Companion planting for blackberries aids the fruiting shrubs in several ways. Depending on the plants chosen, blackberry companions can help repel pests, deter disease, attract bees for better pollination, accumulate nutrients in the soil and stop competitive weeds. Blackberries themselves make good companions as well, when planted near crops such as grapes. Don't forget, too, that sometimes what you plant before your main crop can be just as important as what you plant beside them.
Perhaps more than any other plant mentioned in connection with blackberry, tansy is frequently recommended as a blackberry companion. The yellow-flowering herb possesses a reputation as an especially good friend of blackberries, grapes, cucumbers and roses. Because its leaves contain a potent mix of camphor, borneol and thujone, tansy repels and in some cases kills all manner of "chompers"--striped cucumber beetles, mice, ants, flies, Japanese beetles and many flying insects. Do not plant tansy too close to blackberries, as it spreads aggressively.
Strong-smelling garlic famously protects roses from aphids, but in fact all members of the rose family, including blackberries, benefit when garlic grows nearby. Garlic not only repels aphids, but also beetles, borers, spider mites and weevils. Grow garlic between blackberry rows, between bushes or about a foot in front of a blackberry border.
While grapes don't specifically help blackberries, blackberries definitely come to the aid of grapes when the two grow in alternating rows. A kind of insect called grape leafhopper poses a serious threat to grape crops. But blackberry shrubs host the mortal enemies of the grape leafhopper--a parasite that attacks and kills the grape leap hoppers.
Other Beneficial Herbs
Several flowers and herbs fight disease and repel insects for a wide variety of food crops, including blackberries. Notable examples include borage, marigolds, chives, geraniums and rue. Bee balm and other flowering herbs help attract bees, which aids in blackberry pollination.
The second year after blackberry plants become established, sow an annual variety of oats in the fall. The oats compete with the blackberries for water and nutrients. This seemingly counter-intuitive method actually helps "harden off" the blackberry's roots in preparation for the winter, according to the University of Maine Cooperative Extension Service. For a permanent cover that will suppress weeds and control erosion, the extension service recommends a bluegrass-fescue grass mix.
In "Gaia's Garden," author Toby Hemenway discusses an expanded idea of companion planting, this time involving several plants which become interdependently beneficial. In the community in which Hemenway places blackberries, this guild includes white oak trees for wildlife, leaf mulch and storing large reservoirs of moisture in soil; hazelnut for food; serviceberries, thimbleberries and blackberries for edible fruit; strawberries for fruit and as a mulching groundcover; and sweet cicely and American vetch to add nitrogen to the soil and repel pests.
A good pre-planting strategy for blackberries involves sowing oats, millet, buckwheat or rye to the future blackberry patch. These crops help add humus and nutrients to the soil when allowed to grow for one year, and then tilled in a few months before planting the blackberries. To rid the soil of white grub and weeds, plant and harvest wheat or corn the year before planting blackberries.