of true leaves, resembling those of the mature plant and the primary formation of a root system begins. The root development that takes place at this time is key to the rate at which the plant will continue to grow. Providing the proper environment for the roots will ensure that your crop will have a chance to flourish. As I have said before, the main advantage of hydroponic systems is how they maintain optimal root health!
Once the root system can support further growth, the vegetative stage begins. Because growth during this period is primarily focused on stem, branch (also referred to as "frame") and foliage, plants need large amounts of nitrogen (N) that is required for the production of chlorophyll. The most substantial growth over the lifecycle of the plant occurs in the vegetative stage, and will continue unless interrupted by a change in the environment or lack of water and nutrients. The final stage of a plant's lifecycle is its reproductive stage. Because the plant's objective is now to reproduce and thus carry on evolution, most of its energies are devoted to the manufacture of flowers, seed, and fruit. The primary nutritional requirements begin to shift at this time from a high-N diet to a low N, high P-K diet (remember our discussion on macro-nutrients!). This is due to a considerable slowdown in vegetative growth while reproduction takes place. This change prompts the gardener to switch his or her nutrient solution from a vegetative formula to a flowering or "bloom" formula. Many hydroponic nutrients now come as a two-part system for exactly this reason. In some plants, reproduction is triggered by a change in the length of daylight, this characteristic is called photoperiodism. It is this characteristic which governs when these plants may be sown and harvested if growing outdoors. If you are growing indoors, be sure to provide the proper photoperiod for your crop or they may never fully develop. Changing the length of artificial daylight can trick the plant into flowering early. For example, commercial growers use this "trick" to deliver flowers to markets out of season, and at premium prices.
If you do plan to grow indoors you may have to play "bee" by pollinating the flowers on your plants manually since the insects that would normally do this in nature will not be there. For tomatoes and peppers, a delicate touch with a brush on each flower will help the plant pollinate itself to produce fruit. There are commercially available "plant shakers" that vibrate the flowering plants every so often to accomplish the same result. I have found that the breeze from a strong circulating fan is often sufficient to cause pollination indoors as well. If this sounds like too much work for you, choose a variety that is bred for the greenhouse as they will usually be of the self pollinating type and require no extraneous effort on the part of the grower.
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