Mulch Matters

Brunnera 'Jack Frost'

The organic mulch around this moisture-loving Jack Frost Brunnera will keep me from having to water it as often in the summer time and will cut down on my weed pulling duties.

First, the basics: a mulch is anything that is placed over soil generally for the purposes of inhibiting weed growth, insulating the soil, conserving moisture, and contributing to the aesthetic value of a garden.  The most common mulches are organic ones, that come from dead, or dying, plant materials, but there are also inorganic mulches, such as rock and plastic, that also can be useful in certain situations.  A mulch can also provide the added benefits of reducing the spread of pathogens, preventing erosion, and reducing heaving – when freezing and thawing of the soil pushes up unestablished plants.

With that said, there’s a whole lot more to mulching than meets the eye.  There’s much to mulch, if you will.  Different gardening conditions may call for different types of mulches, or possibly no mulch at all, and deciding how to mulch will also depend on your gardening goals and possibly the availability of the mulch, budget and more.  Don’t worry, this is not meant to be an exhaustive writing on mulch, but I thought I would share some things that I think are the most important and relevant rules of (green) thumb for mulching.

Anytime is an acceptable time to apply mulch, but it may make most sense to do so in the spring or fall when most weeds are sprouting.  You may also want to bear in mind the effect the addition of mulch will have on the soil temperature as well.  When added in spring, the soil will take longer to warm up.  When added in the fall, the soil will remain warm for longer.  When applying mulch, generally a thickness of 2 to 3 inches of the mulch is desirable.  The needed thickness can vary depending on the density and weight of the particular mulch used, but generally this is thick enough to keep weeds down, insulate the soil, and conserve moisture, but not so thick that it will stress plant roots, or prevent the lighter rain showers from reaching the soil below.  You should only need to reapply mulch when the mulch has decomposed a bit into the soil and is no longer fulfilling its mulch duties.  Depending on what material is used, people that add mulch every year may end up adding too much mulch and stress surrounding plants.  Another benefit of mulching around trees or woody plants is that it will keep grass and weeds away from the trunk, keeping you and your mower/weed wacker also away, which are common causes of injury to trees.  However, it’s very important that you keep the mulch about 6 inches or so away from the trunk.  You do not want to pile any organic mulch right up around a plant’s trunk, as it is an invitation to disease and rodents, among other potential problems.

One of the most common organic mulches available is shredded bark or the larger bark chips.  WSU has a great article on wood chip mulch here.  Avoid using mulches with dyed bark or that are exclusively the large bark chips (chunky style).  The large chips will take a very long time to decompose and also don’t suppress weeds as well as a finer-textured mulch would.  The best mulch is one that is able to improve your soil over time.  A fine-textured organic mulch will help improve your soil texture as earthworms and helpful microorganisms and fungi break it down and incorporate it into your soil.  This protective layer of mulch will also help prevent the pounding rains from compacting the soil, and keep the soil from developing a hard, outer “crust”, which can lessen water absorption and create more runoff.  Bear in mind though that as an organic mulch breaks down it will deplete some of the nitrogen from the top part of the soil, so this could have an effect on shallow-rooted plants, such as groundcovers, annuals or vegetables.  Amending or fertilizing your soil properly beforehand will help, or simply keep the mulch further away from plants that are shallow-rooted or not yet established. Grass clippings can also be used as a mulch, although many people may not like the look of grass clippings as such, and it would require continual replenishment.  You would not want to use this as a mulch if there are grass or weed seeds in the clippings.  I’ve heard from some gardeners that grass clippings are great for their vegetable gardens.  A leafy vegetable, such as lettuce or kale, would benefit the most from a nitrogen-rich topdressing in which your main goal is to get the plants to produce lots of foliage and quickly.  However, when the weather is dry and you’re not cutting off more than a third of the grass blades at a time, I’d recommend just leaving the clippings on the lawn to keep that fed organically.

One of the best mulches you can use is compost.  A good compost will add macro and micro nutrients to your soil, as well as beneficial organisms, and will incorporate rather quickly into the soil.  Regular topdressings of compost is one of the best things you could do for your soil.  A happy soil makes for happy plants.  And, as Bob Ross has attested, our landscapes will all benefit from having happy trees.  The downside to using compost, however, is that it is not the best mulch option for preventing weed growth for a long period of time, so it may need to be applied somewhat regularly to keep them down.

Groundcovers can also be used as a mulch and may be one of the better options at improving the aesthetic value of a garden.  The ones referred to as cover crops or green manures can do wonders to help improve soil quality.  Legume cover crops, such as vetch or crimson clover, have the ability to fix nitrogen into the soil, feeding nearby plants.  Cover crops are usually tilled into the soil to create a fertile tilth, which should be done a couple weeks prior to planting your vegetable or fruit garden, wildflower meadow, laying of sod, etc.  Cover crops can work great as a temporary mulch.

Hosta 'June'

This beautiful Hosta was completely eaten this last winter from the bottom up.

One of the only downsides to using an organic mulch is that it can attract slugs, snails, rodents and other pests, depending on the material used.  Since we’ve moved into our home in 2009, I’ve been busy adding beds/borders to our yard and amending them with organic matter.  Of course the moles love to dig in this earthworm-rich, friable soil, as opposed to the hard clay we have elsewhere, and we now have a pretty large network of mole tunnels throughout our yard.  In hindsight, I really should have been much more diligent about chasing them off the property.  At the time they were starting to become more noticeable, I had just read an article on moles from a local gardening source about how they often got a bad rap.  Yes, they loved to eat worms, but for the most part they would leave your plants alone and help with soil aeration, so it said.  Well, apparently I also needed a lesson from the school of life, as I later discovered that voles and mice will take advantage of a mole’s hard work and travel through these underground “highways,” and they may not be as kind to your plants, as they stop for nearby snacks along their route.  Unfortunately, an organic mulch layer also provides opportunity for the voles and mice to more easily dig their shallow holes to get closer to plants to eat them.  This last winter I lost seven Hostas and an ornamental grass, ‘All Gold’ Japanese forest grass, to voles or mice.  These plants were mostly eaten from the bottom up and were in areas that were well-mulched and that had nearby mole tunnels.  Some gardeners have had more luck when they’ve removed the mulch layer, which I plant to do in the winter time around the Hostas.  It may also be helpful to enclose the root ball with hardware cloth when planting Hostas, or any other plants that you are concerned may be eaten by these critters.  With that said, certain mulch materials will actually help repel some pests.  For instance, cockroaches, ants and termites, among others, do not like cedar chips because of the chemical Thujone that they contain.

I generally would not recommend using an inorganic mulch, but there are some circumstances when they might be appropriate.  Black sheets of plastic work very well to warm the ground up, and I’ve seen this method used in lavender farming.  Rocks or boulders seem to be essential to a landscape.  Rocks may be useful as a mulch as they too can help warm the soil, and small rocks could provide for better drainage around a plant’s root crown, an area susceptible to rot.  I would never recommend using the rubber mulches that are shredded/ground up and meant to look like organic ones.  Think of your mulch as a soil enhancer and as potential worm food and this will steer you clear of the ones that are not so garden worthy.

You may also need to take into consideration the effect a particular mulch may have on your soil’s pH, however, it seems the importance of this caution may be exaggerated by many.  New studies are showing that even though a mulch substance can have an extreme pH level, such as oak leaves and pine needles/straw, which have a very acidic or low pH level, they do not have a substantial effect on the soil’s pH, at least not at the root level of most established plants. I hope this information is helpful to you.  I’d love to hear any tips that you might have on mulching.