In Carol Deppe's book The Resilient Gardener, (which I really like for the most part; I strongly disagree with her on some things) she mentions that deep, permanent mulch does not work in Oregon.

She had a dispute about this with an East Coaster who had recently moved to Oregon. So they did a side by side comparison; and the potatoes planted in the mulched bed failed.

She says it is because the mulched soil never warms up. So she tills up the soil and leaves it bare to warm up, then spreads around a little mulch in midsummer to retain moisture.

Since I just placed a large amount of sheet mulch, this has got me worried. If Oregon has trouble warming up deep mulched beds, Colorado will have enormous problems. Spring is late and cool here, and nights stay cool right through the summer.

What is really going on here?

My experience here in Colorado is that mulch definitely prevents the soil from warming up in the spring. I have found that cold soil, even well into May and June, are my biggest hurdles to plant productivity. I always was skeptical of raised beds because they dry out so much, but have moved to using raised beds because warming up the soil is so critical for optimal yields and harvest season.

So yes, I think she is onto something, and yes I think permanent mulch would be a problem in Colorado. By the time it is mid-summer, I have a complete living mulch going in my beds, so there is minimal need for any other mulch. The exceptions are tomatoes and berries, which I do mulch with woodchip compost in mid-summer, with good results.

I'm a big user of mulch, but I live in Hawaii. So my situation is quite different from yours. But regardless of location, one needs to understand how mulch works.

1- Type of material used for mulching can make a big difference. Each type reacts differently with the environment, so you get different results. In my experience, newspaper/cardboard tends to keep rainwater from penetrating if the layers are too thick. More than a dozen sheets of newspaper and the mulch becomes a water blocker. BUT once the ground is wet, thick paper can tend to keep the moisture there. That's why an old pile of newspaper left abandoned in the garden has wet soil under it even during a drought. Get the newspaper layer thicker than a half inch and the dry ground under it doesn't get wet even in a heavy rain. Thick newspaper is glued together (Chemically? Biofilm? Mechanically?) making it virtually impossible for water to filter through it. Instead the water finds a channel to drain away.
Fine, fresh grass clippings can be a good mulch but only if thinly applied. Applied thickly it initially heats up, often killing or damaging the plants you are mulching.Then as it cools down, it forms a gley-like layer which can prevent water from percolating through it while also robbing the soil microbes of oxygen. So for best results I apply grass clippings in light layers, adding a bit more each week or two.
Chopped weeds make a good mulch even if applied several inches deep. I find that they are stemmy and coarse enough not to compact. Thus they allow air circulation while allowing water penetration.
Tree leaves are highly variable because some are fine, some coarse. Some tend to compact while others do not. Some rather quickly affect the soil ecology, changing the pH, causing rapid depletion of nitrogen, or adding compounds that effect other plants. I include pine needles in this group. For some crops, pine needles may be beneficial while for others they are not. And the applied thickness has a bearing on leaf mulch too.
Wood chips are a popular mulch. They too have their pros and cons. Applied to the soil surface and not disturbed, I find them to be fine for my flower beds. But if I flip them in while replanting the beds, then I run into trouble. At least the wood chips that I have access to will deplete the nitrogen rather rapidly if I don't take steps to avoid it.
Compost is a popular mulch with me. My crops have short turn over times, so once a crop gets harvested I lightly turn over the mulch, thus providing fertilizer to the bed. I've had good success using coarse compost as a mulch.
2- Depth of mulch is a major factor. I've already mentioned this. Some mulches are just as good at excluding moisture as they are at retaining it. Some plants thrive with deep mulches while others do not. It's not only a moisture thing but also an oxygen/gasses thing.
3- Time of year applied. While my ground temperature doesn't vary as wildly as it does on the mainland, I still must be aware of it for certain crops. Mulch will prevent the sun from heating up the soil.....which s good during hot summers. But if the soil is too cool for a certain crop, then applying mulch is a mistake. Mulch will prevent the soil temperature from increasing, thus the crop will fail or do poorly. Therefore I will not apply thick mulch when I plant beans until the plants are 6 " high. For soybeans and okra, I will only use light mulching in order to keep the soil warmer unless we suddenly go into drought. Less moisture in the soil causes the temperature to increase, thus it will be warm enough so now I need to focus on moisture retention.....something that mulch can do pretty well.

I always use mulch to protect the soil microbes, even when I'm trying to warm the soil up. I never allow the soil surface to go completely bare or dry for long. A sparse mulch will help protect the soil while still allowing for warming. I don't want to sterilize my soil. I made that mistake once of allowing the soil to sit exposed to the sun for several weeks and as a result it took months to get that soil productive again (I choose not resort to chemical fertilizers).

Specifically about potatoes, a crop under mulch could fail for a variety of reasons. Soil temperature, initial moisture content, type of mulch, depth of mulch all have a bearing. A local gardener tried growing potatoes under a thick mulch like I do and she failed. Came to find out that she did not get the seed potatoes to start sprouting first PLUS she was using store bought tubers. Store bought ones are often chemically treated to retard sprouting. And if potato seed is not pre sprouted, it may rot while it is awaiting to break out of dormancy. I've had excellent results producing potatoes under a 6 inch deep compost mulch where I reapply more mulch 39 days into the growing period. The potatoes end up under a 10-12 inch mulch. They are very productive and easy to harvest this way. I can harvest every 60-120 days depending upon the variety. By the way, I don't normally have problems with me in the garden, but if one grows potatoes under just mulch, mice can eat your spuds before they have a chance to grow.

Hope this gives you some ideas.

Gilbert Fritz wrote:

Since I just placed a large amount of sheet mulch, this has got me worried.

What are you trying to grow and why did you sheet mulch?

Also, before you get too worried, are there examples of things growing really well in your area that are heavily mulched, like say forests? OTOH, some trees/plants suffer if too heavily mulched (water having a hard time getting through to the soil)

I was hoping to plant vegetables. What should I do, till the soil? (If not mulching, or not mulching till mid summer. ) I am converting an existing lawn.

Don't till. To some extent earth worms will till for you.

Here in the Northeast, people move the mulch aside a week or so before planting to warm up the soil. You could cover the planted seed with a cloche to warm the soil too. Once the plants come up, you can put the mulch back.

What did you use for mulch?

I used a mix of wood chips and manure, with an under layer of cardboard. I should have said that it is not exactly a lawn; just a mowed field.

Here is something I just realized; toby hemenway also lives in Oregon. He uses sheet mulch almost exclusively for vegetables, including tomatoes. And his description of the climate matches Carol Deppe's. Also, Patricia Lanza (Lasagna Gardening) developed her technique of deep, permanent mulch for vegetables in the cool, short summer climate of Vermont. How can they get away with it?

Would it help if I added lots of fish emulsion or blood meal to the mulch come spring? For one thing, I have heard that cool soil is mostly a problem because of nitrogen unavailability. And the nitrogen should also actually warm up the soil due to decomposition.

Would it also help if I added Biochar to the surface? Or would that just heat up the mulch surface?

One thing that's going on is that different ecosystems sometimes require different interventions. One thing that I've noticed here in northern New Mexico is that the forests have far fewer viney, throrny, understory species and that there is much more space between trees here than in the other ecosystems where I have lived--central Missouri, Maryland. Thus the mulch in the forest is thinner than what would be expected where there is two to four times the precipitation. Another is that it is hard for lowlanders to understand the effect of lower nighttime temperatures. I'm at higher elevation than you, thus even thinner atmosphere and more heat loss at night. Over the year temps at night average 30+ degrees less at night. This effect is at it's extreme in May/June when we often have day time temps nearing or above 80 dropping to the 40's and even 30's at night. While the sun shines bright, much of the gain in soil temperature is lost each night. On the other hand, soil moisture is precarious as precipitation is low and spring breezes constant and strong. One strategy is to pull mulch off when you want soil temperatures to rise and use light row cover over young plants day and night. Most of the sun's energy penetrates the cover, but evaporation of soil moisture and night time convection loss are reduced. On colder nights a heavier layer of row cover can increase your protection of soil temps. When the soil has warmed and night time temps hit the 50's--early to mid July here--take the row cover off and replace the mulch. I generally leave the mulch on as temps drop in fall as it holds soil heat and use row cover to protect plants from frosty and freezing or near freezing air temps.

Different conditions also affect the balance of benefits to detriments of not tilling. I'm working in an environment where the ground is not ordinarily covered with native broadleaf and grass species. Before I started gardening here, the ground was more bare than covered with vegetation. Thus the soil has minimal organic material and soil life. Thus tilling causes less damage. On the other hand, the seeds and seedlings I plant can greatly benefit from an initial loosening of the sandy, silty, compacted soil. At first I double dug, but have move to a broadfork so that I am not turning the soil over. I mix several inches of aged manure and compost into the top half foot of soil and innoculate with commercial products and compost tea. I have almost a quarter acre of gardens and in preparing these raised beds I saw maybe a dozen earthworms. A local extension agent tells me earthworms are present in the area but their niche is more frequently occupied by ants in this climate. I have more earthworms now, but I feel that I wouldn't have such an increase if I hadn't tilled in all that organic material. Or at least not in the time frame I desired.

I don't have experience in the SW converting existing lawns so I'm not sure what I'd do in your situation. I did want to note that when you disturb soil here, three of the plants that typically move in--amaranth, lambs quarters and purslane--are more nutrient dense than most vegetables that I grow and are known to be edible and to have been depended on by the locals. I harvest lambs quarters when it is just a couple of inches tall and sell it at the farmers market.

Gilbert Fritz wrote:Would it help if I added lots of fish emulsion or blood meal to the mulch come spring? For one thing, I have heard that cool soil is mostly a problem because of nitrogen unavailability. And the nitrogen should also actually warm up the soil due to decomposition.

Would it also help if I added Biochar to the surface? Or would that just heat up the mulch surface?

I think of sheet mulch as newly-made soil rather than what a gardener means by mulch, and that you haven't really mulched your garden yet, if that makes sense. I would just plant it out and see what happens. Maybe throw some cloches made from old gallon milk jugs over the things that like it the hottest, like peppers. Denver is like SLC, where you go from cold spring to grueling heat almost overnight, tomatoes won't set because it's too hot, etc. The soil will get warm enough, just not by March 17 pea-planting day when we are crazy to get our hands dirty again. The only thing to watch is not to get too excited to put seeds in too early. But that's true whether you had tilled up the field or sheet mulched.

You'e done a big experiment, I wouldn't complicate it further. If you really want to do something just to do something, recycle a chunk of clear plastic film and lay it over half the garden for a couple weeks before you plant. Then do test plantings in both sides, see whether at the end of the year it made the slightest difference. I was just reviewing my garden notes for last year-two different plantings of Brussels sprouts, seeded 6 weeks apart, completely the same height and productivity in October. But I wouldn't worry about it. Your sheet mulch is likely to retain so much more moisture than the ground and that will make much more difference in our climate over a season than starting a week or two earlier.

Soil warming can be done using different angles/orientations, compost, etc. Biological activity contributes to heat. Test have been done in orchards and found cover crops and mulch will cool off more than in bare soil, leading some citrus farmers to clean their soil of all life other than the trees. Obviously, that seems wrong. Thick enough mulch during warm weather is considered a fire hazard in parts of California because it has the potential to self-combust. It's also a known fact that mulch and cover crops hold more water than bare soil and water is a temperature stabilizer - think of the ocean. There's also albedo - the color. Dark things warm faster than lighter things. Fresh mulch will be cooler than compost-mulch. With all that factored in, perhaps it's a matter of preference. I bet you could take all the warming factors, stick them together with seeds started early in doors and make a nice crop. But, maybe that's just me. Or, maybe that's my potato patch this spring.