Question and Discussion: Eliminate debris to reduce pests and diseases (clean garden practice) VS mulching, chop & drop and creating wildlife habitat: which is better?
Question from one of our students:
“I’ve read that one major way to inhibit garden pests organically is to eliminate debris to get rid of places the pests hide. But you taught us the value of mulching and chop and drop which also make sense. So I have been putting freshly chopped leaves down in the fall. I think it helps with weeds and retaining moisture and must also add organic content. But I am still having trouble with the squash vine borer… and had to throw away the squash plants early because of the vine borer and also a virus in the zucchini. Do you have any thoughts on that? I also practice crop rotation.”
Clean Garden Practices: Typically, this refers to removing all organic material from the garden in Fall to reduce places for pests and diseases to overwinter. The aim is to reduce pest and disease populations by reducing the places they can overwinter.
Organic and Green Mulches: Dead and living organic material, plant materials, placed deliberately on the soil surface to increase soil moisture, regulate temperature, save water, increase soil carbon, repair soil fertility, increase soil life and wildlife in the garden.
Chop and Drop: A Permaculture term for the action of cutting organic matter, plants, weeds, and grasses and using them for mulch in the garden rather than removing them to the compost pile. When uses as the basis for managing the soil, such a system is academically referred to as “slashmulch,” and is an ancient gardening practice from around the world.
The Diversity Resiliency Principle/Theory: A Theory in Ecology, sometimes referred to as a principle, that states: the greater diversity in an ecosystem, the greater degree of resilience (from climate factors, disturbance and pest and disease pressures) is conferred to the individuals in the system. Greater diversity = Greater health.
Ecological pest and disease control: Fending off pests and diseases, in part, by increasing wildlife habitat, habitat for pollinators and generally relying on the “Diversity Resiliency Principle.”
Unfortunately, there’s no good short answer to this complex question. Most gardeners will pick one of the two approaches that best matches their temperament and goals. but advanced gardeners may create their own hybrid to support their specific goals. My review of the history of this discussion found a strong move away from clean garden practice over time, with studies showing many benefit of mulching, debris and “messy garden practice” (as some termed the action of avoiding fall clean-up) including on populations of beneficials, and crop resistance to disease and pests. Many mainstream horticulture sources now advocate against strict “clean garden” practice. Meanwhile, most of the evidence supporting clean garden practice appears annecdotal, or dependent upon chemical management of pests.
Personally, I favor the more natural approach, as I believe current research demonstrates it better matches my goals of achieving sustainability, maximizing wildlife value, saving time, and still producing high yields of high quality produce.
A very timely question as gardeners in the northern hemisphere contemplate whether to follow the “fall clean-up” regimen, or newer advice to leave debris and mulch to create wildlife habitat.
First of all, these are some very common questions regarding the practice of chop and drop and mulching. And once you’re asking these kinds of questions and making these kinds of observations, in my opinion, that’s when you’re on the path to expert old-world gardening! You can only take the conventional color by numbers approach so far, but once you start coloring outside the lines you can really maximize the value of your garden, if you’re willing to trouble-shoot and experiment a bit.
First we’re going to look at the conflict between chop and drop and clean garden practice, then we’ll get into the specifics about squash as an example.
So, clean garden practice VS. ecological pest control: The truth is both of these methods work. Clean garden practice reduces SOME garden pests and diseases, those that overwinter in debris.
Or at least there’s annecdotal evidence of this, though an hour search through extension and master gardener recommendations on fall clean up from a dozen different sources found 0 studies documenting the effect of clean garden practice. With this, I turned to my collection of horticultural and agricultural textbooks, my copy of Integrated Pest Management, and again found no sources documenting a positive impact of clean up. However, this classic did site a few studies where summer pest populations (such as spider mite) were predicted based on sampling populations of overwintering insects, but there are several reasons this doesn’t quite clearly support clean garden practice.
What I found instead, was a history of changing opinions and practices, with a textbook from the 30s recommending VERY thorough clean-up, even to the point of mercilessly removing every last fallen leaf! Of course, as studies have increasingly demonstrated the value of leaving debris (such as leaves which fertilize soil) these old-fashioned practices have consistently loosened up. (If anyone has studies showing or implying the benefits of clean garden, please send them my way!)
Anyway, whether it’s proven or not, it is logical to assume that removing debris reduces the numbers of certain pests that overwinter in that debris. The downside is it also gets rid of the pest-predators and beneficials, which often live in the same debris! Hey, living with your food is a good idea! For most of our existence humans did the same thing. Of course beneficial organisms do the same. And we do indeed now have plenty of good research to show that leaving debris increases populations of beneficials! And, researchers are concluding that plant residues and no-till systems are correlated with a “lack of
serious disease and pest pressures.”
If we want to take a more forest-like approach, we can mulch deeply, and leave debris in the garden, which also leaves more pests in the garden, but also provides habitat for beneficial organisms that fight pests and diseases. This is important, because pest populations typically grow much faster than the predator populations needed to provide natural control. Often, unless there’s already a population of predators in the garden, our crops are destroyed by the time they ever show up! But OVER TIME providing habitat builds up an army of beneficials to help fight pests and diseases for you. Mulch and debris are fantastic beneficial habitat. For example, comfrey leaf debris has been found to host large numbers of beneficial insects. Hollow plant stems are the main over-wintering spot for predatory wasps and native pollinators. Corn debris hosts a beneficial bacterium that also predates on powdery mildew (though some disagree), and even inhibits weeds in squash, two reasons why the Native American 3 Sisters planting works!
In addition to increasing soil life and beneficial organism populations, there are very many benefits of deep mulching, especially “chop and drop” mulching with plants grown in the garden:
– it saves water
– it conserves soil moisture which helps plants regulate their health and prevents problems like splitting, damping off and blossom end rot,
– it increases soil carbon
– it increases soil fertility
– it prevents erosion,
– it reduces weed germination and weeding work
– it can replace importing compost
The down side of deep mulching is that it DOES provide habitat for some pests to live in, both overwinter, and during the growing season. It sometimes can take time for the needed beneficials to establish themselves in the garden and provide that ecological control and balance.
In the short term, clean garden practice probably reduces SOME BUT NOT ALL pests and diseases (just those that overwinter in plant debris.) But when these pests do find the garden (which they always do) there will be no predators to hold them in check or for plants to call on for protection. Modern research shows plants really do “call” for help, secreting hormones that attract predators of the pests and diseases attacking them, but this only works if the predator populations are also built up in the garden.
In that between time, deep mulching and chop and drop will probably greatly reduce gardening work and required imported inputs like fertilizer and compost, but the pests may mean somewhat reduced overall yield. My personal experience and sense is that most of the time, a deep mulched garden will still have a higher “return on investment,” meaning a greater yield for the time, money and effort put into the garden.
So, my take is that clean garden practice might work better for gardeners who are prioritizing maximum yield over “return on investment” and looking to do 90% of their own pest prevention through spraying, etc. Or for those who don’t have time to let a more balanced ecology develop over a few years time (though it might not take that long at all!) An ecological approach may take time to build up enough diversity to give the plants all the tools they need, so that when they call for help, there are predators nearby. But it will quickly start saving time and imports, and eventually, it can radically reduce the pests and diseases in the garden.
Most gardeners will choose one of these two approaches, since they seem opposed. But I think expert-level creative gardeners can actually maximize by using both, selectively, depending on which pests they’re dealing with and which crop they’re growing. For example, black rot on grapes is spread through “corpses” of infected grapes and plant tissues. When dealing with these tissues, I move them away from the grapes, rather than chopping them in place. But clean garden practice won’t help much with slugs, since some species (and life-cycles) overwinter in soil rather than in surface debris while fireflies (which, in their larval form, eat slugs!) do overwinter in grassy debris! So, if slugs are the problem, clean garden practice over winter may hurt beneficials more than the pest. But during the season, slugs largely live on and in debris and mulch. So if a slug population get’s very destructive on a crop like small seedlings of greens, I might temporarily remove the mulch until the greens can fend off the slugs better. Or I might just put down some crushed egg shells or slug traps around the vulnerable plants.
Squash bugs, however, overwinter in debris (and other sheltered places like rocks, logs, or buildings) so clean garden practice MIGHT help control populations. Parasitic wasps and flies feed on squash bugs, though, and these too overwinter in debris….
Overall, my approach is to trust ecological controls and good variety selection as my “baseline” approach, allowing diversity to build up over time to reduce pests and disease in the garden. This also keeps my gardening very simple! And it reduces my overall “input” time. If a particular pest gets out of control, I try to learn about its specific life cycle and natural predators, so I can apply a remedy that really makes sense. In some cases, that remedy might be clean garden during a part of the year near that particular crop. In this way, my gardening slowly evolves, becoming very subtle, very in-tune with the specifics of my own site, the beings I share it with, my crop preferences, and my own needs and way of living.
Regarding the specific question of squash, the biggest factor is variety and species. Not all squash are the same! For example, the pepo species (summer squash, zukes, acorns, etc.) are much more susceptible to both disease and pest pressures through most of North America, while the Mochata species (my favorites are the Seminole pumpkin, Tonda podana, and neck pumpkins) are much more vigorous and disease/pest resistant. Maximas (brodee galeux, hubbards and buttercups are my favorites) are typically considered inbetween, but that may depend somewhat on location, with them performing poorly in our gardens in Illinois, but better than pepo for us in Michigan) My own personal solution is to grow all of these in deep slashmulch, but plant pepo and maxima species in successions and plant mochata species (especially seminole) once. Pepos usually produce much faster than other species, and you can get a good yield before the plants succumb. I find this preferable to fighting a long losing battle to keep them alive against wilt, powdery mildew, squash bugs, cucumber bettles and vine borer. With all those enemies, I assume I’m going to lose the battle and my goal is simply to get as many squash as I can over the season, not to keep the plants alive the longest. Mochatas take longer to produce longer-lasting squash with deeper flavores and more nutritional density, but they can also tend to fend off most of these attackers better if given good nutrition, as you experienced. Since Maximas tend to also require a long time to mature, but fend poorly against pests, they take the most work to grow and tend to require diotomaceous earth for squash bugs and BT (a liquid bacteria that kills the larvae) for borers. Unless I’m in the mood to do that extra work, I let somebody else grow maximas. (There is also the C. Angyosperma species (like cushaws) which I’ve heard would be similar to Maxima, but I have no experience growing them.)