Disease in Hops; Mulch or No Mulch?

I recently talked with a gardener who is managing a Hops plant for me and learned, to my dismay, that he had removed all of the mulch around the base of the mulch. His cited reasons for doing this were that, according to research he had done, “hop plants are especially susceptible to getting diseases from mulch”. I wrote him a letter to explain the shortcomings of this “no-mulch” management paradigm. This post is an edited version of that letter:

My Hop Plant, 2015

I am going to make the case that the overall benefits of mulching far outweigh the potential drawbacks. I will also explain why the belief that “mulching under a hop plant will cause disease” is fundamentally flawed. Here I go….

The effects on plant and soil health of mulching versus not mulching:

Issue

With Mulch ……

With Bare Soil….

Soil Organic Matter (which is responsible for holding most moisture and nutrients in soil)

Organic Matter is made from decomposed mulch. The Soil Organic Matter under mulch is constantly increasing as microorganisms eat the mulch from below and incorporate it into the soil.

There is no source of Soil Organic matter if the soil is bare. Soil Organic Matter will decrease over time since it is not being replenished. This means less water and nutrients held in the soil.

Soil Temperatures

Mulch acts like insulation for the soil, moderating the temperature. The soil is kept cooler on hot summer days, which prevents evaporation and possible burning of roots. The soil is also kept warmer during the winter, which allows the roots to be active and growing which will strengthen the plant the following season.

Soil temperatures fluctuate quickly without mulch. The soil gets very hot when the summer sun is hitting the soil, which kills most life in the top few inches of soil and also sucks out moisture quickly. Soil gets very cold in the winter, which could potentially kill some roots, weakening the plant in the future. Soil temperatures fluctuate widely between the day and night, these unstable conditions are hard for organsims to live in; this reduces soil biodiversity.

Soil Compaction

Mulch provides ample food for the microorganisms in the soil. Microrganisms are responsible for creating non-compacted, airy soils in nature where there is no tilling machinery: they create glues and fibres which bind soil particles together into larger particles, these larger particles do not pack as tightly as smaller particles (think of Olives in a jar versus Rice in a jar.) which allows more air and water flow through the soil.

Bare soil provides no food for soil organisms and makes it difficult for them to live. Without microorganisms creating larger soil particles the soil will become more and more compacted as the soil particles break down over time. Tilling is the only option…. which does not work under a hop plant since they have perennial roots.

Water infiltration rate and infiltration depth

Water can infiltrate a mulched soil quickly because the soil will be less compacted. This means that when it rains all of the falling water is immediately absorbed into the soil instead of staying on the surface where it causes erosion problems. Mulched soil (if the soil has been well managed for several years) will also be loose to a greater depth which allows rainwater to more easily reach the deep roots of a perennial plant, like Hops. *Note: certain mulches can repel water, this is bad. Make sure you dont use that kind of mulch.

Bare soils become more and more compacted over time… especially at the surface layer of the soil. When rain hits the compacted soil most of it will simply stay on the surface and run downhill, instead of being soaked up by the soil. If there is somewhere for this surface water to go it will start to erode the soil (losing soil = bad). If not then it will create puddles. These puddles prevent air from getting to the soil below… this leads to anaerobic (no oxygen) soil conditions which are bad for many reasons(see below). The roots also require oxygen, if there is a puddle over the soil the roots will drown (die).

Soil Aeration (anaerobic vs aerobic)

Anaerobic = No Oxygen, nutrients lost as gasses, bad smell, soil compaction, disease causing organisms thrive

Aerobic = With Oxygen, nutrients remain in the soil, no smell, soil decompaction, disease causing organisms are outcompeted by beneficial organisms.

Mulch, as mentioned above, allows soil microorganisms to naturally decompact the soil. Decompacted soil (loose soil) is the only place Aerobic conditions can occur.

Bare Soil, as mentioned above, creates compaction over time. Compacted soil quickly becomes Anaerobic.

What about the experts who say that “mulching under a Hop plant will cause disease” or “Hop plants are especially susceptible to diseases from mulch”?

These statements are based on reductionist thinking. Although reductionist thinking is very usefull in certain fields of study (technology for example), it has a very poor track reccord at predicting the complexity of nature. Here are two counter points to the advice above:

      1. These statements are most likely based on either scientific research or personal observation done on commercial hop farms. Almost 100% of commercial hop farms are not managed according to the principles of ecology. Hop plants under commercial management are susceptible to disease simply because of poor soil management practices. A healthy hop plant will be disease free, regardless of whether or not there is mulch underneath it!

        1. Commercial hop farms are typically monocultures. Monocultures (one plant species) will always have more disease problems than “polycultures”(many plant species)

        2. Commercial hop farms, even if they are Organic, are heavily fertilized. Fertilizers, even organic fertilizers, kill beneficial soil organisms. The soils have also likely been tilled at some point, killing even more beneficial organisms. It is also likely that pesticides have been applied as well. These missing beneficial soil organisms are natures way of protecting the plant roots from disease. They are not present in commercial hop farms, therefore the plant roots are vulnerable to disease.

        3. Commercial hop farms also have many other factors which would contribute to diseases becoming a problem: compacted soil, anaerobic soil, lack of bug diversity, lack of micronutrients, lack of genetic diversity, etc.

        4. ** Even if the advice about mulch came from small scale gardeners I would still doubt it because, sadly, most small scale gardeners are misinformed about soil management. Poor soil mismanagement, whether on a large scale or in a backyard garden, will still have the same results: death of beneficial soil organisms, opening the door for disease.

      2. Commercial varieties of Hops are actually very similar to the original wild hop plant, domestication has not altered them very much. Therefore they should behave very much like wild hop plants:

        1. Wild hop plants are widespread in Europe. They grow primarily on the edges of forests. They are a “late successional plant” which means they will almost never naturally grow on bare, or recently disturbed, soil. They always have mulch!

        2. Wild hop plants grow very vigorously and are generally free of disease

        3. Wild hop plants have evolved to thrive in this natural environment over millions of years, they are highly adapted to it. It is unlikely that a few hundred years of dommestication would be able to undo millions of years of evolution.

        4. Therefore hop plants must be well adapted to growing in the presence of decaying organic matter (mulch), because that is the only place they grow in the wild.

The principles I have talked about here apply to all plants (except pioneer, weedy plants… which actually like bare soil) and all soils. Diseases on plants are actually just symptoms; they are signs of an unhealthy plant. And likewise, unhealthy plants are usually a sign of an unhealthy soil. Mulch is one of the best ways to repair an unhealthy soil.

Experiment:

You are free to form whatever opinion you would like. If you are at all in doubt about what to believe I would recommend performing this experiment to find out the truth for yourself:

  1. Establish 2 test plots near to each other with similar soil and environmental conditions.
    • Test Plot #1: Keep the soil in this test plot constantly covered with at least 1 inch of mulch (wood chips are the ideal mulch for hop plants)… No fertilizers (even organic fertilizers), No pesticides of any kind, No tilling
    • Test Plot #2: Keep the soil in this test plot completely free of all organic matter (mulch) and all weeds, No fertilizers, No pesticides, No tilling
  2.  Observe the results over several years. Which plant is more vigorous? Which looks healthier? Which produces more? Which has more disease?
  3.  If you do this you will not need to rely on the advice of anyone else; you will know the truth for yourself.

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