Blackberries (Rubus fruticosus), with their dark green leaves and jewel-like black fruits, thrive in moist, warm conditions and grow in U.S. Department of Agriculture plant hardiness zones 6 through 10. In fact, they grow so well that they are considered a weed by many gardeners in the Pacific Northwest. Whether you're trying to eradicate a blackberry patch or simply dispose of annual pruning debris, think twice before throwing blackberry canes into your compost heap.
Home Composting Piles
Blackberry thorns can be fierce opponents, ranging in size from 1/8 to 1/4 inch. Immature thorns are green and pliable, but they quickly mature to sharp, woody points. Like most small twigs and sticks, blackberry thorns don't decompose quickly in the compost bin. In fact, they might not decompose at all. If you manage your compost pile religiously and you're willing to wait several months, the thorns might disappear. If you take a more casual approach to composting, it's probably best to leave the blackberry canes -- and those nasty thorns -- out of the compost pile.
Seeds and Pathogens
In addition to thorns remaining in the compost heap, blackberry seeds and pathogens might survive the composting process to reemerge in your garden. Blackberry seeds have a hard outer coating and can remain viable in the soil for several years before sprouting again, advises Valerie Jean Rose of the Washington State University Extension. Wild blackberries can carry viruses, such as verticillium wilt, that you don't want in your garden.
Instead of adding blackberry canes and thorns to your compost pile, use a municipal composting service if your community provides one. These services chop plant material into very fine pieces and compost them at high heat so even challenging materials, such as blackberry thorns, break down. Another option is to burn the canes or tie them with twine and dispose of them curbside. Leave a note alerting your trash collector of the sharp thorns.
When it comes to dealing with blackberries, your best option is to plant thornless cultivars, such as "Black Pearl" (Rubus fructicosus "Black Pearl") or "Black Diamond" (Rubus fructicosus "Black Diamond"). To eradicate a blackberry patch, try mowing it repeatedly to kill not only the canes but the perennial roots, or dig up individual plants and their roots. Both methods take time and repeated efforts, according to the Journal of Pesticide Reform.
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