Plants need sunlight to conduct the carbohydrate-making process called photosynthesis in their green leaves and stems. Seeds do not contain green pigment, but merely a dormant embryo. Seeds do not need to bask in sunlight to germinate. However, the warmth from sunlight can create a soil environment more conducive for germination. The requirements for seed germination vary among all plant species. Usually, direct sunlight is harmful to the process of germination and initial development of the sprout.
Regardless of plant species, the seed needs three basic requirements to germinate. It needs soil moisture to cause the embryo to swell and produce its first root and leafy shoot. The seed also must have access to air to facilitate gas exchange as the embryo initially sprouts from the seed coat. Lack of air around soil particles and the seed can lead to rot. The seed also needs a conducive soil temperature to grow. Depending on the plant species, some seeds sprout only in cool soils under 60 degrees Fahrenheit, while other plants sprout only in warmer soil.
Why Sun Isn't Needed
Seeds do not need to be exposed to sunlight in order to germinate. Inside the seed there are minute amounts of carbohydrates to provide the nourishment to sustain germination and initial growth of the tiny plant. Sunlight isn't required until the seedling breaks through the soil with its leaves, which naturally produce chlorophyll and become green once exposed to sun rays. Some species of plants, however, may germinate better if the seeds are exposed to light of any kind. The light triggers receptors in the seed to germinate. For example, the length of daylight can be the catalyst for the seed to sprout, as it is an indicator for the season of year. Some plants have other triggers, such as exposure to winter cold or intense heat from a fire -- both allow the penetration of moisture after the hard seed coat is finally cracked open.
Harmful Sunlight Effects
Direct sun rays on the soil around or above a seed can make the environment less conducive for germination. The primary concern is the drying effect of sunlight on the top 1/2 to 1 inch of soil. Sunlight can offset the soil moisture levels around a shallowly planted seed. In some species of plants, direct sunlight can warm the soil to levels that inhibit germination. This is particularly true in summer, when the sun's rays are more intense and warm or dry the soil much more quickly than during fall, winter or spring.
Unless a seed packet or germination instructions for a plant specifically provide instructions, assume the seeds do not need to be planted in soil that is basking in sunlight. Only once the seedlings sprout from the soil is light needed, as it promotes growth of leaves. Direct sunlight may not be best initially, as it could dry out the soil and kill the sprouts. Bright indirect light may prove best until the seedlings display green pigments and their first unfurled leaves.
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