MVH Studies on Various Forms of mulch

Grass clippings (Fig 1) - Poor suitability as a mulch for hops. It can easily blow away with hard winds and must be constantly reapplied. It can harbor disease and molds, especially as it decomposes. As it compacts down over time, it forms a hard cap on top of the crown that holds in excess moisture. When lifted off of the hop crown, excessive moisture can be found along with significant mold development and possibly crown rot/decay (Fig 1).  

Hay/Straw (Fig 2) - Poor suitability as mulch for hops. It easily blows away and must be constantly reapplied. The decay process can also create mold and spread disease in a similar manner as grass clippings. If too much is applied, a hard cap can be created and produce the same problems as grass clippings.

Cedar Chips (Fig 3) - Fair suitability as a mulch for hops. It takes more wind to blow them away, but they still can blow away fairly easy and must be regularly applied. It was found that drenching the chips at the time of application helped compact them down enough to last a little longer on hills before reapplication was necessary. Cedar contains natural oils that can help repel some pests. No adverse affects from those oils were noted in any hop plants or varieties. Furthermore, they did not create a situation where too much moisture was retained on top of the crown. However, it remains uncertain what effects cedar chip decomposition may have on hop crowns, or how long decomposition takes. Cedar chips can be expensive as well. Finally, if cedar chips are covered with any other form of mulch, they tend to retain and hold excess moisture and can create disease and hop crown decay problems.   

Cyprus Mulch (Fig 4) - Good suitability as a mulch for hops. It takes very strong winds to blow this material off of the hop hills, depending on the brand and its shred consistency. It provides adequate cover for moisture retention in soil without creating a problem of crown rot or decay. Periodic applications may be necessary throughout the season, but Cyprus mulch appears to be very cost effective. Like Cedar chips, it remains uncertain what effects Cyprus mulch decomposition can have over a hop crown, or how long decomposition takes. Also, Cyprus mulch does not harbor disease like some forms of mulch, and has been shown to be disease and pest resistant.  

Shredded Hardwood Mulch from local sawmill company (Fig 5)- Poor suitability as a mulch for hops. Initially this product showed promise like Cypress mulch. It does not blow away in hard winds and at first provided very good water permeability without leaving the crown hills water logged. However, as the wood decayed over 3 to 4 months, it became hard packed and resembled the hard caps that grass clippings created. This situation created areas for mold and crown decay to form. Furthermore, some types of deciduous trees can harbor the Verticillium wilt pathogen which could then be transferred to the hop plant.

Landscape Fabric/Black Plastic/Cardboard (Fig 6) - Poor Suitability as a mulch for hops. While MVH has not experimented with these forms of cover, we have seen its results elsewhere. Black plastic prevents water from reaching the roots. It also deteriorates within 2 to 3 years and is easily cut up by mowers and weed eaters. Similar problems with deterioration and maintenance impediments can occur with landscape fabric and cardboard. Furthermore, all forms of this kind of mulch or weed preventative create a situation in which the ground below the mat warms up and forces the hop roots and rhizomes to grow right at, or on top, of the ground surface.Moisture retention produced by these mats then creates a prime situation for crown rot, disease development and soil compaction.  

Plain Dirt - Fair suitability as a mulch for hops. In denser hop yards where the plant canopy can shade the ground, and/or rain events during the growing season rarely exist, plain soil as a covering can be fine. However, in regions and yards where the plant canopy is not as thick or it rains a lot, the soil can easily dry out between rains and erode away to expose the crowns. For the winter months, plain soil may be the best source for protection for over-wintering buds.  Also, it has been found that in climates that are wetter, more dirt over the tops of crowns can cause significant crown rot and/or disease due to the crowns constantly staying muddy.  In some cases, no covering can be better.   

While no single form of mulch appears to be an all around excellent choice for weed prevention, cold protection, soil erosion control or moisture retention, a combination of products appear to work well at MVH. Our choice is to use Cyprus mulch for moisture retention and ground erosion control during the growing season. If further protection is needed for winter, the Cyprus may be removed and plain soil used to lightly cover the crown buds. We do not use, nor recommend using compost to cover hop crowns. The nitrogen content that many composts possess or may create from decomposing can cause an increase in disease presence and pressure, especially if applied at a time when the hop plant(s) cannot take up and utilize the excess nitrogen. Furthermore, if the compost is not heated to the proper temperatures during its decomposition, diseases may not be destroyed and can then be spread to the hop crown.