Jim wrote:

My mother was wondering what kind of plants would grow good under full
sized rhododendrons that are in zone 6. We live in the North East.

Rhododendrons have shallow roots so never cultivate the soil around a
rhododendron and it is not a good idea to plant other plants under a
rhododendron. If you stay outside the drip line of the rhododendron,
you can plant companion plants that have deep roots.

Rhododendron and Azaleas are ericaceous plants, members of the heath
family, which is called the "acid loving" family. Their special cultural
requirements and shallow root structure make them incompatible with some
plants. Astilbe is an Asian perennial herb which grows well with
Rhododendron and Azaleas. Kalmia, Mahonia, Pieris, and Viburnum are
evergreen shrubs which compliment Rhododendron and Azaleas. Many of the
viburnum will grow tall and can be used behind Rhododendron or Azaleas.

Since rhododendrons have shallow roots and love partial shade, it is a
common problem to find a suitable shade tree. Four trees are well known
for having deep roots, being fast growing, and providing the "high
shade" that is often recommended for rhododendrons and azaleas. These
are Ginkgo biloba or the "Maidenhair tree", Metasequoia glyptostroboides
commonly know as the "Dawn redwood", Nyssa sylvatica or the "Sour Gum",
Quercus rubra or the "Northern Red Oak". Even with well behaved trees
such as these it is always good to dig down about 5 inches between
adjacent rhododendron or azaleas to prevent the intervention of other
roots into their root space. It is not uncommon to find tree roots
growing well beyond the drip line of a tree, so trees in the immediate
vicinity are not the only concern.

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