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The relationship between light and plant growth is complex. Some seeds will germinate only in light, some in darkness and for some the requirement for light is related to the temperature. Once a plant starts to grow it does require some light, but this is not a given amount for all plants. You can infer what a seed is likely to need based on where it grows naturally.
Light and Germination
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Some seeds just need exposure to a certain number of foot-candles before they will germinate. A brief period of bright light is the same as a longer period of lower levels of light. Others have more specific light requirements. Some are even sensitive to wavelength. As a general rule, small seeds require light to germinate. You should plant light-requiring seeds on the surface of the soil and press them in, not cover them.
Light and Temperature
Some seeds are sensitive to a combination of light and temperature for germination. These are often alpine plants such as Primula denticulata. If the temperature is warm, the seeds will sprout in any light. However, if the temperature is cooler than normally required for germination, the seeds will sprout if there is sufficient light. The additional light breaks the dormancy of the seed, assuring that the plant will have time to mature before the brief alpine growing season is over.
Seeds Requiring Darkness
Some seeds require darkness to germinate. These are generally large seeds with thin seed coats, although there are exceptions. Both garden peas and marijuana seeds require darkness. Fewer seeds require darkness than those that require light. These seeds should be planted more deeply when they are planted or, if they are planted in a seed tray, the tray should be covered with something that will keep out the light until the seeds have sprouted.
Growth of Seedlings
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Once the seeds have germinated, the combination of light and temperature determines their growth. These requirements can also be inferred from where they grow naturally. If there is not enough light, the seedlings will grow taller but will be thin and leggy. They will be less likely to survive transplanting. If there is too much light for plants that naturally grow in low light, they will be weak and begin to burn and turn brown around the edge of the leaves.
- Royal Tasmanian Botanical Garden; Seed Germination; July 2009
- Plant Physiology; Seeds and Seed Germination; Ross Konig; 1994
- "Sowing a Better Garden"; John Kelly; 1988
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