I am moving to Pennsylvania, and I would love to plant some rhododendrons. Ideally, I would like to have them show from the window. Of course, the places where they would show the best from inside the house face South and West. Is it possible to plant them facing South if there is partial shade from a tree? My backyard faces East/Northeast -- would that be the optimal side to plant them on?
2 Answers
Short answer: on average, north or east sides of your home are better, but the amount of shade and moisture availability are the most important factor.
There are several different Rhodedendron species. Probably one of the most robust choices for your area, a native species which I would recommend, is Rhododendron maximum, but most species have similar preferences. They tend to love shade, and can tolerate even heavy shade, but tend to grow best under the shade produced by trees with a high canopy, with some light coming through the canopy.
But they also need sufficient moisture, and organic matter in the soil. They do best in mesic conditions (neither too dry nor too wet), but they can survive okay on drier sites. Also, they do well where the soils are deep, acidic, and have a thick layer of organic material that decomposes slowly and forms a peaty humus.
These sorts of conditions are more likely on the north and east face of a house or other structure, because of the shade from the building, but other factors, like topography, tree cover, and soil texture can also affect the conditions, so in some cases, such as if the south side of the house is more shaded, they might actually grow better on that side. Part shade from one tree will help, but will not be optimal if it's a dry slope away from the house.
You can help improve conditions by leaving a lot of leaf litter in place under them, to create their preferred soil conditions.
If you want a similar-looking evergreen shrub, also native to your area, but tolerant of drier conditions, I would recommend mountain laurel, Kalmia latifolia. It actively prefers slightly drier conditions and thinner soil with less organic matter than Rhododendron. It still prefers some shade and some organic matter in the soil.
Basically I would look at the conditions you have and decide where you want the shrubs, and then plant Rhododendron if it is in shadier, moister conditions, and Mountain laurel if it is slightly drier but still has part shade.
If you have a spot in your yard where it is drier and exposed to full sun, neither of these plants would be appropriate. In general, in most of Pennsylvania the winters are cold enough that broadleaf evergreens need some shelter and only survive on moister, shadier sites. You can get away with slightly more in the warmest (southeasternmost) parts of the state, and perhaps in that narrow strip of land a few miles long, right next to Lake Erie, but throughout most of the state, broadleaf evergreens are restricted to more sheltered sites.
I recommend looking up county-level range maps for a species on BONAP before planting it. You see Rhododendron maximum:
http://bonap.net/MapGallery/County/Rhododendron%20maximum.png
and mountain laurel:
http://bonap.net/MapGallery/County/Kalmia%20latifolia.png
are both native throughout nearly the whole state. They are probably going to be more reliable than most other similar species.
I live in zone 5, and any of the Rhododendrons coming out of Minnesota's breeders are hardy here and into zone 4. This white paper gives more details on how to grow them as well as a handy list of hybrids for zones 4 and 5. It's a bit old, so I'm sure there are newer varieties on the market, too.
One thing I'd emphasize is answer is that Rhodies must have acid soil, so invest in a pH test (I'd recommend that you go through Penn State's Extension service). Soil tests usually also tell you whether the soil sample is loam, silt, or clay This is helpful because rhodies tend to not do well in clay, as they prefer well-drained soil.
You can add a couple amendments to lower pH if necessary: elemental Sulfur and Aluminum sulfate (some sources cite coffee and vinegar as possibilities, but in my experience they don't work well at all, and vinegar is sometimes used as a weedkiller).
Both Rhododendrons and Kalmia are understory trees/shrubs, so as @cazort notes they will not do well in full sun. Note as well that rhodies are affected by juglone, which is produced by black walnut tress, so don't plant them underneath a black walnut, if you have one on your property.
