Ask Mr. Smarty Plants

Apocynum cannabinum (Indianhemp), also known as "dogbane" is native not only to Bergen Co., but just about the whole state of New Jersey, as seen in this USDA Plant Profile.  So, you're not alone if that is any comfort. A number of varieties occur across the continent. This species can become a serious weed as it is aggressive and difficult to control. It is also a dangerous plant to have in your garden. From our page on this plant in the Native Plant Database: "POISONOUS PARTS: All parts, fresh or dry. Highly Toxic, May be Fatal if eaten. Symptom: Cardiac arrest. Toxic Principle: Resins and cardiac glycosides."

So, getting rid of it is a good goal. It is perennial, so just preventing it seeding out is not going to be enough, but it's a start. This plant is starting to bloom now-do whatever you can to keep those blooms from becoming seeds. Pull the whole plant out if you can, or keep trimming it as low as possible to keep it from re-blooming.  The problem with just pulling it out, of course, is that you are probably also be pulling out some of your milkweed, and spraying it with herbicide will have the same effect, killing the milkweed just as surely as the dogbane. These plants may be found growing as colonies due to a long horizontal rootstock that develops from an initial taproot

So, you have a multi-pronged problem; you must not only prevent it from re-seeding but also get out the root from which new plants can grow, as well. From this site, primitiveways.com, Dogbane, we learned that the milkweed you brought the dogbane in with is likely Asclepias syriaca (common milkweed), and extracted this information:

"Stems and leaves secrete a milky sap when broken. Sprouts emerging from the underground horizontal rootstock may be confused with Common Milkweed (Asclepias syriaca) emerging shoots. But note that they are not related to milkweeds, despite the milky sap and the similar leaf shape and growth habit. The flower shape is quite unlike that of milkweed flowers and the leaves of hemp dogbane are much smaller than those of common milkweed. When mature, these native plants may be distinguished by the branching in the upper portions of the plant that occurs in hemp dogbane, and also the smaller size of hemp dogbane compared to Common milkweed."

We noted that the common milkweed is native to exactly the same areas as the dogbane, so it's no wonder they got mixed up.  You will have to be careful not to put herbicide on the milkweed, or leave the dogbane roots behind, thinking they are milkweed.

From our Native Plant Image Gallery:

Apocynum cannabinum

Apocynum cannabinum

Apocynum cannabinum

Apocynum cannabinum

Asclepias syriaca

Asclepias syriaca

Asclepias syriaca

Asclepias syriaca