Light deprivation is great low-to-no cost way to control some weeds in your garden. It’s done by covering an area with black plastic sheeting, landscape fabric, scrap sheet metal, a tarp, old carpet, or other kinds of light barriers for a few months. Alternatively, you can use carbon rich materials like paper and cardboard held in place with heavy objects or covered with mulch.
The idea is that if the weeds underneath the cover can’t access sunlight, they can’t photosynthesize. If they can’t photosynthesize, they can’t do the necessary maintenance to keep their roots, stems, and leaves in good working order. Over time, overall plant health will start to decline. In that weakened state, insects and soil microlife will begin to nibble on those weeds until they eventually die.
It’s a little gruesome to imagine in detail. But the method is really effective on fast-growing weeds with fibrous root systems or rot prone crowns. Unfortunately, there are also many weeds that can wait out short-term light sieges.
Light Seige Resistant Weeds
Here are a few examples of weeds that can’t be easily killed with light deprivation.
- Herbaceous perennial weeds with the capacity to go dormant (e.g., cold hardy or drought hardy plants) can use their dormancy aptitudes to survive without light for more than 6 months.
- Weeds with deep storage roots or tubers can survive a light siege by continuing to do critical maintenance using those nutrients stored in their underground plant parts for years.
- Weeds that spread quickly by rhizomes can extend beyond light blocking barriers and continue to feed the root systems underneath the light blocking barrier indefinitely.
For these light-seige adapted survival weeds, targeted digging or repeat mowing to weaken root systems, followed by long-term light deprivation are necessary.
Weed Seed Light Blocking
Light deprivation is also used to prevent certain weed seeds from germinating. This is normally done by applying a few sheets of paper followed by several inches of compost, garden soil, or other mulches to bury weed seeds.
This method is really effective against seeds that require light or that germinate only at shallow planting depths. It won’t prevent deep-germinating seeds from starting. However, by increasing the soil depth they have to grow up through, those weeds will be weaker when they emerge. Then, you can cover them with a big rock or repeat mow them down to finish the job.
Solarization vs. Light Deprivation
Just one cautionary note: Be careful not to confuse light deprivation with solarization.
Solarization is an in-ground method of pasteurizing your soil used only to treat severe cases of fungal pathogens. It’s done over a long period of time, including the warmest months of the year, using tightly applied clear or black plastic that scorches the soil. Solarizing is detrimental to soil life and often causes enormous amounts of weeds within a few months of removing the plastic sheeting.
Light deprivation, by contrast, should not interrupt the airflow to soil or cause the soil to heat up. The soil below should also be kept moist so it doesn’t overheat in dry hot weather.
Light deprivation kills plants while beneficial insects and microlife in the soil remain unharmed. In some cases, light deprivation even increases soil life populations as they feast on the dying plants.
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