Raised garden beds offer a streamlined approach to gardening, providing better control over soil quality and drainage. The first challenge involves the existing ground cover, whether it is a lawn or weeds. Proper site preparation beneath the frame is a fundamental step that dictates the long-term health and maintenance of the bed. Addressing what to do with the grass beneath the structure is necessary for successful gardening.
Deciding Whether to Dig or Suppress
The necessity of physically removing existing grass depends on three variables: the type of grass, the depth of the bed, and the gardener’s timeline. Shallow beds, typically under 12 inches deep, are vulnerable to grass and weeds growing up into the cultivated soil. In these cases, the upward growth pressure from the underlying turf is significant, making an aggressive preparation method more effective. Deeper beds (18 inches or more) apply more weight and distance, which helps smother the vegetation.
The specific species of grass is the most important factor in this decision. Perennial running grasses, such as Bermuda grass or quackgrass, are highly tenacious due to their extensive network of rhizomes or stolons. These systems allow the plant to regenerate from small fragments, meaning they will likely push through new soil and barrier material. For these aggressive warm-season grasses, physical removal is often the most reliable way to guarantee eradication.
Cool-season grasses like fescue or ryegrass lack the deep, spreading rhizome structure and are much easier to manage. Similarly, thin annual weeds can generally be suppressed without digging. Digging allows for immediate bed setup if planting must occur right away. Suppression methods require a waiting period of several weeks or months for the underlying vegetation to fully die off.
Methods for Site Preparation and Grass Removal
For situations requiring immediate planting or where aggressive grass species are present, physical removal, or sod busting, is the most direct approach. This method involves using a sharp spade or a sod cutter to slice beneath the root system of the grass, typically at a depth of one to two inches. It is important to ensure all root material and runners are completely removed from the perimeter of the bed area to prevent regrowth. While highly effective and fast, this process is physically demanding and removes a portion of the native topsoil, which must be accounted for when filling the bed.
Alternatively, non-physical suppression techniques use light and oxygen deprivation to kill the vegetation in place, allowing the organic matter to decompose and enrich the soil beneath the bed. Sheet mulching, sometimes called lasagna gardening, is a popular suppression method that involves layering biodegradable materials directly over the grass. This typically starts with several layers of wet cardboard or six to eight sheets of newspaper, which block sunlight. This base is then covered with organic materials like compost and mulch, which begin the decomposition process.
Solarization is another suppression method that uses heat to sterilize the soil, which is particularly effective during the warmest months. This involves covering the area with a clear plastic sheet, which traps solar radiation and superheats the soil to temperatures that kill grass and weed seeds. Solarization may kill grass in several weeks, but requires high ambient temperatures to be effective. For both methods, the required waiting period is the trade-off for avoiding the labor of digging.
Choosing and Installing a Weed Barrier
After the existing grass has been either physically removed or thoroughly suppressed, the final step involves installing a bottom barrier before filling the bed with soil. The primary purpose of this layer is to prevent any remaining weed seeds or grass roots from growing up into the new, nutrient-rich soil. It also helps separate the native subsoil from the imported garden mix and maintains good drainage.
The most common choice is cardboard, which is readily available and decomposes completely within four to six months, allowing earthworms to move freely between the bed and the native soil. When using cardboard, remove all tape and glossy print, as these materials do not break down easily.
Landscape fabric offers a more durable, longer-lasting barrier that allows water to pass through. However, it can restrict the movement of beneficial soil organisms like earthworms and may hinder the growth of deep-rooted plants.
For installations in areas with burrowing pests like gophers or voles, a layer of hardware cloth or wire mesh is recommended. This metal mesh acts as a physical deterrent against animals but does not function as a weed barrier, so it is often paired with cardboard or fabric. Regardless of the material chosen, the barrier should be laid flat, with edges overlapped by several inches to prevent gaps, before the bed is filled with the growing medium.