to mulch or not to mulch

July 7, 2008 9:21 PM

Should I or should I not use mulch in an organic/biodynamic garden in Hawaii, and WHY? help me analyze pros and cons of mulching...

My situation:
We're intensively growing vegetables in mound/raised beds on a 1/4 acre for a CSA. We're in a hot, dry part of Hawaii. It is summer and the sun is bright many hours of the day. Weeds/grass grow fast and inundate beds. Our soil/Hawaiian soil is not generally deficient in nitrogen. Our boss is opposed to mulch because it robs the soil of nitrogen and other nutrients as it breaks down. We have conceded to not use mulch next to the actual plants but wish to lay it deep in the paths (which will not be tilled for 6 months-1 year or more) to control weeds.

So, is mulching good or bad, what say those with experience farming/gardening and also the experts/books you have read? I've had some difficulty finding info about mulching in biodynamic texts, so if anyone can give me citations from a text that would be great as well.

posted by dahliachewswell to home & garden (8 comments total)

I am a dedicated mulcher.

Mulch conserves moisture in the soil.

If mulch is applied at a depth of at least 2-3 inches it acts to prevent weed growth by smothering the weed seeds so that they don't germinate.

Mulch acts as an insulator for the soil.

We've had a very wet spring here, so I have had to scrape the wood mulch back from around the roots of my plants just so they can dry out. Typically this is not a problem!

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My method of mulching is to mulch around perennials and trees with cypress or cedar mulch.

In my vegetable gardens, I mulch with partially-decomposed compost.

I would not use "raw" grass clipping as mulch in a humid environment. It's too easy to spread fungus. So, I let my grass clippings, leaves, kitchen scraps, etc. get to a semi-decomposed stage and then I put this into my garden. Periodically, I will work this into the soil where it completes the decomposition process and enriches the soil structure.

You can google "lasagna gardening" where even sheets of carboard and newspapers are used as mulch - this may be an option for your pathways as the cardboard and newspaper will lay flat.

posted by Ostara at 10:20 PM on July 7, 2008

As a basic rule of thumb, wood based mulches (e.g. paper, cardboard, sawdust, bark, woodchips, etc) last longer, but are low-nitrogen and will pinch it from the soil as they break down; grass-based mulches (e.g. cane mulch, lucerne, grass clippings, etc) don't last as long, but are high(er) in nitrogen and won't draw it from the soil.

If you're growing seasonal veges, then you probably don't want a long lasting mulch anyway - you want something that'll break down quickly enough that you can turn it back in before planting the next crop. I shouldn't imagine cane mulch would be hard to find in Hawaii...
posted by Pinback at 10:27 PM on July 7, 2008

Certain carbon based mulches use soil nitrogen as they break down. However, you can mulch with something that breaks down more slowly, like wood chips, (as opposed to shedded bark or cardboard) and remove them at the end of the planting season. You're going to need to replenish the soil nitrogen with compost and green manures after the growing season anyway, so that's the route I'd take. Generally speaking, properly managed vegetable gardens are going to do much better with mulch than without. The best mulching practices help maintain consistent soil temperature, conserve water, prevent water splashing onto crop leaves, makes weed management easier, and prevent soil compaction. Just be sure to remove any wood chips and re-use or compost them. You do not want to till them into your soil.
posted by oneirodynia at 10:42 PM on July 7, 2008

Personally, I'd mulch with something like pea-straw and then plant a green-manure crop at the end of the growing season. Nitrogen deficiency isn't good, but neither is having your crop succumb to weeds and drying out.

However, what about a non-organic mulch like rock slabs or pebbles - they'll suppress weeds, prevent evaporation, and can be removed (although not as easily as the dreaded black plastic). I'm sure I've seen (but I can't find the book - they're still all packed after the move) a garden using chunks of black lava as mulch with the added benefit that it scavenged a little extra moisture from dew in the morning. YMMV but it's something to think about.

I've also used carpet as a temporary mulch when I needed to kill off weeds in a sensative part of the garden - it's easy to lay and remove and you might be able to find something more environmentally sensative if you use coir or sisal matting (less glue and chemical dyes etc).
posted by ninazer0 at 12:12 AM on July 8, 2008

Our boss is opposed to mulch because it robs the soil of nitrogen and other nutrients as it breaks down

This, like most bullshit, is oversimplified. Regularly adding organic matter to your soil by topping it with mulch is generally one of the best things you can possibly do with it.

The initial rot process for carbon-rich mulches like woodchips certainly does bind nitrates in the soil it touches, making those unavailable to plant roots. However, this process only occurs where the mulch is actually touching the soil. Typically, this is not where the roots are (except for the roots of newly germinated things, like weeds, that you don't really want to feed anyway). The same initial rot process can make the soil immediately underneath slightly more acidic than it would otherwise have been, depending on what's used for mulch. Pine needles are especially effective at acidifying soils.

However, once mulch has been in place for a year or so, the bottom layers start breaking down even more and turning into humus - and humus, my friend, is gold for soils. All that nitrogen bound by the mulch during initial rot becomes well and truly available to plants once the mulch has turned into humus.

So the Right Thing, if you're the least bit concerned by the prospect of temporary nitrogen deficiency, is to lay down a layer of something nitrogen-rich, like maybe chicken shit (sorry, "dynamic lifter") or blood and bone, throw around a bit of dolomite to counter any tendency to acidify, and then use a wood or bark chip or straw mulch over that. The chicken shit burns emerging weed seedlings and provides nitrogen to be absorbed by your mulch, which will eventually return it to the soil as humus. The mulch layer also tends to contain the chicken shit stink quite effectively, especially if it contains a layer of something fine like cut grass.

If you're going to cut grass as part of your mulch, don't use too thick a layer, or it will turn slimy and very very slippery underfoot; also, don't use clippings from really invasive species like kikuyu, or they will strike root in your mulched bed and become nuisance weeds. Overall, in a hot climate, your first year's mulch layer should end up about four inches deep.

The next year, check the mulch depth (it will have shrunk a lot) and top it back up to four inches again with a new layer of chicken shit and woodchip. There's no need to dig up the old mulch - just put new mulch right over the top. Earthworms (which will be plentiful after a year under mulch) will do all the digging over that's required.

When you're planting new things into a mulched bed, don't bother digging it over first. Just scrape a flower-pot-sized hole in the mulch and dump a flower-pot-sized amount of weed-free potting mix in there to make a little mound for your new plant to start in.

If you mulch properly, the only way you'll have trouble with grass is when it comes in from the edge of the beds and sneaks runners under the mulch. Good bed edges will help a great deal with that.
posted by flabdablet at 1:20 AM on July 8, 2008 [3 favorites]

What flabdablet said. I've worked with a series of community gardens for a few years, and we've made extensive, enthusiastic use of the free wood chips available from the city forestry division in all of our gardens. These are located primarily on abandoned lots which have marginal soil, at best, and many are mostly dirty fill (brick dust, sand, glass and other crap) left after a fire or demolition. We do, however, also use a little horse bedding from the local racetrack as well (is a nitrogen amendment, when useful, absolutely out of the question?), but even in cases where we don't, any temporary dip in nitrogen availability is far, far outweighed by the short- and long-term benefits of 1) mulch and 2) more humus.
And, yeah, don't mix them into the soil, lay them on top! Big pieces are fine!
posted by pullayup at 7:31 AM on July 8, 2008

I did a little test this year with some peas mulched and some peas not mulched. I was very impressed at the way the mulch kept the soil under it moist. I use hay but I know some people are anti-hay.

Why not try some mulch on half a row and see what it does for you?

I'm going to guess that techniques that work in Hawaii might be very different than what works for other places. Check UofH Agricultural Extension
posted by bdc34 at 9:13 AM on July 8, 2008

The many benefits of mulch, as outlined by the first comment, far far far outweigh the minimal nitrogen binding that you'll get with surface contact.

Flabdablet has it all spelled out.

If your boss is really that concerned, just spread around some blood meal (it's high in nitrogen) before you mulch, to take care of any potential nitrogen deficiencies. Then you can have all the moisture retention and weed-beating properties of mulch, while being assured of healthy plants. One of those rare situations in which you can have your cake and eat it too!
posted by GardenGal at 12:08 PM on July 8, 2008