Himalayan Blackberry (Rubus armeniacus) is an introduced semi-shade-tolerant bramble that readily colonizes bare ground and damaged soils and spreads by bird dispersal. Large areas of lowland Salish Sea have reverted to blackberry after we cut forests, damage soils, and reduce native seed banks an seed rain. Over time, blackberry forms dense impenetrable thickets that may be six or eight feet tall. A recurring revegetation scenario is attempted conversion from blackberry to young native forest. Blackberry grows back rapidly from crowns, rhizomes and seed bank, and can outgrow native shrub vegetation, and tip roots from arching perennial canes. Efficient strategies must understand the strengths and weaknesses of our native flora, soil conditions, the rampant productivity of blackberry, and natural patterns of native forest development.
Objectives of Blackberry Conversion
Blackberry can occasionally colonize seasonal wetlands with high water level fluctuation on hummocks but generally the presence of blackberry indicates hydrology that supports native forest, and so either coniferous or deciduous native forest is typically the target of blackberry conversion. In some cases, native canopy trees are not desired (because of infrastructure, shading, or view preservation) and so the target is some kind of alternative shrubland. Like all revegetation, blackberry conversion involves some combination or cycle of disturbance, propagation, and tending. Because of the rampant regrowth potential of blackberry, the initial disturbance shapes the arc of restoration.
General Components of a Blackberry Conversion Strategy
The following components are combined into strategies
- Mowing - Blackberry conversion usually begins with mowing, with a brush hook or ditch-bladed scythe by hand, a walking brush cutter, or attached to a tractor or skid-steer.
- Hand Grubbing - Blackberry crowns produce the most vigorous regrowth, and so many conversion strategies involve manual removal of knobby root crowns, most efficiently accomplished using a forked hoe.
- Mass Grading - projects that involve extensive earwork can use grading to disrupt rhizome networks and bury patches.
- Weed Fabric Methods - nursery grade woven weed fabric can be purchased in massive rolls, and used to cover over mowed backberry, exhausting roots. Plants can be propogated on seams. vigorous canes can push up fabric or escape from seams.
- Deep Mulch - an alternative to weed fabric, deep mulching with ramial chips can greatly reduce regrowth, from rhizomes, while improving soils, and improving legibility of follow-up treatments.
- Herbicide - herbicide application is an alternative to hand grubbing. However herbicide may not translocate completely into the rhizome network resulting in regrowth from rhizomes, suggesting multiple applications.
- Rapid Canopy Closure - dense planting of deciduous canopy trees such as willow, cottonwood, alder can outpace blackberry regrowth, and practically eliminate blackberry during stem exclusion.