Winterizing a raised garden bed involves preparing the soil for the dormant season to maintain its structure and fertility. This preparation protects the exposed soil surface from harsh elements, preventing damage to the delicate soil ecosystem and loss of nutrients. Gardeners often use plastic sheeting as a readily available and inexpensive material for this purpose.
Deciding If Plastic is Necessary
The decision to use plastic depends entirely on the local climate and the specific winterization goal. In regions with consistently high winter rainfall or heavy snowmelt, plastic can be beneficial by protecting bare soil from leaching and erosion. Without a cover, water percolating through the soil can wash away soluble nutrients, like nitrogen, which are valuable for the next growing season.
A secondary benefit of using plastic, particularly clear or black sheeting, is the potential for passive solarization during warmer winter days. This temporary heat buildup can help suppress persistent weed seeds and pathogens near the soil surface. The plastic also helps retain consistent soil moisture, which benefits the dormant microbial communities. This method is most effective when the soil is left completely bare and the primary concern is managing excessive moisture or actively suppressing weeds before spring.
Hazards of Improper Plastic Use
Using an impermeable plastic cover without careful management introduces significant risks to the soil’s biology and physical structure. The most serious concern is the creation of anaerobic conditions, where the lack of oxygen can harm beneficial soil microorganisms. If the plastic is sealed too tightly, or if the soil is already saturated, water cannot evaporate, leading to stagnant, waterlogged conditions within the bed.
Excessive trapped humidity under the plastic promotes the growth of undesirable fungi and molds. This moist, sealed environment can become a breeding ground for plant diseases ready to emerge in the spring. Another potential issue is the risk of “cooking” the soil if a period of strong solar gain occurs. Plastic greatly amplifies solar heat, and this intense temperature spike can eliminate beneficial soil microbes and disrupt the delicate balance of the soil food web. Finally, prolonged exposure to ultraviolet (UV) light can cause low-quality plastic to degrade, resulting in microplastic fragments contaminating the garden soil.
Non-Plastic Winter Protection Options
When the goal is to protect the soil without the risks associated with non-breathable plastic, several organic alternatives offer benefits. Applying a thick, uniform layer of organic mulch, approximately three to five inches deep, insulates the soil effectively. Materials like clean straw, shredded autumn leaves, or aged compost are excellent choices for this purpose.
These organic mulches decompose slowly over the winter, adding organic matter and replenishing soil nutrients naturally. They insulate the soil to stabilize temperatures, which helps prevent the freeze-thaw cycles that can heave small plants or damage soil structure. Unlike solid plastic, these materials remain permeable, allowing for gaseous exchange and preventing the excessive buildup of moisture that leads to anaerobic conditions. Another effective alternative is to use woven landscape fabric or specialized permeable row covers, such as agricultural fleece, which offer a physical barrier while still allowing air and moisture vapor to pass through.
Practical Steps for Covering Raised Beds
If a gardener determines plastic is the best choice for their specific situation, such as for a short-term solarization project, proper installation is necessary to mitigate the risks. Begin by ensuring the raised bed has excellent drainage pathways to prevent water accumulation beneath the cover. The plastic sheeting should extend beyond the edges of the bed and be secured tightly with heavy objects like bricks, rocks, or soil to prevent wind from lifting or tearing the material.
For simple ground cover, black plastic is useful for blocking light and suppressing weeds, while clear plastic is more effective for generating heat to warm the soil or perform solarization. It is important to create a slight air gap between the soil and the plastic to allow for some circulation and moisture release. The cover should also be lifted and vented periodically during mild spells to allow trapped humidity to escape and to refresh the oxygen supply in the soil.