Mulch that does not blow away
Which mulches stay put in Front Range and high-plains wind, how deep to lay them, and how to keep them off trunks so voles stay out.
Mulch in a humid climate is mostly a weed and moisture question. Mulch in this climate is those things plus a wind question. A bale of light straw spread on a dry afternoon will be against the fence by evening, and the bed you thought you protected will be bare when the chinook finishes. The right mulch is the one that is still there on Thursday, that does not crust the soil, and that is not tucked against the bark of anything you hope to keep.
What leaves, and what stays
Leaves, cocoa hulls, and dry straw move. They are usable only if something holds them: a light net, a row cover, dampening them as you spread them, or a bed sheltered by a wall. Straw that has been wetted and walked will knit a little.
Straw thrown on a ridge will not. In a vegetable bed, straw is still useful as a summer mulch and as a winter cover over carrots, because you can pin it. Plan on the pins.
Wood chips and shredded bark that have started to compost stay put better because the pieces lock. They are the default mulch for paths, around trees, and under shrubs. Fresh chips robbed from a deep pile can be coarse and can sit as a mat that sheds water.
Wet them in place. A two-to-four-inch layer is a mulch. An eight-inch layer is a disposal site, and it keeps rain out of the root zone while giving voles a roof.
Gravel and rock stay put, which is why they cover so many Western yards. They are a mulch for a true xeriscape path or a planting that wants sharp drainage and heat, including some Plant Select ice plants and penstemons that rot in wood fines.
They are a poor mulch for vegetables. They warm the soil, which can help in May and can cook roots in July, and they add nothing to organic matter.
A yard that is only gravel is not the xeriscape Denver Water described. It is a heat island with a hose. The planting alternative is on the xeriscape page.
Compost as a surface layer feeds the soil and stays if it is finished and slightly moist. It disappears into clay by the end of the season, which is a success, and then you add more. It is not a weed barrier by itself.
Plastic and landscape fabric under rock keep weeds down for a season or two and then grow weeds on top of the debris that collects, while roots and water both get confused. Fabric can be a fair tool under a permanent path. It is a bad tool under a vegetable bed or a perennial border you intend to dig. If you already have a yard smothered in fabric, the maintenance is pulling it up where plants are failing, not adding another layer.
Depth, stems, and winter versus summer
Two to four inches of wood chips around a shrub, kept back several inches from the bark, is the pattern.
Mulch against the trunk stays wet, invites rot, and is the winter home of voles, which girdle bark under the cover of mulch and snow.
The wildlife page is the animal side of this. The same rule applies to vegetable stems and to the crowns of perennials. Mulch the soil, not the plant.
Summer mulch goes down after the soil has warmed and the seedlings are up, to hold moisture and to blunt the crust that forms on clay after a downpour. A mulch on a cold, wet clay bed in April keeps the bed cold.
That can be what you want for a spinach bed and what you do not want for a tomato. Warm-season crops need the soil to warm. Mulch them later than the peas.
Winter mulch goes down after the soil has started to freeze, especially around garlic, strawberries, and perennials that heave. The point is to keep the ground frozen and the temperature steady, not to tuck the plant in warmly so it breaks dormancy in a January chinook.
A mulch applied in October on a still-warm bed does the opposite. Strawberries are the crop where this timing is most often missed. The berry page ties it to the cultivar type.
In spring, pull winter mulch back from crowns so they can dry and grow. You can leave the material in the paths. A mountain bed that stays under a heavy mulch into June is a bed that never warms, in a season that could not spare the delay.
Wind, slopes, and the vegetable row
On a slope, lay mulch in contour bands, not in a smooth sheet that slides. Chips hold on a grade better than straw. A berm of chips on the downhill edge of a bed stops the rest from creeping.
Check after the first good wind and after the first thunderstorm. The Colorado Vegetable Guide notes that a lot of the year’s water arrives in a few hard rains.
Those rains float a dry, lightweight mulch and then drop it in the path. Wetting the mulch as you lay it is not fussiness. It is how the first storm fails to rearrange your work.
Vegetable beds that use drip can take a finer mulch once the lines are in place. Do not bury the emitters so deep you cannot find a clog. A thin compost or straw layer over a visible drip line is the compromise.
Raised beds in full wind may need the straw netted even when a sheltered bed does not. If you are re-spreading the same straw every week, change materials or change the exposure. The bed is telling you it is in the wrong place, which is a wind siting problem.
Rock mulch next to a south wall, under a plant that wants heat, is legitimate. Rock mulch over the root zone of a chlorosis-prone maple, reflecting heat and covering a compacted clay, makes June worse. Alkaline soil problems are aggravated by roots that stay hot and dry, or cold and wet. Mulch is part of that water story, not a decoration on top of it.
A yearly rhythm
- Late spring, after planting and after the soil has warmed: lay summer mulch on vegetables and renew chips on paths.
- Midsummer: check depth, add where wind or watering has thinned it, pull it off anything it has touched.
- Autumn, after freeze-up: add winter mulch to garlic, strawberries, and heave-prone perennials. Drain irrigation first.
- Early spring: pull winter mulch off crowns. Top-dress compost where the soil needs it. Do not till a deep wood-chip layer into a seedbed the same week you sow carrots. Chips that have not broken down tie up nitrogen at the surface if you bury them fresh. Leave them on top, or compost them first.
Checklist
- Choose chips or compost where wind is strong. Hold straw down or skip it.
- Keep mulch off trunks, crowns, and vegetable stems.
- Use gravel for drainage-loving plantings and paths, not as the vegetable-bed surface.
- Mulch vegetables after soil warmth arrives, not before, if the crop is tender.
- Put winter mulch on after the soil freezes.
- Pull it back in spring.
- On slopes, lay mulch so it cannot slide in the first storm.
- If it blows away twice, change the mulch or the site.
Sources
- CSU Extension, Colorado Vegetable Guide, on wind, downpours, and low organic matter.
- CSU Extension, Iron Chlorosis of Woody Plants, on organic mulch and compaction.
- Denver Water xeriscape principles, which include mulch as one of the seven practices.
- CSU Extension mountain vegetable guidance, on soils that start very low in organic matter.