Will Weeds Die Without Sunlight?

Sunlight and Seeds

• Not all seeds need the simulation of sunlight to germinate. In fact, some seeds need complete darkness for successful germination. But once seeds become seedlings, they need light, or they will soon die. Germinating seeds, bulbs and even tubers like potatoes, which find themselves in darkness, will put all of their energy into reaching the light because they can’t live without it. If sunlight is effectively blocked, seedlings will nonetheless expend all of their energy trying to reach light. When their energy stores are gone, they die.

Sunlight and Photosynthesis

• Once seedlings or mature plants leave darkness behind, they rapidly unfold or produce green leaves and begin the process of photosynthesis, thanks to the green plant pigment chlorophyll. Starting with inorganic substances, plants produce glucose and use it as a “building block” for many other organic substances crucial for life on earth. But without sunlight, none of the life-building work accomplished by plants -- even weeds – can take place. Blocking sunlight stops photosynthesis, which stops the plants’ ability to produce energy. Without food, plants die.

Blocking Sun: Organic Mulch

• Linda Chalker-Scott of Washington State University Extension suggests a simple form of “lasagna gardening,” or layering organic matter atop lawn or weeds to kill vegetation while composting. She eliminates all the complicated, hard-work steps -- double digging the soil, and then layering cardboard and green organic matter -- and just adds 12 inches of arborist wood chips to let everything “cook.” A very thick layer of organic matter is all that’s needed to cook and kill unwanted weeds, says Chalker-Scott. Gardeners can achieve the same effect with very thick layers of leaf mold, grass clippings and pine straw -- any organic matter that will effectively block the light.

Blocking Sun: Inorganic Mulch

• You can also try inorganic mulch such as black plastic sheeting to block sun and kill weeds. Because water and air cannot permeate this mulch, water the soil thoroughly before laying down the black plastic – so weed seeds will germinate rather than lie dormant. Black plastic and similar inorganic mulches generally work better for newly germinating weeds and annuals because perennial weeds with well-established roots may muster the strength to break through the plastic into sunlight. Black plastic absorbs solar heat and rapidly warms the soil, an advantage before planting vegetables and flowers in spring. But that’s a drawback if you're using it for permanent mulch since the heat will cook the roots of desirable plants, too.