March 13, 2013

Let there be light

Plants need full-spectrum light from the sun to undergo photosynthesis – the process a plant uses to convert energy from sunlight to chemical energy. When starting plants indoors, it is important to replicate the light that the plants would be receiving if grown outdoors.

The least expensive lighting method is to place seedlings in a south-facing window (most sun exposure). However, this is not a sufficient amount of light to support the growth of strong plants since seasonal changes, cloud cover, and window treatments can affect the amount of sunlight that enters the window. This indirect light results in weak and spindly looking seedlings that reach and bend for light.

Many gardeners use artificial lights to promote plant growth. Garden catalogs display several different grow stands with a wide range of costs, depending on capacity and complexity. However, the average home gardener can make their own effective grow stand using shop lights, fluorescent bulbs, and a shelf. 

Factors that affect the rate of photosynthesis are the type (quality) of light, intensity of light, and duration of light. It is important to understand how to regulate these factors when growing transplants indoors under artificial light.

Fluorescent bulbs come in cool, warm, and full-spectrum lighting. Cool bulbs produce a bluish cast of light while warm bulbs produce a reddish-orange cast of light. Full-spectrum lights produce a balance of blue and red light that is more similar to natural sunlight; however, full-spectrum lights are more expensive and do not last as long standard fluorescent bulbs. An excellent alternative to full-spectrum bulbs is to grow seedlings under a two bulb fluorescent fixture using one cool and one warm fluorescent bulb. Together, these bulbs will provide sufficient light to support the growth of strong and stocky plants.

The intensity of light that a plant receives is determined by the wattage of the bulb and by how close the plant is to the light source. Lights that are placed too close to plants will burn them; lights that are placed too far from the plants are less effective for promoting growth. It is best to suspend the light so that is it 3-4 inches from the top of the plants. As the plants grow, the light will need to be adjusted so it is better to suspend the lights from adjustable cables/chains. Since fluorescent bulbs emit the most intense light in the center of the bulb, it is important to rotate the plants 1-2 times per week to ensure that all plants get an even amount of light.

The length of time that light is available will determine how the plant grows. Most plants require 14-16 hours of sunlight a day to provide sufficient fuel needed for the plants to grow stocky stems with an abundance of leaves. Less than 12 hours of sunlight will signal the plant that winter is coming and to focus its energy on flower production rather than green growth. More light is not necessarily better. Plants need rest too!  Do not provide more than 18 hours of light to plants growing indoors. As important as light is for plant growth, plants also need at least 6-8 hours to respirate – another important part of a plants growth process.

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