Buy only the gap between the depth you want and the depth still on the bed. Every inch of top-up over 100 square feet takes 8.33 cubic feet of mulch, which is 0.31 cubic yard or a little over four 2 cubic foot bags. If the old layer already measures 3 inches of shredded bark, the right amount to add is zero.
That zero is the answer most often skipped. Penn State Extension puts it bluntly: applying new mulch over old mulch in successive years is the same as applying too deep a layer all at once. The University of Maryland Extension reports beds where mulch has built up to five, six or more inches, which is past every ceiling in the extension guidance below. A spring routine of "two bags per bed, same as last year" is how that happens, because last year's mulch has not all gone anywhere.
Measure what is left before you buy anything
The number that drives the whole calculation is the remaining depth, and it is rarely uniform. Penn State's advice for checking an overmulched tree is to dig carefully with a hand trowel to see how deep the layer is, and the same method works for any bed. Push the trowel straight down until it meets mineral soil, pull it toward you, and read the depth of the mulch band on the cut face with a ruler.
A few habits make the reading worth trusting:
- Measure at five or more spots per bed, including the front edge, the middle, the back, and right next to any shrub or tree. Mulch migrates: rain and foot traffic thin the edges, while the area under shrubs collects leaves and last year's top-up.
- Count only material that still looks like mulch. The dark, crumbly layer at the bottom that you could not tell apart from compost is already soil organic matter. Clemson's Home & Garden Information Center notes that wood chips are usually completely decomposed within 2 years, so on an old bed that bottom band can be thick and still provide little cover.
- Break the crust before you measure. Penn State and Clemson both describe matted or crusted layers that shed water. Clemson describes fungal mats that repel water. Break them apart with a rake before you read the depth, and note Penn State's point that a light raking alone may be all the bed needs.
- Write the readings down by zone, not as one average. The zoned method in the uneven-bed example below shows why.
If most readings already sit at or above your target depth, stop. Rake to freshen the surface and come back next season. Illinois Extension's tree-mulching sheet says the same thing in its own words: if enough mulch is present, refresh its appearance with a rake and break up matted layers rather than adding more.
Target depth by mulch material
Extension services agree that finer material should go on thinner, but the exact numbers differ by a half inch to an inch from one publication to the next. The table collects the targets and ceilings from the extension publications listed under Sources, so you can pick a target with its source in view.
| Material | Recommended depth | Ceiling stated | Source |
|---|---|---|---|
| Most organic mulches, general rule | 2 to 4 in | No more than 4 in; less on poorly drained soil | Penn State Extension |
| Most organic mulches, general rule | Keep at the original depth | Should not exceed 3 in | University of Maryland Extension |
| Most organic mulches, beds and trees | 2 to 4 in on well-drained sites | Not stated beyond the range | Illinois Extension; MU Extension G6960 |
| Fine-textured or double-shredded mulch | 1 to 2 in | Implied by the range | Penn State Extension |
| Fine wood and bark mulches | 2 to 3 in | Not more than 3 in | Clemson HGIC 1604 |
| Bark (fine-texture bark) | 2 to 3 in | Bark should not exceed 3 in | Oregon State EC 1629 |
| Wood chips and arborist mulch | 3 to 4 in | Wood chips should not exceed 4 in | Oregon State EC 1629 |
| Screened, uniform-particle products | 3 in | Not stated | Oregon State EC 1629 |
| Coarse mulch, pine bark nuggets | 3 to 4 in | Up to 4 in | Clemson HGIC 1604; Illinois Extension |
| Composted materials | 2 to 3 in | Not stated | Illinois Extension |
| Pine needles | 2 to 3 in | Not stated | Clemson HGIC 1604 |
| Whole leaves | 2 to 3 in | Not stated | Clemson HGIC 1604 |
| Shredded leaves, grass clippings | 2 in | Never deeper than 2 in | Clemson HGIC 1604 |
| Straw or hay (vegetable and perennial beds) | 4 to 6 in, because it packs down | Not stated | Wisconsin Horticulture |
| Small rocks | 1 in | Not stated | Clemson HGIC 1604 |
| Around shallow-rooted shrubs such as boxwood | 1 in | No more than 1 in | University of Maryland Extension |
Two practical rules fall out of the table. First, a 3 inch target for ordinary shredded hardwood or bark sits inside or at the ceiling of every general rule above, which is why it is the default on most calculators. Second, if you want 4 inches, that depth is supported for coarse chips and nuggets, not for fine shredded products, where Clemson and Penn State both point to thinner layers because fine particles let less air through.
Planting density also changes the target. The UC Master Gardeners of Placer County suggest 2 to 4 inches where plants are close together and 6 inches in open spacing, and Oregon State notes that as plant cover increases, mulch needs decrease, with shrub beds planned to reach full canopy cover in 3 to 5 years. A mature bed where the shrubs shade the soil may need only a thin refresh, or none.
Soil drainage pulls the target down too. Penn State specifically says to use less mulch on poorly drained soil, and Oregon State links deep mulch plus overwatering to waterlogged soil and poor growth. On heavy clay that stays wet, take the low end of the range.
The top-up formula, and the 8.33 cubic feet rule
The arithmetic is one subtraction followed by the standard volume formula:
- Top-up depth (in) = target depth minus measured remaining depth. If the result is zero or negative, add nothing.
- Cubic feet = area (sq ft) × top-up depth (in) ÷ 12.
- Cubic yards = area (sq ft) × top-up depth (in) ÷ 324.
The 324 comes from 12 inches per foot times 27 cubic feet per cubic yard. Clemson's fact sheet states the same constant from the other direction: one cubic yard covers 324 square feet at a depth of one inch.
From those constants come a handful of figures worth keeping in your head:
- 1 inch over 100 square feet = 100 ÷ 12 = 8.33 cubic feet, or 0.31 cubic yard.
- 1 cubic yard = 27 cubic feet = 13.5 bags of 2 cubic feet or 9 bags of 3 cubic feet.
- A half-inch top-up uses half as much as a 1 inch top-up. Depth scales the volume linearly, so a bed that lost 1 inch since last year needs exactly one third of what it took to lay 3 inches from scratch.
That last point is where the savings sit. A 300 square foot bed laid at 3 inches takes 75 cubic feet (2.78 cubic yards). If it measures 2 inches a year later, topping it back to 3 takes 25 cubic feet, 0.93 cubic yard, a third of the original order.
Top-up volume table: cubic yards by bed area
Find your bed area in the left column and the top-up depth (target minus what you measured) across the top. Values are cubic yards, calculated as area × depth ÷ 324 and rounded to two decimals.
| Bed area (sq ft) | 0.5 in | 1 in | 1.5 in | 2 in | 3 in |
|---|---|---|---|---|---|
| 50 | 0.08 | 0.15 | 0.23 | 0.31 | 0.46 |
| 100 | 0.15 | 0.31 | 0.46 | 0.62 | 0.93 |
| 150 | 0.23 | 0.46 | 0.69 | 0.93 | 1.39 |
| 200 | 0.31 | 0.62 | 0.93 | 1.23 | 1.85 |
| 300 | 0.46 | 0.93 | 1.39 | 1.85 | 2.78 |
| 500 | 0.77 | 1.54 | 2.31 | 3.09 | 4.63 |
| 1,000 | 1.54 | 3.09 | 4.63 | 6.17 | 9.26 |
The 3 inch column is there for a bed whose old mulch is fully gone (or that you have stripped). For any other bed, you will be reading one of the left-hand columns. A 1,000 square foot set of beds that has thinned by an inch needs just over 3 cubic yards, while the same beds laid from bare soil at 3 inches would need more than 9.
Bulk suppliers usually sell by the cubic yard, and many have a minimum or a delivery charge, so check how they round. A 0.62 cubic yard top-up for a 200 square foot bed may cost more delivered in bulk than it does in bags, even though bulk is cheaper per yard.
The same top-ups in 2 and 3 cubic foot bags
Bags are the more common way to buy a top-up, because the volumes are small. The table gives cubic feet first, then the bag count for 2 cubic foot and 3 cubic foot bags, rounded up to whole bags.
| Bed area (sq ft) | 0.5 in | 1 in | 1.5 in | 2 in | 3 in |
|---|---|---|---|---|---|
| 50 | 2.1 cu ft: 2 / 1 | 4.2 cu ft: 3 / 2 | 6.2 cu ft: 4 / 3 | 8.3 cu ft: 5 / 3 | 12.5 cu ft: 7 / 5 |
| 100 | 4.2 cu ft: 3 / 2 | 8.3 cu ft: 5 / 3 | 12.5 cu ft: 7 / 5 | 16.7 cu ft: 9 / 6 | 25.0 cu ft: 13 / 9 |
| 150 | 6.2 cu ft: 4 / 3 | 12.5 cu ft: 7 / 5 | 18.8 cu ft: 10 / 7 | 25.0 cu ft: 13 / 9 | 37.5 cu ft: 19 / 13 |
| 200 | 8.3 cu ft: 5 / 3 | 16.7 cu ft: 9 / 6 | 25.0 cu ft: 13 / 9 | 33.3 cu ft: 17 / 12 | 50.0 cu ft: 25 / 17 |
| 300 | 12.5 cu ft: 7 / 5 | 25.0 cu ft: 13 / 9 | 37.5 cu ft: 19 / 13 | 50.0 cu ft: 25 / 17 | 75.0 cu ft: 38 / 25 |
| 500 | 20.8 cu ft: 11 / 7 | 41.7 cu ft: 21 / 14 | 62.5 cu ft: 32 / 21 | 83.3 cu ft: 42 / 28 | 125.0 cu ft: 63 / 42 |
| 1,000 | 41.7 cu ft: 21 / 14 | 83.3 cu ft: 42 / 28 | 125.0 cu ft: 63 / 42 | 166.7 cu ft: 84 / 56 | 250.0 cu ft: 125 / 84 |
Each cell reads "cubic feet: 2 cu ft bags / 3 cu ft bags". Rounding up to whole bags adds a small cushion on its own: the 100 square foot, 1 inch cell needs 8.33 cubic feet but buys 10 cubic feet in 2 cubic foot bags or 9 in 3 cubic foot bags.
For spot repairs it is easier to think in coverage per bag than in volume per bed. One bag spread to a given top-up depth covers this much ground:
| Top-up depth | One 2 cu ft bag covers | One 3 cu ft bag covers | One cubic yard covers |
|---|---|---|---|
| 0.5 in | 48 sq ft | 72 sq ft | 648 sq ft |
| 1 in | 24 sq ft | 36 sq ft | 324 sq ft |
| 1.5 in | 16 sq ft | 24 sq ft | 216 sq ft |
| 2 in | 12 sq ft | 18 sq ft | 162 sq ft |
| 3 in | 8 sq ft | 12 sq ft | 108 sq ft |
| 4 in | 6 sq ft | 9 sq ft | 81 sq ft |
A worn strip along a path that is 2 feet wide and 18 feet long (36 square feet) and needs an inch back takes one 3 cubic foot bag, or two 2 cubic foot bags with some left over.
A 12 by 20 foot shredded-hardwood bed, start to finish
Take a rectangular foundation bed, 12 feet deep and 20 feet long, mulched with shredded hardwood. The owner wants 3 inches, the common default and inside the 2 to 3 inch range Clemson gives for fine wood and bark mulch. Trowel readings at six spots: 1, 1, 0.5, 1.5, 1 and 1 inches. The spread is narrow, so one zone is fine, and the reading to use is 1 inch.
- Area: 12 × 20 = 240 square feet.
- Top-up depth: 3 − 1 = 2 inches.
- Cubic feet: 240 × 2 ÷ 12 = 40 cubic feet.
- Cubic yards: 40 ÷ 27 = 1.48 cubic yards (the same as 240 × 2 ÷ 324).
- Bags: 40 ÷ 2 = 20 bags of 2 cubic feet; 40 ÷ 3 = 13.33, so 14 bags of 3 cubic feet.
Cross-check with the 8.33 rule: 2 inches over 100 square feet is 16.67 cubic feet, and 240 square feet is 2.4 times that, or 40 cubic feet. The two routes agree.
Had the owner skipped the trowel and laid "a fresh 3 inches", the order would have been 60 cubic feet, 30 bags of 2 cubic feet, and the bed would have ended at 4 inches of fine shredded material. That is above Clemson's limit for fine mulch and above Maryland's 3 inch ceiling for organic mulch generally, and the extra 10 bags would have been money spent on a problem.
The mulch calculator's own tip suggests adding 5% for settling. Applied here, 40 × 1.05 = 42 cubic feet: 21 bags of 2 cubic feet, still 14 bags of 3 cubic feet because 14 bags already hold 42.
Uneven beds: zone the bed instead of averaging
Most beds that have been mulched for a few years are not uniformly thin. Edges wash out and the soil under shrubs collects material. Averaging all your trowel readings into one number gets the total volume close but puts the mulch in the wrong places, and it hides any zone that is already too deep.
Take a 300 square foot bed with a 3 inch target, split into three zones from the trowel readings:
| Zone | Area | Measured depth | Top-up needed | Volume |
|---|---|---|---|---|
| Front edge along the lawn | 100 sq ft | 0.5 in | 2.5 in | 20.83 cu ft |
| Middle | 150 sq ft | 2 in | 1 in | 12.50 cu ft |
| Back, under shrubs | 50 sq ft | 3.5 in | None (0.5 in over) | 0 cu ft (2.08 cu ft surplus) |
| Bed total | 300 sq ft | 33.33 cu ft to add |
Zoned, the bed needs 33.33 cubic feet (1.23 cubic yards), which is 17 bags of 2 cubic feet or 12 bags of 3 cubic feet. The back zone already holds 2.08 cubic feet more than its target. Rake that half inch forward into the middle zone before you open any bags and the purchase drops to 31.25 cubic feet: 16 bags of 2 cubic feet or 11 bags of 3 cubic feet.
Now the averaged method. The area-weighted average depth is (100 × 0.5 + 150 × 2 + 50 × 3.5) ÷ 300 = 1.75 inches, so the averaged top-up is 1.25 inches over 300 square feet, which is also 31.25 cubic feet. The totals match only because the averaging silently assumes you will move the surplus from the back zone. Spread 1.25 inches evenly across the bed instead and you end with 1.75 inches at the front edge, 3.25 inches in the middle, and 4.75 inches under the shrubs, well past every ceiling in the depth table, right where roots and stems are.
The rule: zone the bed, top up each thin zone to the target, and treat any zone over target as a source of mulch to move, not a place to add more.
Tree rings and the clearance you leave
Around trees and woody shrubs, the top-up has two dimensions: depth, and the bare gap at the trunk. All six publications in the table say to keep mulch off the bark. They disagree on how big the gap should be:
| Publication | Gap from trunk or stem | Gap from building walls |
|---|---|---|
| Clemson HGIC 1604 | 2 to 3 in from stems of woody plants | 6 to 12 in |
| UC Master Gardeners of Placer County | 6 in from trunks | 12 in |
| University of Maryland Extension | 3 in from the base of the trunk | Not stated |
| Illinois Extension | At least 4 in from the trunk | Not stated |
| Penn State Extension | 3 to 5 in for young trees, 8 to 10 in for mature trees | Not stated |
| Purdue Extension (2023 county article) | About a 1 to 2 in gap | Not stated |
Penn State's mature-tree figure is the widest, and it ties the gap to exposing the root flare, the swelling where trunk meets roots. If you can't see the flare because mulch covers it, Penn State says uncovering it is essential, and the top-up order for that ring is zero until the old mulch has been pulled back.
Ring size varies too. Clemson suggests a 3 to 4 foot diameter circle for a newly planted tree and, for established lawn trees, about 2 feet of circle diameter for each inch of trunk diameter. Oregon State uses a 4 foot circle, 3 to 4 inches deep, for young lawn trees. Illinois says to extend toward the dripline but treats a 3 foot radius ring as the maximum for most trees. Purdue's 2023 advice calls for at least a 3 to 5 foot radius.
Example inputs: a lawn tree with a 4 inch trunk, mulched with arborist wood chips. Following Clemson's 2 feet per inch of trunk gives an 8 foot circle, radius 4 feet. Oregon State says wood chips should not exceed 4 inches, so the target is 4. The trowel reads 2.5 inches, so the top-up is 1.5 inches. Leave Penn State's 8 inch mature-tree gap.
- Ring area: π × 4² = 50.27 square feet.
- Bare circle at the trunk: trunk radius 2 inches plus an 8 inch gap is a 10 inch radius, 0.833 feet. π × 0.833² = 2.18 square feet.
- Area to mulch: 50.27 − 2.18 = 48.08 square feet.
- Volume: 48.08 × 1.5 ÷ 12 = 6.01 cubic feet, or 0.22 cubic yard.
- Bags: 4 bags of 2 cubic feet or 3 bags of 3 cubic feet.
The bare circle barely changes the order, about 0.27 cubic feet here, so skip that subtraction on small trunks. If you follow Illinois's 3 foot radius instead, the ring shrinks to 26.09 square feet net and the top-up to 3.26 cubic feet, roughly two bags of 2 cubic feet. For circles of other sizes, the circle calculator returns the area from a radius or diameter.
When you top up a ring, rake the new material outward from the trunk and keep the layer level or sloping slightly away from the tree. Purdue's urban forestry specialist advises a layer sloping away from the tree. A top-up dumped in the middle of the ring and spread by eye tends to end deepest exactly where it should be thinnest.
When the old layer has to come out
Extension services do not agree on whether old mulch normally needs removing. Purdue's urban forestry specialist told homeowners not to worry about removing the old layer, because it works its way into the soil and adds organic matter. Penn State says some old mulch may need to be removed before adding a new layer. The disagreement is smaller than it looks: both cap the finished depth, and removal only becomes necessary when the old layer alone is at or over that cap. This decision table sorts the situations the sources describe:
| What you find | What to do | Source for the rule |
|---|---|---|
| Remaining depth below target, layer loose | Add the gap only (target minus remaining) | Penn State; Maryland ("replace as needed to maintain original depth") |
| Remaining depth at target, surface faded or crusted | Rake to break up matted layers; add nothing | Penn State; Illinois; Clemson |
| Remaining depth over target | Remove or spread the excess to the dripline; add nothing | Penn State; Maryland |
| Mulch touching bark or covering the root flare | Pull it back from the trunk first, then measure | Penn State; Maryland; Illinois |
| Fine leaves or leaf mulch fully decomposed | Dig the decomposed layer into the soil, then lay new mulch | Clemson HGIC 1604 |
| Thick wet layers that stay soggy, plants yellowing | Reduce the depth; Clemson links excess fine mulch to suffocated roots, yellowing leaves and poor growth | Clemson HGIC 1604; Oregon State |
| Shallow-rooted shrubs (boxwood) under more than 1 in | Thin to 1 in | University of Maryland Extension |
| Signs of voles under deep mulch | Thin the layer and keep it off the trunks | Maryland; Penn State; Illinois |
Removal does not have to mean hauling material away. Maryland's sheet suggests working old mulch gently into the planting bed to improve the soil, or moving it to other garden areas. Penn State's photo sequence of an overmulched red maple makes the point well: no mulch left the site; the volcano was pulled back and spread out to proper depth, and the material in the volcano was enough to mulch the tree correctly.
Example inputs for an overmulched bed: 150 square feet under shrubs, trowel readings averaging 6 inches, target 3 inches. The excess is 3 inches, so 150 × 3 ÷ 12 = 37.5 cubic feet, or 1.39 cubic yards, needs to go somewhere. That is close to 19 bags' worth of 2 cubic foot mulch already on site. If a neighboring bed of 225 square feet measures 1 inch and wants 3, it needs 225 × 2 ÷ 12 = 37.5 cubic feet, and the two beds settle each other's accounts with a wheelbarrow and nothing bought.
How often each material needs topping up
How much a bed loses each year depends on the material, the climate and the soil life, and none of the extension publications cited here gives a percentage per year. They do give replacement intervals, which set how often you should expect to measure and top up:
| Material | Replacement interval or behavior | Source |
|---|---|---|
| Bark mulch | Reapply every 2 to 4 years | Oregon State EC 1629 |
| Wood chip mulch | Reapply every 1 or 2 years | Oregon State EC 1629 |
| Wood chips (general) | Usually completely decomposed within 2 years | Clemson HGIC 1604 |
| Finely shredded mulches | Decompose faster, need replenishing more often | Penn State Extension |
| Grass clippings and leaves | Decompose very fast, need frequent replenishing | Clemson HGIC 1604 |
| Straw | Decomposes rapidly, must be replenished to keep weeds down | Clemson HGIC 1604 |
| Gravel and pebbles | Rarely need replenishing | Clemson HGIC 1604 |
| Most organic mulches in beds | Add new mulch yearly to keep up depth | UC Master Gardeners of Placer County |
Oregon State explains the bark-versus-chips gap: bark is higher in lignin, which resists decomposition, while wood chips contain more air space and settle more. That is why a bed of arborist chips can look an inch thinner after one season while a bed of bark nuggets looks almost unchanged.
Timing matters as much as amount. MU Extension suggests replenishing in late March or early April, before new weed growth appears. Illinois Extension's tree sheet says to wait to replenish until the mulch has decomposed. Read together, a spring check is the time to measure, and the measurement decides whether anything goes down.
Weed control is the usual reason people want a thicker top-up, and Oregon State gives the number: mulch needs to be at least 3 inches deep to smother existing weeds and stop germinating seedlings, and small-seeded weeds generally can't emerge through a 3 inch layer. Going past 3 or 4 inches buys no extra weed control in that guidance, only the root and trunk problems described above.
Settling, fluffy bags and the 5 percent allowance
Fresh mulch from a bag or a pile is loose. Your trowel readings on the old layer are taken on material that has already settled. So a top-up that exactly fills the measured gap will read a little thinner a few weeks later, once rain and gravity pack it.
How much thinner depends on the product. None of the extension publications cited here puts a percentage on settling. Oregon State does say that wood-chip mulches settle more than bark because they contain more air space, and that bagged mulch usually contains fewer coarse particles than bulk mulch. CalculatorVillage's mulch calculator suggests adding 5% for settling. Treat that as a planning allowance rather than a measured figure, and expect to lean on it more for coarse chips than for fine bagged bark. On small orders the rounding to whole bags often covers it already, as in the 12 by 20 bed above, where 14 bags of 3 cubic feet held both the 40 cubic foot need and the 42 cubic foot padded figure.
The bigger risk runs the other way. Overestimating settling and adding "a bit extra every year" is how a bed creeps from 3 inches to the five, six or more that Maryland describes. If you are unsure, add the measured gap, wait a few weeks, and measure again. A second small top-up of two bags costs less than pulling an inch back out of a bed.
Running the numbers in the CalculatorVillage tools
The mulch calculator works from bed size and a depth, and it returns cubic feet, cubic yards and counts of 2 and 3 cubic foot bags. For a top-up, enter the top-up depth, not the target. In the 12 by 20 example, enter 12 by 20 feet and 2 inches, and the calculator returns 40 cubic feet, 1.48 cubic yards, 20 bags of 2 cubic feet and 14 bags of 3 cubic feet. Entering 3 inches would return the from-scratch order of 60 cubic feet, the over-deep mistake described above.
For zoned beds, run the calculator once per thin zone with that zone's own top-up depth and add the results. Skip zones at or over target. Round only the final total to whole bags, since rounding each zone separately can add a bag or two.
The gravel and mulch calculator adds weight and cost. It converts cubic yards to tons with a preset density for wood mulch and prices by the ton, and both presets are editable planning examples. If your supplier quotes mulch per cubic yard rather than per ton, use its cubic yard result and multiply by the per-yard price yourself. Sizing other materials for the same project works the same way: the sand calculator and the sod calculator for new lawn areas use the same area-times-depth logic for the beds and lawn edges you are building next to the ones you are refreshing.
If the mulch top-up is one line on a longer list of spring jobs, the homeowners hub collects the material guides that take the same measure-first approach, including how many bags of concrete a post or pad needs. And for the one bed you are about to fix, the best next move is the trowel: five readings per bed, written down by zone, before you type the gap into the bed-volume calculator for mulch.
Sources
Each extension publication below was checked on October 31, 2026. The volume tables and the worked example are plain arithmetic done by CalculatorVillage that day, at 12 inches to the foot and 27 cubic feet to the cubic yard.
- Clemson Cooperative Extension, Home & Garden Information Center, Mulch, HGIC 1604 (Marjan Kluepfel and Robert F. Polomski, revised June 20, 2016): depths by material, 2 to 3 inch limit for fine wood and bark and up to 4 inches for coarse nuggets, 2 inch limit for shredded leaves and clippings, 1 inch for small rocks, clearance from stems and walls, tree-ring sizing, replenishment by material, matted mulch, and the 324 square foot per cubic yard per inch constant.
- Oregon State University Extension, Mulching woody ornamentals with organic materials, EC 1629 (Neil Bell, Dan M. Sullivan and Tom Cook, published March 2009, reviewed 2024): 3 to 4 inch wood chip depth, 3 inch and 4 inch ceilings for bark and chips, measuring existing depth before reapplying, reapplication intervals, settling of wood chips, 3 inch weed-control depth, bagged versus bulk particle size.
- Penn State Extension, Mulching Landscape Trees (David R. Jackson, updated April 12, 2018): 2 to 4 inch range and 4 inch ceiling, 1 to 2 inches for fine or double-shredded mulch, checking depth before replenishing, repeated top-ups equal one over-deep layer, trowel check, trunk gaps for young and mature trees, spreading excess to the dripline.
- University of Maryland Extension, Excess Mulch Problems (updated March 14, 2023): accumulation to five or six inches, 3 inch ceiling and 3 inch trunk gap, replacing only to maintain the original depth, working old mulch into the bed, 1 inch limit for boxwood, voles.
- University of Illinois Extension, Tree Care Basics: Proper Mulching Techniques (Sarah Vogel and Jenny Lee, modified March 2024): 2 to 4 inch range, 2 to 3 inches for composted and 3 to 4 inches for coarse mulch, 4 inch trunk gap, 3 foot radius ring, raking instead of adding when enough mulch is present.
- Purdue Extension, Whitley County, Arbor Day and Mulching Trees (May 2023): advice from urban forestry specialist Ben McCallister on leaving the old layer, a 2 to 3 inch sloped layer, ring radius and trunk gap.
- University of Missouri Extension, Mulches, G6960 (Christopher J. Starbuck, revised September 2017): 2 to 4 inch layer for most products, replenishing in late March or early April.
- University of Wisconsin-Madison Extension, Mulches for Home Gardens and Plantings: 3 inches for most organic mulches, 4 to 6 inches for straw or hay because they pack down.
- UC Master Gardeners of Placer County, UC Agriculture and Natural Resources, Mulching Basics (presentation PDF): 2 to 4 inches in close plant spacing, 6 inches in open spacing, 6 inch trunk and 12 inch building clearance, adding new mulch yearly to keep up depth.