rose milkweed and river birch
Thank you for the question. Swamp milkweed, also called rose milkweed, is a native perennial that is highly adaptable to moist or dry conditions and full to part sun conditions. River birch needs full sun and a moist, slightly acidic soil to do the best but is also adaptable to variable conditions. These two plants should be able to coexist if the tree canopy allows enough sun to reach the plants and they get enough moisture during the first few years of establishment since the growing conditions are similar.
It's always a challenge to grow plants under landscape trees because the trees will always out-compete the plants for water and nutrients. This means young plants will need to be watered and tended more than if they were in a garden away from the trees.
The other challenge is to preserve the health of the tree roots if you choose to plant under the canopy. Most tree roots are in the top 12 inches of soil where they get oxygen, nutrients, and water. If you plant here, minimize root disruption as much as possible and avoid thick layers of mulch more than 4 inches deep. Destroying lots of feeder roots, cutting large structural roots, and applying thick layers of mulch can result in the death of the tree several years later.
You can carefully dig small holes between roots, insert plants and carefully replace the soil and the trees should be fine. Remember, establishment of new plants takes several years. Read through the following publications about river birch, milkweed, and planting under shade trees.
Thank you for contacting Extension.