Filling a raised garden bed

Asked April 8, 2020, 9:02 AM EDT

I live in Ocean Pines, MD near the shore. I just built a new 1' high raised garden bed that I am filling with materials that I have around our home. The bed is placed on gravel (with a couple inches removed from the inside of the bed). I placed a thin layer of cardboard and about 1-2" of rotting wood in the bottom to help fill the bed. Is it okay to add boxwood clippings and leaves to this next layer to help fill the bed? I'm saving bagged "garden soil" and peat moss (as well as hoping to get some compost) for the top growing area. I will be planting mostly leafy greens this season. I know I'll need to add a bunch of compost next year.

Worcester County Maryland

1 Response

We have information published on raised bed construction and filling; here is the page on soils: https://extension.umd.edu/hgic/topics/soil-fill-raised-beds. You want as uniform a soil as possible in terms of particle size and consistency in order for water to drain evenly through the soil and not leave certain layers or pockets too wet or dry.

Raised beds are so shallow compared with in-ground growing that the entire depth of the bed should be devoted to growing space (i.e., room for roots and soil) rather than including any "fillers." The smaller the volume of soil, the more competition the plants will experience, the faster the soil will change temperature (not ideal for warm soil lovers), and the faster it will dry out. Cardboard can be useful for smothering lawn or a bed of weeds upon which a raised bed were being built, but should not be left in place, otherwise as it breaks down slowly and impedes water and oxygen availability to roots and beneficial soil life.

Rotted wood can be useful organic matter, but still-decaying wood is risky to use as it will cause the settling and collapse of patches of soil as it degrades over time. This leaves roots with less space to grow in and may bury plant stems as water and wind re-levels the soil, leading to stem infections or plant death for sensitive plants. Planting on top of buried woody debris is similar to the practice of hugelkultur, but that is done on a much larger scale, and has its share of challenges and few advantages over level-ground gardening. It would be best to fully compost your wood instead before adding it to this bed. Plant clippings also should be composted first; while they could be used as a compost-in-place mulch, this may be more difficult in a vegetable garden if small plants are installed and the clippings aren't of uniform size. (You don't want bulkier woody plant clippings to be blown around on top of vegetable seedlings, for instance, which could cause them serious damage.) Dried leaves are probably less problematic as a topdressing, especially if they can be chipped into smaller pieces by a mulching-style lawnmower.

Topsoil and compost are the main components of raised bed fill. You can probably have bulk or bagged soils delivered at present.

Miri