How much mulch for black medic, dandelions?

myrmayde(5b Western Montana)August 15, 2014

How thick of a mulch does it take to keep down black medic, dandelions, and other common weeds? Can I put mulch down without pulling the weeds first? I'm on the verge of getting carpal tunnel syndrome from weeding so much. I can't keep wasting my time doing things that are inefficient, ineffective, or downright counterproductive.

You could say that my major problem is that my flower border is too big, at about 4,000 square feet. Reducing it to half or a quarter would help, but I still like the way it looks and want it this size, so I'm trying to salvage the situation. I've planted six trees and ten shrubs so far and would like to double or triple those numbers eventually. My purpose for this garden is to create a habitat for birds, bees, butterflies, moths, etc.

In 2011 when I started this garden where nothing but lawn had been, I sowed cover crop seeds. I didn't weed that year, and the next spring I chopped it all down and put it in the compost pile. I'm sure a lot of weed seeds, particularly black medic, got into the soil that year. According to this web page (, black medic can produce three crops of seeds per year, the seeds can remain viable for twenty years, they can germinate in three days, and the most germination is in the fourth year after they're "sowed." This explains why I was able to keep up with the weeding in 2012 and just barely in 2013 but I'm losing the battle this year. In many places there are many black medic plants per square foot and they're growing so thickly that they stand up, supported by each other, rather than sprawl on the ground.

Dandelions are my second major weed problem this year. I use a dandelion fork but never seem to get the whole root, so a couple weeks after finishing weeding and mulching a bed, it's full of little dandelions, seemingly every single one I pulled out.

I've been reading the GardenWeb forums archives for five years, and I've learned a lot. But apparently I've been in denial about how much mulch it takes to block sunlight from hitting the native soil. I tried half an inch; that just made the weeds healthier. I tried an inch; that just maybe slowed them down a tiny bit. Obviously I need to try at least two inches. Do you think that will do it? I've read that more than three inches is too much, so I hope there is an upper limit at some point. Each inch will cost me $1,000, which explains why I haven't been putting enough of it down.

I didn't mulch in 2012, 2013, and earlier this summer because I had sowed 1.5 million flower seeds, most of them native wildflowers that can take up to two years to germinate. I had great germination of lupines in 2012, so there's a lupine about every square foot. Some (disappointingly few) of the 130 or so other species that I sowed last May, last December, and this May germinated and are hidden among the weeds.

I now realize that my bare native soil must be treated as if it were radioactive waste or flesh-eating bacteria. It is and always will be so full of weed seeds that it must never be allowed to see the light of day. Now, after I weed an area, I mix half compost and half sphagnum moss and sprinkle that just thickly enough that I can't see the soil. Then I dump as much "Soil Pep" (partially composted pine bark fines) as I can over that, but it's probably not even an inch in all places. I even dump it over the top of the lupines, because many weeds grow right inside lupine clumps. If I pull a weed from an already mulched area, I try not to let any native soil drop on the mulch, and I add some more mulch.

Before I started mulching, I had weeded about three-quarters of the beds, and all of the weeds came back with a vengeance in two or three weeks. I've weeded and mulched about a quarter of the beds since then. Now I've been told that I'm right on the edge of having carpal tunnel syndrome and I have to stop hand-weeding for a while.

I use "small western bark" on the paths, which I like, so if I were to start using something like that on the beds, I'd have to switch to something else for the paths so there would be a contrast.

Ironically, although I needs many cubic yards of expensive mulch, I already many cubic yards of partially composted yard debris that I'm afraid to use, four huge piles of stuff. Three piles are layers of pulled weeds and grass clippings. The other pile is spent lupine plants. The piles take about year, more or less, to decompose in place. I also have five compost containers that rotate, which produce finer compost from food scraps, leaves, and grass. When I've put the compost on the ground in the past, hundreds of weeds sprouted in those spots. So I've been using the more finished compost to cover the rougher piles before winter, so that they look like berms of soil to the neighbors. They've become places to dispose of excess materials, but I haven't been able to spread this stuff around to shade out weeds, because it would just create more weeds. (Sometimes the grass clippings have Weed-B-Gon in them as well.)

I could spread chopped lupine plants around as a mulch, but only in the areas that they grew in, because I'm trying to keep different colors of lupines in separate areas. Should I try that? One drawback is that the lupines have powdery mildew, and I've read that you should remove mildewy vegetation before the winter.

At this point I've finished hand-weeding one area (out of seven), and I'm trying to finish another and weed one more by September 9, when I want to sow seeds in those three areas. I bought a border fork to loosen up the soil before I weed, and I water it heavily as well. I'll add a little compost on top of the mulch and sow in that. Will that work?

I've given up on weeding the other four areas this year. Those areas are so thick with lupines that there isn't room for any other flowers. When the lupines turn brown I'm planning to weed-whack everything, rake it up, put it on the compost pile, and dump a bunch of mulch on the beds. Do you think this will work to suppress weeds?

I had a soil test done in 2011: pH 7.5, adequate fertility and organic matter. Clay loam, good drainage, good texture, dark color, lots of worms and insects.

These are the weeds I've identified (from Weeds of the West, University of Wyoming, 2012, 640 pages), in order of how numerous they are:
black medic (Medicago lupulina), annual or short-lived perennial
dandelion (Taraxacum officinale), perennial
prickly lettuce (Lactuca serriola), biennial
wild mustard (Sinapis arvensis), annual
common groundsel (Senecio vulgaris), annual or biennial
wild buckwheat (Polygonum convolvulus), annual
netseed lambsquarters (Chenopodium berlandieri), annual
Canada thistle (Cirsium arvense), perennial
western salsify (Tragopogon dubius), biennial
tumble pigweed (Amaranthus albus), annual
wild oat (Avena fatua), annual
large crabgrass (Digitaria sanguinalis), annual
annual bluegrass (Poa annua), annual
rattail fescue (Vulpia myuros), annual
shepherd's purse (Capsella bursa-pastoris), annual
tumble mustard (Sisymbrium altissimum), annual
field pennycress (Thlaspi arvense), annual
yellow sweetclover (Melilotus officinalis), annual or biennial
hemp dogbane (Apocynum cannabinum), perennial
redstem filaree (Erodium cicutarium), biennial
common chickweed (Stellaria media), annual
broadleaf plantain (Plantago major), perennial
hairy vetch (Vicia villosa), annual
spotted knapweed (Centaurea stoebe), biennial or short-lived perennial
common mullein (Verbascum thapsus), biennial

Here is a link that might be useful: Medics

Thank you for reporting this comment. Undo

To be effective at controlling unwanted plants ("weeds") mulches need to be thick enough to block access to sunlight which might mean a mulch 3 to 6 inches thick. Black Medic is, for most of us, an annual that reproduces prolifically from seed. Seeds, from Black Medic, often will germinate in mulches. Dandelions often will regrow from bits of root left behind, but also grow from the prolific seeds each plant produces and those seeds often find mulches a good place to germinate and grow. So while mulches will aid in suppressing unwanted plant growth they can also be good seed nurseries.

    Bookmark   August 16, 2014 at 7:11AM
Thank you for reporting this comment. Undo
myrmayde(5b Western Montana)

Thanks for replying, kimmsr. This all seems more and more like a no-win situation this year. And yet someone on GardenWeb said that she produced one little bucket of weeds per summer out of a 3,550 or so square foot flower garden. Maybe she used four to six inches of mulch, I don't know, but I thought it was two or three.

Also, I went on a garden tour recently, and four people had flower gardens about the size of mine, and they seemed to be using about one inch of Soil Pep. (Either they told me that's how much they used, or I dug a little back with my fingers to find out.) These gardens were being maintained by one person, and they were from 13 to 20 years old (the gardens, that is; the people were much older!). Possibly the explanation for this is that over the years they had depleted the seeds in the soil bank by sprouting and then killing them, obviously without letting any of them form more seeds.

The real Catch-22 for me is that I actually want seeds to sprout in my mulch, but only my flower seeds, not the weed seeds. With an extremely thick, rough-textured mulch (say, six inches of arborist's wood chips), how on earth does anyone manage to have self-seeding flowers, or clumping shrubs and grasses, or spreading ground covers, or bulbs? How does anyone direct-sow seeds? It almost seems that you'd need to cover the native soil with thin layers of weed-free soil/compost/fine mulch repeatedly, and continually hand-weed, to get this to work.

If the mulch, which is being used to cover the weed-seed-filled soil, accumulates weed seeds itself, it would make sense to me to add more mulch to cover the first mulch, maybe twice a year. So if I put down too much mulch at first, I wouldn't be able to add more until some of the first had decomposed (and then too many weed seeds would accumulate in it). There must be an optimal amount of mulch to add per year, after taking into account how much of it decomposes each year. This should lead to the level of mulch slowly increasing every year. I need to figure out what that amount should be when I'm using Soil Pep (or small bark chips on the paths and around trees). On the other hand, at the very beginning I need to add enough to somewhat effectively suppress weeds, not so little that it merely encourages them.

Then there's the price. All total, including the vegetable garden, I actually have 5,715 square feet to cover with either mulch (Soil Pep in my case: 4,525 square feet) or wood chips (for paths and around trees: 1,190 square feet). They're both $4.49 for a two-cubic-foot bag. (I'm actually leaving out the cost of compost, sphagnum moss, rock phosphate, greensand, and elemental sulfur, because I consider those as more conditioners or fertilizers than mulches and they have very little bulk. But they still cost a lot of money.) At one inch I'd need 189 bags of mulch and 50 bags of chips, for a total of $1,073. At two inches I'd need 377 bags of mulch and 99 bags of chips, for a total of $2,137. And so on: three inches would cost $3,210, and four inches would cost $4,279.

Five or six inches is out of the question for physical reasons. The beds and paths aren't raised, contained, edged, or separated, so the material would just spill over at the edges. I have many flowers, including tall ones like hollyhocks and delphiniums, that only get five or six inches tall the first and even second year. (Weird, I know.) I can't completely bury my flowers; that would defeat the whole purpose. I guess two to three inches per year is all I can handle, both money-wise and time-wise. Our car can hold 12 bags, so each inch takes 20 trips. It takes a lot of time just to buy, load, haul around, and distribute all of that material.

I guess what I want to know is: If I add one inch of Soil Pep twice a year, and less than that decomposes each year, so that eventually I have an ongoing level of three inches on average, will that make much difference in the amount of time I have to spend weeding? Should I leave my dead lupine vegetation lying around to add to the mulch and not worry about the powdery mildew?

    Bookmark   August 16, 2014 at 2:13PM
Thank you for reporting this comment. Undo

I think that you should post your questions in the Soil, Compost, & Mulch forum, where you will receive good advice from people who mulch everything. I also think that you need to find another source of mulch, if at all possible. Do you have tree companies near you? Can you get wood chips from them? I'm in NU, and I just paid $18 a cubic yard delivered for double ground hardwood chips, which were an absolute delight to deal with. Third, if you don't want to edge your beds, just have the mulch taper down as you approach the edge, so that the area just by the lawn has only a little mulch; it will get more weedy, but it will be easy to get at.

    Bookmark   August 16, 2014 at 5:18PM
Thank you for reporting this comment. Undo
myrmayde(5b Western Montana)

Thanks for replying, lisanti. All good ideas. I found out that our municipal compost company doesn't deliver, and the company I bought in bulk from four years ago is out of business. But I'll look around for a cheaper product.

    Bookmark   August 16, 2014 at 5:45PM
Thank you for reporting this comment. Undo

Many of us use sun blockers such as newspaper, cardboard, etc., under the mulch material to extend the mulch material, but look around for free sources of material. Many times the local tree trimmers have dropped loads of chipped trees off for me.
Many gardeners use what I believe are environmentally unsound "weed" control practices to get that "weed" free garden.

    Bookmark   August 17, 2014 at 6:48AM
Sign Up to comment
More Discussions
Idea for homemade herbicide: think it'll work?
So I have been brewing kombucha at home. A bunch of...
Lawn Weed
Hi Guys, I live in Central Coastal Queensland and I...
Fixed nitrogen in the soil
Is nitrogen fixed in the soil good or bad?
Gary Cotterill
Parsley-looking weed?
I planted a bunch of herb seeds in my tiny urban backyard...
Weed Identification (Florida)
Good afternoon, I have a problem with a weed (type...
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™