Rhododendrons 



Are for New Hampshire 



Joe B. Parks 



How many shrubs can you name that will 

 grow in much of New Hampshire, are ev- 

 ergreen, and have a spectacular flower- 

 ing season? Not many, I'll wager. In fact, 

 rhododendrons are the only ones 1 can 

 think of. Moreover, they are among the 

 easiest, most versatile of evergreens to grow. So why 

 aren't they in every yard? 



I suggest that it may be that too many people have 

 been burned too many times by plants that failed to sur- 

 vive. Rhododendrons are so spectacular that, in full 

 bloom, they are a comparatively easy sale, it is possible 

 that the problem lies with unscrupulous (or perhaps just 

 unknowledgeable) discounters. They are able to pass off 

 onto the unknowing public, at low prices, plants from 

 warmer climates that have a poor chance of surviving 

 New Hampshire winters. Or the problem may lie in the 

 tremendous number of hybrids (over 10,000) and the lack 

 of sufficient information as to which ones are suitable for 

 our climate. This lack of information puts even a knowl- 

 edgeable nurseryman at a disadvantage when it comes to 

 rhododendrons. Hopefully, this article and the accompa- 

 nying list will be of some help. 



The genus rhododendron is large and widely distributed 

 throughout the world. Rhododendron habitat ranges from 

 near the Arctic circle for species such as R. brachycarpum 

 to the tropics for the vireyas and extends from sea level 

 to above 13,000 feet. Many species prefer some shade, 

 some do best in full sun while still others do equally 

 well in sun or shade. Most, if not all, prefer an acid soil 

 which is well-drained, but affords plenty of moisture. 



Thus, since those in the marketplace are usually hy- 

 brids, there's no quick-and-easy way to ascertain their 

 needs or hardiness. Also, as we all know, a plant that is 

 "hardy" in New |ersey or along the Connecticut South 

 Shore is something different from one that is hardy in 

 New Hampshire. The end result of all of this is, for many 



a conscientious nurserymen (and the public), a decision 

 to play it safe — stay away or stick with known cultivars 

 such as 'Roseum Elegans', 'Boule de Neige' and the like. 

 The public, as well as the nurseryman, is the loser. 



Though most are aware of rhododendron's special re- 

 quirements, perhaps a few comments about their needs 

 might still be in order. Their needs for an acid soil are 

 well-known. Also well-known is the fact that (except for 

 areas along the Connecticut River and an occasional 

 house foundation) New Hampshire's soils are acid — 

 though those that are below pH 4.8 are really too acid. 

 What is not so well-known is that our soils tend to be 

 sadly deficient in magnesium and calcium. Both nutrients 

 are required by rhododendrons. To avoid raising the pH, 

 I use gypsum (calcium sulfate, instead of lime to provide 

 calcium. For magnesium (number four on the list of plant 

 nutrients), I use Epsom Salts (magnesium sulfate) at the 

 rate of about a half teaspoonful per five-foot plant every 

 couple of years. I also provide 5-10-10 fertilizer early in 

 October. Because rhododendrons prefer a well-drained, 

 but moist, soil high in organic matter, I use large quanti- 

 ties of rotted wood chips for planting and mulching. Peat 

 moss can be used equally well for planting, but is unde- 

 sirable for mulching. A problem that is often not under- 

 stood is the fact that although rhododendrons prefer a 

 moist location, they do poorly with wet feet. So in areas 

 that do not drain well, 1 raise the planting bed above the 

 surrounding soil . 



For the last 15 years I have maintained a rhododen- 

 dron hardiness record based on annual reports from 

 growers in New Hampshire and Massachusetts. It now in- 

 cludes almost 5000 entries on over a thousand species 

 and cultivars. Reported minimum winter temperatures 

 range from 2F to 5F. Enough data has accumulated on 

 some species and cultivars to enable some assessment 

 of how well they survive New Hampshire winters. From 

 that data, I have culled a list of 30 plants that can be ex- 



APRIL * MAY 1996 



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