Sunlight supplies the complete range of wavelengths and can be broken up by a prism into bands of red, orange, yellow, green, blue, indigo, and violet. Blue and red light, which plants absorb, have the greatest effect on plant growth. Blue light is responsible primarily for vegetative leaf growth. Red light, when combined with blue light, encourages flowering. Plants look green to us because they reflect, rather than absorb, green light.
Knowing which light source to use is important for manipulating plant growth. For example, fluorescent cool white light is high in the blue wavelength. It encourages leafy growth and is excellent for starting seedlings.
Incandescent light is high in the red or orange range, but generally produces too much heat to be a valuable light source for plants. Fluorescent grow-lights attempt to imitate sunlight with a mixture of red and blue wavelengths, but they are costly and generally no better than regular fluorescent lights. Duration, or photoperiod , refers to the amount of time a plant is exposed to light. Photoperiod controls flowering in many plants Figure Scientists initially thought the length of light period triggered flowering and other responses within plants.
Thus, they describe plants as short-day or long-day, depending on what conditions they flower under. We now know that it is not the length of the light period, but rather the length of uninterrupted darkness, that is critical to floral development. Plants are classified into three categories: short-day long-night , long-day short-night , or day-neutral, depending on their response to the duration of light or darkness.
Short-day plants form flowers only when day length is less than about 12 hours. Many spring- and fall-flowering plants, such as chrysanthemum, poinsettia, and Christmas cactus, are in this category.
In contrast, long-day plants form flowers only when day length exceeds 12 hours. Most summer flowering plants e. Day-neutral plants form flowers regardless of day length. Examples are tomato, corn, cucumber, and some strawberry cultivars. Some plants do not fit into any category, but may respond to combinations of day lengths. Petunias, for example, flower regardless of day length, but flower earlier and more profusely with long days.
You can easily manipulate photoperiod to stimulate flowering. For example, chrysanthemums normally flower in the short days of spring or fall, but you can get them to bloom in midsummer by covering them with a cloth that completely blocks out light for 12 hours each day. After several weeks of this treatment, the artificial dark period no longer is needed, and the plants will bloom as if it were spring or fall. This method also is used to make poinsettias flower in time for Christmas.
To bring a long-day plant into flower when day length is less than 12 hours, expose the plant to supplemental light. After a few weeks, flower buds will form. Temperature influences most plant processes, including photosynthesis, transpiration, respiration, germination, and flowering.
As temperature increases up to a point , photosynthesis, transpiration, and respiration increase. When combined with day-length, temperature also affects the change from vegetative leafy to reproductive flowering growth. Depending on the situation and the specific plant, the effect of temperature can either speed up or slow down this transition.
The temperature required for germination varies by species. Generally, cool-season crops e. Sometimes horticulturists use temperature in combination with day length to manipulate flowering. For example, a Christmas cactus forms flowers as a result of short days and low temperatures Figure If temperatures are high and days are long, cool-season crops such as spinach will flower bolt. However, if temperatures are too cool, fruit will not set on warm-season crops such as tomato.
Low temperatures reduce energy use and increase sugar storage. Thus, leaving crops such as ripe winter squash on the vine during cool, fall nights increases their sweetness. Adverse temperatures, however, cause stunted growth and poor-quality vegetables. For example, high temperatures cause bitter lettuce. Thermoperiod refers to daily temperature change. Plants grow best when daytime temperature is about 10 to 15 degrees higher than nighttime temperature.
Under these conditions, plants photosynthesize build up and respire break down during optimum daytime temperatures and then curtail respiration at night. However, not all plants grow best under the same range between nighttime and daytime temperatures. Temperatures higher than needed increase respiration, sometimes above the rate of photosynthesis. Thus, photosynthates are used faster than they are produced.
For growth to occur, photosynthesis must be greater than respiration. Daytime temperatures that are too low often produce poor growth by slowing down photosynthesis. The result is reduced yield i. Some plants that grow in cold regions need a certain number of days of low temperature dormancy. Plants require mostly blue and red light for photosynthesis, but for flowering, infrared light is also needed.
Incandescent lights produce mostly red and some infrared light, but very little blue light. Fluorescent lights vary according to the amount of phosphorus used by the manufacturer. Cool-white lights produce mostly blue light and are low in red light; they are cool enough to position quite close to plants. Foliage plants grow well under cool-white fluorescent lights, while blooming plants require extra infrared light.
This can be supplied by incandescent lights or special horticultural fluorescent lights. Most plants tolerate normal temperature fluctuations. In general, foliage plants grow best between 70 degrees and 80 degrees F. Most flowering plants prefer the same daytime temperature range, but grow best when nighttime temperatures range from 55 degrees to 60 degrees F.
Lower nighttime temperatures help the plant: recover from moisture loss, intensify flower color and prolong flower life. Excessively low or high temperatures may cause: plant stress, inhibit growth, or promote a spindly appearance and foliage damage or drop. Cool nighttime temperatures are actually more desirable for plant growth than high temperatures. A good rule of thumb is to keep nighttime temperatures 10 to 15 degrees lower than daytime temperatures.
Atmospheric humidity is expressed as the percentage of moisture to air. This is important to plants in modifying moisture loss and temperatures. The same covering prevents radiative heat loss at night. The cover will also help prevent weeds and water loss from the soil.
You can do things to the soil itself. Light tilling will break up the soil and help promote excess water drainage. Changing the soil consistency by mixing in other soil types can help. You can use raised beds, which run warmer. And for some warm-season plants like tomatoes, you can start them inside and transplant them out when the soil is warmer and the threat of frost is past. Smart Gardening. Garden Maintenance. Book Reviews. Beneficial Wildlife. Edible Gardening.
Fruiting Plants. Vegetable Plants. Planting Edible Gardens. Tending Edible Gardens. Container Gardening. Container Plants. Container Design. Garden Design. Lack of sufficient water causes droopiness or wilting in plants. However, excess water can also cause wilting. Plants cannot perform photosynthesis in the absence of sunlight. If photosynthesis does not occur, plants cannot prepare starch, and they eventually die. Autotrophic plants contain a green pigment called chlorophyll, which is necessary for photosynthesis.
Chlorophyll traps heat from sunlight and initiates photosynthesis. Plants grow in fertile and nutrient-rich soil. Plants cannot grow in infertile soil because there are no nutrients for the plant to feed on, so there is nothing to promote growth and maintenance of the plants. Depending on their habitat, different plants require different types of soil to grow.
For instance, cactus grows well in sandy soil. Each soil type differs in its nutrient content and water-retention capacity. Decomposition of fallen leaves, animal and bird droppings, and dead animals and birds enrich the soil with organic matter. This replenishes the nutrient content of the soil periodically. When cultivating plants for agriculture and indoor use, people often add fertilizers or compost to soil to increase its nutrient content.
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