Mulch and mulching

Mulched with arborist chips

Mulching a garden, especially with bark or wood chips, is a trend that has developed relatively recently in gardening.  A hundred years ago few ornamental gardens used mulch except to protect plants over the winter.  Even 50 years ago mulch was generally something like straw placed on the rows in vegetable gardens.  Now one seldom sees a public landscape without mulch around some plant or another.  Most homeowners now buy mulch every year to place around trees and flower beds and garden mulch is a multi-million-dollar business.  But is mulch really necessary or good for all garden areas?  Is mulch a beauty treatment for gardens or something to replenish the soil?

There is no doubt that when mulch is properly used it helps control weeds, keeps soil moist and breaks down to make valuable organic matter in soil. Mulch eventually turns into compost, and then humus.  Decomposing mulch adds some nutrients to the soil, but it is not fertilizer. To many gardeners, mulch also improves the look of the garden.  It may not be necessary, but mulch can be very useful.

Types of mulch

Landscape /Arborist mulch is what results when complete trees, except for the largest portion of the trunk, are chipped up.  This material is coarse, and the chunks are not evenly sized.  It may have leaves and needles in it.  It’s not the most attractive type of mulch but it is probably the best mulch to improve your soil. Using arborist mulch is much like sheet composting.  Many experts believe arborist mulch is far superior to bark nuggets or shredded bark.

Tree bark nuggets and shredded bark are often used for mulch.  They look nice but don’t offer much in the way of nutrients when they decompose.  Bark mulches tend to shed water rather than absorb it, which can be a problem in some settings.  They break down quickly and must be replenished frequently.

Recycled wood chips are chipped up pallets and construction debris.  This is the type of mulch that is often dyed red or black or other colors.  The dye is harmless although it sometimes leaches out in rain and stains other things. Recycled wood may contain wood that has been treated with pesticides or fungicides.  It may be a good use for old wood but it’s probably the least desirable type of wood mulch for gardens, especially when used around food crops.

 

Shredded bark mulch

Other organic mulches include pine needles, (sometimes called pine straw), coir, peat, compost, straw, old hay, cocoa bean shells, and other waste products from food and fiber production.  These mulches all have advantages and disadvantages.  Compost of course, is ideal mulch if you have large quantities of it.  Most people don’t have enough, which is why they use other types of mulch.

Pine needles will not make your soil acidic, at least in any meaningful way but they are slippery when wet and may wash away in heavy rain. Hay and straw can bring weed seeds with them. They break down quickly but aren’t too attractive.  Cocoa bean shells may be toxic to dogs if they eat them.  Most of the advantage to these other organic mulches comes if they are cheap and easy to obtain in your area.

Inorganic mulches include stones, pea gravel, plastic sheeting, and recycled rubber products.  Stones, like those white landscape rocks sold in bags, or red lava rock do not improve the soil.  They are a pain in the butt when they get into a lawn area that needs to be mowed. When leaves and twigs get into them they look messy and are very hard to clean.  Most professional gardeners don’t use them anymore.  Pea gravel may have its place on paths, as it drains well. It’s wise to use landscape fabric under gravel mulch to keep it from sinking into the soil.

Stone mulch

Recycled rubber doesn’t break down, at least for a very long time. It can look attractive.  However, it probably leaches various chemicals into the soil-the research is still on-going- and doesn’t improve the soil. I wouldn’t recommend it except for playgrounds and the occasional path.

What about things like cardboard, old carpet, newspapers and landscape fabric or plastic sheeting?  I used to recommend layers of newspaper, cardboard, or carpet to smother weeds and preserve moisture.  However new research finds that this practice prevents a good exchange of air with the soil surface and impedes soil microbes in their important work.  It can also compact the soil and sometimes keeps it too wet.  It’s better to use mulch that is loose and chunky which allows air and water flow through it than layers of paper or carpet.

Plastic sheet mulch has its place in some forms of vegetable and fruit production.  It’s temporary in this use and is removed between crops. Black mulch helps heat the soil up in spring, red mulch can improve tomato and strawberry production.  Generally, irrigation hoses need to be underneath the plastic for the system to work well.  Plastic mulch is not a good idea for landscape plants and flower beds.  It doesn’t allow enough water to get to plants or conversely may keep plants too wet. Air exchange with the soil is impeded. In the landscape the plastic may be covered with other mulch, which further impedes water and airflow.

Landscape fabric is perorated or woven material that is supposed to allow water through it while impeding weeds.  It really shouldn’t be used under mulch however, in applications other than paths.  Like paper and cardboard, it does impede airflow, and adds nothing to the soil.  It can be difficult to remove or work through if you want to add plants, divide plants or remove plants later. 

Where and how to use mulch

It is good to mulch around trees and larger shrubs if they are in a lawn because it keeps someone from getting too close to the trunks when mowing or weed whipping and damaging them.  In this case the mulch should not touch the trunks of trees or shrubs and should not be more than 6 inches deep.  Mulch deeper than that may keep moisture and air from getting to tree root systems.

Spread mulch evenly – nothing looks more amateurish than volcano like mounds of mulch around trees.  The diameter of the mulched area should be proportionate to the size of the tree or shrub.  A three feet circle of mulch around a sapling looks right but a large tree needs a larger mulched area.  If your trees are in a natural setting or set among groundcovers and smaller shrubs they don’t need mulch.

Mulch is also good on path areas, such as in vegetable gardens, where you don’t want something like paving stones.  It’s environmentally friendly to use something that absorbs water on paths rather than something that causes it to run off into storm drains or other areas. Even shredded rubber and gravel mulches on paths allow water to percolate through them.  Mulches help keep weeds down in paths but unless you lay down landscape fabric before putting on the mulch you’ll have weeds popping through.  Even on landscape fabric some weeds will start growing but they are easy to pull.

Mulching around plants in the vegetable garden has advantages and disadvantages.  It can keep some crops cleaner and it makes weeding easier.  But in the spring you want the vegetable garden soil to warm up and dry out so applying mulch then isn’t a great idea.  Young plants heavily mulched in early spring may be slow to grow.  Applying mulch in early summer, when the soil has warmed up, will help keep weeds down and keep the soil from drying out as fast. 

However, if your area is seeing a lot of rain and you have clay soil you may want to skip the mulch so more water is lost to evaporation.  Don’t pile mulch too deeply as it may keep water from actually reaching the soil in drier conditions. Three inches in vegetable gardens is a good depth.  Put the mulch down when the soil is moist, but not when its saturated/flooded.

Use mulch that decomposes quickly in vegetable gardens, so it will improve your soil by adding organic content.  Straw, hay, grass clippings, shredded leaves and landscape chips break down faster than say – recycled pallet wood chips.  Current research indicates that mulch breaking down does not use enough nitrogen to cause a problem for plant growth.

Sometimes mulch in the vegetable garden can harbor pests like mice and voles, which may then cause snakes to frequent the garden.  Snakes are harmless to humans 99.99 % of the time, but for some gardeners the idea of mice or snakes in the garden will make them want to leave out the mulch.  Straw and hay mulch seem to have more mice and vole problems.

Mulching around and in perennial flower beds is probably the most controversial mulch application.  When bare ground is showing around perennials in early spring and weeds are growing rapidly gardeners are tempted to improve things with mulch.  However, many things should be considered before adding that mulch.

Just like in the vegetable garden, mulch added early in spring can slow down the growth of plants by keeping the soil cool and wet.  Once plants reach their mature size there will be less bare ground to see and weeds may be smothered or shaded out by heavy foliage in many cases.  Add mulch later- in early summer- and just pull weeds for a time in the spring and your plants may grow better and be healthier.

What about the mulch you applied last year or the leaves that blew into the garden and that were left there to rot, should they be removed in the spring?  The answer is – it depends.  If the layer isn’t too deep you may only need to remove mulch from around plant crowns.  If it is a deep matted layer of leaves or other matter the top layers may need to be raked out and added to the compost pile.  Loose woodchips are rarely a problem.  If flooding has occurred or it’s been very wet, then raking the mulch off the flower bed for a while may let the soil dry out and prevent plant roots from rotting.

The type of plants growing in your garden and your soil type can figure in when you are deciding to use mulch or not.  If you have sandy soil and are trying to grow plants that need constant moisture, then mulch is probably a good idea.  However, if you have clay soil and are trying to grow plants that like drier conditions, such as most herbs, you would do well to leave off the mulch.  And remember that while mulch can preserve soil moisture, mulch that is too deep may keep plant roots from receiving any water from light rains or irrigation.

Some flower gardeners compromise and use a strip of mulch between the plants and the edge of the garden and don’t mulch between plants.  This looks nice and still allows the soil to breathe and water to reach plant roots. 

Mulch can be helpful in protecting the crowns of certain perennials in the winter and in keeping plants from freezing and thawing cycles that lift them from the ground. (Some plants don’t like winter mulch – check your cultural suggestions.)  In this case you should apply the mulch after the plants have gone dormant, the dead leaves have been pruned off and the soil is frozen or soon will be.  You can apply mulch generously then - 6-12 inches in depth.  Just make sure to gradually remove it from the plant crowns in early spring.

All organic mulches decompose and settle and will need to be replenished, usually once a year.  If you are preparing a new bed and want to smother weeds and grass, don’t use plastic or cardboard, use a very deep layer of organic mulch (more than 12 inches), preferably arborist wood chips. Leave the mulch on for a season – or at least a few months.  When you get ready to plant you should have some nice loose, enriched soil down under that mulch.  If it’s still deep you may need to pull some back around the plants you put in but it will be worth it.

While many gardeners have been trained to think that gardens require mulch, they may not, and in some cases adding mulch to the garden is actually detrimental.  Think of what your plants need and require before giving in to the impulse to apply mulch as a “beauty treatment”.  That’s the sign of a great gardener.