How to Manage Pests
Pests in Gardens and Landscapes
Of the 11 species of Rubus in California, four were introduced primarily from Eurasia. Most species of wild blackberry, also called brambles, provide important sources of food and cover for many birds and mammals.
Four species, however, are considered weeds. Two of these are non-natives, cutleaf blackberry (R. laciniatus) and Himalaya blackberry (R. discolor [formerly known as R. procerus]). In addition, two native species also can be weeds under certain conditions. For example, thimbleberry (R. parviflorus) competes with conifers during establishment in reforested areas, and California blackberry (R. ursinus [formerly known as R. vitifolius]) can infest areas adjacent to streams and ditches. Of these weedy species, the most common, vigorous, and troublesome is Himalaya blackberry.
Of the four weedy wild blackberries, thimbleberry is the only nonvining species. It also lacks prickly stems and has a simple leaf with no leaflets. Both Himalaya and cutleaf blackberry have five-angled stems whereas thimbleberry is rounded in cross section, but Himalaya blackberry is easily distinguishable from the other wild blackberries by its five distinct leaflets, each one toothed and usually oval. By comparison, cutleaf blackberry has five very deeply lobed leaflets, and California blackberry has only three leaflets. Not all wild blackberry leaves are deciduous; many remain evergreen. This is an important feature for chemical control in late fall and winter.
Himalaya blackberry has showy flowers that form in large clusters at the end of shoots. Each flower is about 1 inch across with five white or pink petals. The fruits are black and tasty when ripe. New canes are produced each year from the crown (the base of the plant), replacing those that die naturally. New plants start from crown regrowth, rhizomes (horizontal, underground shoots), and seeds that germinate in fall and spring. Reproduction is similar for the other three species.
The scrambling habit of Himalaya and the other vining wild blackberries smothers existing plant growth. In addition, the tangled mass of thorny stems blocks access of humans, livestock, equipment, and vehicles to pastures and waterways. In addition, it can host Pierce's disease and serve as a vector to movement of the pathogen to other agricultural and nonagricultural areas, including riparian sites.
In forest areas, timber-logging operations create large open areas that wild blackberries often invade. When grazed, the thorny stems can injure the nasal passages of livestock. Another undesirable aspect of vining blackberry plants is they are a good source of food and shelter for rats.
Many animal species feed on wild blackberries; consequently, seeds spread easily from one area to another in animal droppings. Wild blackberry seeds have a hard seed coat and can remain dormant for an extended period. Once seeds germinate and grow and the plants become established, expansion of the thicket is almost entirely a result of vegetative growth from rhizomes. Over time a single plant can cover a very large area.
Wild blackberry plants can live for 25 years or longer. They produce vines that arise from a central crown or from buds that form along rhizomes. First-year canes don't produce flowers. In the second year, the canes fruit and die. Tips of first-year canes that contact the ground form roots at the nodes, contributing to the lateral expansion of the plant.
Bumblebees and honey bees are the primary pollinators of wild blackberry flowers. The flowers can be self-pollinated, but cross pollination increases fruit set.
Wild blackberries are able to regenerate from the crown or rhizomes following mowing, burning, or herbicide treatment. This makes them difficult to control, and control measures often require follow-up treatment. Land managers often rely on a combination of mechanical and chemical control methods followed by a prescribed burn to dispose of vegetative material.
Because of the extensive underground root system, digging out the plants in a home landscape is a difficult undertaking. Home gardeners generally must rely on foliage-applied herbicide treatments to control an infestation of wild blackberries. One nonchemical option in the home landscape is the use of a rototiller to till the ground several times after the canes have been removed.
Because repeated tillage easily controls wild blackberries, they aren't a problem in cultivated agricultural systems. A single cultivation, however, can fragment the rhizomes and spread the weed. Bulldozing also can cause resprouting and can spread the weed by fragmenting roots and stems.
Mowing isn't an effective method for controlling wild blackberries. In many cases it stimulates the formation of suckers from lateral roots and induces branching. Despite the lack of long-term control, mowing or chopping can provide short-term canopy reduction that will encourage the growth of grasses and broadleaf plants.
Burning, like mowing, isn't an effective long-term strategy, because wild blackberry plants vigorously resprout from rhizomes. However, like mowing, it also provides short-term canopy reduction.
Because many Rubus species are native or of economic importance, biocontrol isn't a practical control method in California. In Australia, however, blackberry leaf rust (Phragmidium violaceum) has been released for control of the weed. Thus far this program has not been successful, because the rust hasn't caused significant damage to its host. The rust was discovered in Oregon in the early 2000s and appeared to cause some damage to Himalaya blackberry populations. However, it has not maintained that level of injury and hasn't become widespread in California.
Blackberry plants usually regrow following herbicide application; thus, repeated treatments might be necessary for effective long-term control.
Herbicides applied to the soil
In noncrop areas, tebuthiuron (Spike) is registered for use by licensed applicators for brush control. Tebuthiuron is a nonselective urea herbicide that is used for total control (i.e., it eliminates other vegetation in the treatment area) of shrubs, trees, and other weeds. It can be applied in a pelleted formulation at the base of the plant to provide long-term control of wild blackberries.
Herbicides applied to the plant
Herbicides can be used in rangeland, pastures, noncrop areas, along roadsides, and in right-of-ways to control actively growing wild blackberry plants.
To effectively control blackberries during the growing season, an herbicide must be transported within the plant to the rhizomes and new growing points. For this to occur, the herbicide must move in the phloem with the plant sugars produced through photosynthesis.
In early summer during the rapid extension of canes and expansion of foliar tissue, sugars are transported within the plant from the underground storage tissues to the shoots. After midsummer, new growth is reduced in wild blackberry first-year canes (nonflowering shoots), because these shoots are actively transporting sugars to the rhizomes. These sugars are stored for the following year's growth.
In the flowering shoots (second-year canes), movement of sugars from the shoots to the rhizomes occurs later in the season than it does for first-year canes and is most active after completion of fruiting.
Time a foliar herbicide application so that it coincides with the maximum rate of sugar movement to the root system. This will depend upon whether the plants are primarily first-year canes or a combination of both first- and second-year canes.
In a situation where only first-year canes are present (for example when plants have been burned or mowed), the most effective time for optimal herbicide transport to the root system is in late summer. Herbicide application at this time reduces the likelihood of regrowth in subsequent years.
Where the bramble infestation consists primarily of second-year canes or a combination of first- and second-year canes, apply an herbicide in early fall, before plants become dormant. Herbicides applied too early generally result in good kill of the top growth but very little movement of the chemical to the root system. Consequently, the plant regrows.
Plants stressed from drought or grazing don't translocate sugars as rapidly as do actively growing plants. Thus, chemical control of wild blackberry plants under stress is difficult and not recommended.
Herbicides used to control wild blackberry during the growing season include glyphosate, dicamba, dicamba/2,4-D combinations, and triclopyr. Of these, glyphosate (Roundup and other products containing glyphosate) and triclopyr (Brush-B-Gon, Blackberry and Brush Killer) are registered for use by home gardeners.
Glyphosate formulated into a product with 41% active ingredient (a.i.) can provide good to excellent control of wild blackberries when applied in a 0.5 to 1.5% solution (i.e., about 0.6 to 2 ounces of product per gallon of water).
Dicamba alone (Banvel, Vanquish) or plus 2,4-D applied in late summer gives good control of wild blackberries. However, 2,4-D alone provides only fair control and will result in resprouting.
Triclopyr is available to licensed applicators for commercial use in either amine (Garlon 3A) or ester (Garlon 4) formulations.
Basal bark treatment
Concentrated forms of triclopyr (often mixed with commercially available seed oils for better penetration) can be applied to basal regions of wild blackberries with a backpack sprayer using a solid cone, flat fan, or a straight-stream spray nozzle. Thoroughly cover a 6- to 12- inch basal section of the stem with spray but not to the point of runoff. Basal bark applications can be made almost any time of the year, even after leaves have senesced (aged, dried, and fallen from plant). In areas where people frequently harvest the fruit of wild blackberries, a midfall basal bark treatment might be desirable to avoid human contact with the chemical.
Dormant stem and leaf treatment
As an alternative to basal bark treatments, a 1% solution of triclopyr ester can be applied to dormant leaves and stems in late fall and winter in a 3% crop oil concentrate mixture; see product labels for the rate to use to obtain the desired concentration. As with other herbicide applications, spray the plant until it is thoroughly wet but not to the point of runoff. Like basal bark treatments, the timing of this technique prevents human contact with the herbicide during berry-picking season.
DiTomaso, J. M., and E. A. Healy. 2007.Weeds of California and Other Western States. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3488.
DiTomaso, J. M., and G. B. Kyser. Jan. 2008. Pest Notes: Woody Weed Invaders. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 74142.
Pest Notes: Wild Blackberries
Author: J. M. DiTomaso, Plant Sciences, UC Davis
Produced by UC Statewide IPM Program, University of California, Davis, CA 95616
Technical Editor: M. L. Flint
Produced by University of California Statewide IPM Program
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