About MulchBed
Why mulch depth matters more than mulch type
Most gardeners under-mulch, spreading a thin scattering that looks tidy for a week and then lets weeds straight through. Weeds need light to germinate, and a layer thinner than five centimetres simply does not block enough of it. At seven to ten centimetres, most annual weeds are suppressed, soil moisture stays far more stable through dry spells, and soil temperature swings narrow, which keeps roots active for longer. Going much deeper than ten centimetres brings its own problems: it can starve the soil surface of oxygen, keep stems permanently damp, and waste money on material that adds no further benefit. The right depth is therefore a genuine calculation, not a guess, and it is the depth figure that drives everything this tool computes.
How the volume calculation works
Volume is length times width times depth, with the depth converted from centimetres to metres, giving cubic metres. One cubic metre equals one thousand litres, so dividing by the bag size in litres gives the number of bags, always rounded up, because a part bag cannot be bought and a shortfall leaves one bed half done. The calculator also multiplies bags by your local price to give a total cost, which makes it easy to compare a bulk delivery against bagged mulch: bulk is usually cheaper per cubic metre, but delivery charges and the need to move a loose pile by wheelbarrow can erase the saving on small jobs. For beds under two cubic metres, bags are almost always the practical choice.
Measuring beds that are not rectangles
Rectangular beds are simple, but curved borders and island beds need a different approach. For a curved border, break the shape into a series of rectangles and triangles, or lay a hose along the edge and measure its length, then multiply by the average width. For circular beds, use pi times the radius squared, and for a ring around a tree, subtract the inner circle from the outer circle. It is better to overestimate a curved bed by five percent than to run short, since an opened bag of mulch keeps well if stored dry, while a half-mulched bed looks unfinished. Keep a note of each bed's area once measured, so next year's top-up is a one-minute job rather than a re-measure.
Choosing the right mulch material
Bark chips last longest and suit ornamental borders and paths, but they are slow to break down and add little nutrition. Wood chips are cheaper and fine for large areas, though fresh wood can temporarily lock up nitrogen at the soil surface. Straw and hay break down quickly and suit vegetable gardens, where they are dug in at season's end. Compost as mulch feeds the soil directly but disappears fastest and needs topping up twice a year. Gravel and stone mulch suit dry gardens and never rot, but they heat up in sun and make future planting harder. Match the material to the bed's purpose rather than buying whatever is on offer, and check that bark products are not dyed, since dyes can leach in heavy rain.
Applying mulch properly
Weed the bed thoroughly first, because mulch over established weeds only hides them for a few weeks. Water deeply before mulching so the soil underneath starts moist, then spread evenly to the target depth, checking with a ruler in several spots. Keep mulch five to ten centimetres away from stems and tree trunks, because mulch piled against bark traps moisture and invites rot and pests — the classic volcano mulching mistake. On slopes, start at the top and work down, and consider a slightly coarser material that resists washing away. After heavy rain, check for thin patches and top them up rather than waiting for weeds to appear.
How often to top up
Organic mulches lose roughly a third of their depth each year through decomposition and settling, so a seven centimetre layer becomes four to five centimetres after twelve months. Top up once a year, usually in spring after the soil has warmed, rather than waiting until weeds return. Autumn top-ups also work and protect roots over winter, but avoid mulching over cold, wet soil in late autumn, which can lock in the cold. If you use compost mulch, expect to top up twice a year. Keep a simple record of when each bed was mulched and how many bags it took, and the job becomes routine rather than a yearly puzzle.
Mulch and soil health
Beyond weed suppression, mulch feeds the soil food web as it breaks down, with worms and fungi gradually incorporating organic matter into the topsoil. This improves soil structure, drainage and water holding capacity over several seasons, which is why mulched beds need less watering and less fertiliser than bare beds. Mulch also protects soil from compaction by heavy rain, which otherwise seals the surface and causes runoff. Avoid black plastic sheeting under mulch in planting beds, since it blocks water and air movement and eventually tears into fragments. A deep organic layer, renewed annually, does the job better and improves the soil instead of merely covering it.
Cost, bulk delivery and storage
For orders above three cubic metres, get quotes for bulk delivery and compare the delivered price per cubic metre with bagged mulch, remembering that bulk piles need a hard standing area and a wheelbarrow for distribution. Bags are easier to store, carry and spread in small doses, and they keep the garden tidy between jobs. Store spare bags under cover and off the ground, since wet bags grow mould and become heavy to move. If a bulk pile must sit for weeks, cover it with a tarp to prevent weed seeds blowing in and to keep it from drying into a dusty heap. Split a large order with a neighbour if neither garden needs the full load, since fresh mulch is best used within the season.
Mistakes that waste money
The costliest mistake is ordering by eye, which typically underestimates curved and irregular beds by twenty percent or more. The second is buying the cheapest dyed bark, which fades within a season and can stain paths. The third is mulching over dry soil in a drought, which seals in the dryness and forces you to water through the mulch layer anyway. Always water first, measure twice, and buy from a supplier who states the bag volume honestly — some budget bags are labelled by weight rather than volume, and wet bark weighs far more than dry. A tape measure and this calculator cost nothing and save the price of a wasted delivery.
Mulch for vegetable beds and paths
Vegetable beds benefit from mulch as much as ornamental borders, but the material choice and timing differ. Straw mulch suits vegetables because it breaks down within a season and can be dug in at year end, adding organic matter exactly where crops need it, while bark chips on a vegetable bed persist for years and interfere with digging and sowing. Apply straw after seedlings are established and tall enough not to be smothered, and keep it away from stems to prevent rot. Garden paths are a different case: bark or wood chips last longer underfoot, suppress weeds between stepping stones, and define the path edge, but they need a landscape fabric or cardboard layer underneath to stop weeds pushing through from below. For paths, use a coarser grade that resists compaction and displacement, and top up annually after winter, when foot traffic and rain have thinned the layer. Combining bed and path calculations in one order saves delivery charges, so measure both, add them together, and round up to whole bags or a single bulk delivery. Keep a small reserve bag for path repairs after storms, when mulch washes into beds and leaves bare patches that weeds colonise quickly if left.