October 17, 2026

Which fill method is best for filling raised garden beds

The cheapest way to fill a raised bed is a bulky organic base, either logs or layered cardboard and compost, under a uniform mix on top. All bagged mix costs the most and settles the least, so it suits short beds and gardeners who want to plant this week. The base you choose changes how far the bed sinks and how it dries. A probe in each bed shows you the difference.

How much fill a bed needs

Multiply length, width, and depth in inches, then divide by 1,728 to get cubic feet. Our raised beds measure 94 by 39 by 24 inches, which works out to about 51 cubic feet each. That is close to 1.9 cubic yards per bed. Filling that volume from bags adds up fast, which is why most people with deep beds use a cheaper base. Plan on topping up whichever fill you choose, because microbes eat organic matter and the bed shrinks as they do.

All bagged mix

Bagged raised bed mix is usually peat or coir, composted bark, and compost, blended to one texture. Water moves evenly through a uniform mix, because no layer boundary stops it on the way down.

• Cost. You buy every cubic foot, so this is the most expensive option for a deep bed. It is also the fastest, with nothing to collect or wait on.
• Settling. Fine particles pack down after the first few waterings, and the compost in the blend slowly breaks down. Expect a modest drop in the first season and a top-up each spring.
• Drying. The bed dries from the top down at an even rate. Peat-heavy mixes repel water once they dry out completely, so a neglected bed can shed the next watering down its sides.

The ratio of compost to bulk material shifts how fast a bagged bed dries. We cover that in our guide to raised bed soil mix ratios.

A log base (hugelkultur)

Hugelkultur puts logs, branches, and twigs in the bottom of the bed, then covers them with soil and compost. Wood costs nothing if you have trees or a neighbor taking one down.

• Cost. This has the lowest cost per cubic foot, since wood fills the bulk and you buy only the top layer. You spend the savings in hauling and cutting.
• Settling. Wood rots over several years, and the gaps between logs collapse as it softens. The surface sinks unevenly, so one corner can drop while the rest holds.
• Drying. Fresh wood starts dry and pulls water from the soil around it, while soft rotted wood later holds water like a sponge. Air gaps between logs break the capillary path, so the soil on top dries like a shallower bed until the wood breaks down.

Decomposing wood also ties up nitrogen. The fungi and bacteria breaking down high-carbon wood draw nitrogen from the soil around them, so plants rooted near the wood can yellow in the first year. Keep the wood deep and make the top layer rich in compost.

Cardboard and compost layers

Layered fill, often called lasagna gardening, starts with plain cardboard on the ground to smother grass. Alternating layers of leaves, straw, grass clippings, and compost go on top. A compost-rich mix finishes the surface.

• Cost. This is cheap when the leaves and clippings come from your own yard. The cost rises with how much finished compost you buy for the top.
• Settling. Leaves and straw shrink more than any other fill as they rot. A bed filled to the rim in fall can sit well below it by spring.
• Drying. Compost and decayed leaves hold water well, so the root zone stays moist longer. Each boundary between a fine layer and a coarse one holds water above it until the fine layer saturates, which can leave a wet band sitting over a dry one.

Cardboard softens once it stays damp, and roots grow through it within a season. Our guide to compost for raised beds covers how much to blend into the top layer.

What goes in the bottom of any bed

Over grass or weeds, a layer of cardboard is enough. Over burrowing pests, hardware cloth fastened to the frame keeps them out. Leave gravel and rocks out of the bottom. Water stops where fine soil meets coarse stone and sits there, so the base of the bed stays wet long after the top has dried.

What a probe shows about your fill

Three of our raised beds are the same size, and each carries a soil moisture probe. From mid-July to early September in our own garden, raised bed 1 ran 39% to 54%. Bed 2 held between 36% and 57%. The widest range belonged to bed 3, which dropped as low as 32% and climbed as high as 62%. Identical boxes under the same weather dried differently. Fill is one cause, alongside sun, the crop, and how close the probe sits to a layer boundary.

A probe reads the soil around its tines. It cannot tell you whether the logs or leaf layers deep in a bed are wet or dry.

Once each bed has its own mapped probe, Siltwell pushes a notification after the latest two readings from that bed come in under 25%. Every bed uses that same line, so beds with different fills race against one threshold. None of our probed beds crossed it in that stretch, and July's 10.89 inches of rain helped. The alert tells you which bed is dry, and you do the watering.

Which fill for you

Pick all bagged mix for a short bed, a bed you want planted this week, or a site where you can't haul bulk material. Pick a log base for a tall bed and a free supply of wood, and expect to top it up for several years. Pick cardboard and compost layers if you have leaves and clippings and can wait a season for them to settle.

Whichever you choose, wet each layer as it goes in, and fill a little above the rim. Dry organic material sheds water, and soaking it during filling keeps the middle of the bed from staying dry under a damp surface.