Azaleas 



for New Hampshire 



joe Parks 



'zaleas — "Beautiful, versatile shrubs 

 for everyone;" "An abomination 

 foisted on the unwary;" "A disease- 

 and-insect-prone unsightly plant;" 

 "Gorgeous butterflies of the rhododendron world " You 

 are likely to hear these comments and others just as 

 diverse whenever there is a discussion of deciduous 

 azaleas. Who's right? Well, in truth, everybody! The 

 problem is that all deciduous azaleas are not created 

 equal. And it is of considerable importance for you as a 

 retail nursery owner who values your customers to 

 know the differences. 



The problem is that many of the azalea cultivars in 

 the trade in New England are highly susceptible to mil- 

 dew and green sawfly larvae damage. The larvae strip 

 the leaves (and sometimes flowers), leaving only a 

 needle-like midrib; the mildew disfigures the new 

 leaves that grow out. Yes, there are sprays to kill sawfly 

 larvae and prevent mildew but, to my way of thinking, 

 this is an undesirable solution for the average home- 

 owner. Five months of bare branches, a week or two of 

 flowers, a month of stripped leaves and two months of 

 spraying or of disfigured, mildew-covered leaves do not 

 a fine plant make. I suggest that if you handle disease- 

 prone cultivars, you should advise your customers 

 about controls. 



Notice that 1 said 'cultivars' and not 'plants,' for the 

 problem is genetic. Fortunately our Eastern North 

 American (and some exotic) species are highly resistant 

 to both saw fly larvae and mildew. Some hybrid prog- 

 eny also display this resistance, so there are plenty of 

 "good" plants available (see list). Now that the prob- 

 lem — and the solution — is understood, there will be 

 more resistant cultivars coming onto the market. 



I should point out that this problem with mildew and 

 sawfly is not universal. New lersey growers don't seem 

 to think it a problem and when I talked to hybridizers 

 in Oregon last year, they wanted to know why the fuss. 

 Unfortunately, not having the problem, they pay little 

 or no attention to it in their hybridizing. 



Even though there are some problems, deciduous 

 azaleas are eminently suited for New Hampshire. There 

 are resistant species and hybrids hardy from the Sea- 

 coast to the farthest reaches of Coos. There is even a 



resistant hybrid, 'Lemon Drop,' that is hardy to at least 

 -34°F. (Zone 3B). Furthermore, they offer a long season 

 of bloom, starting in April and ending in )uly. Better 

 still is the wide range of flower color which includes 

 reds, pinks, oranges, yellows, lavenders, and whites 



The cultural requirements of deciduous azaleas are 

 much the same as for all rhododendrons. They need an 

 acid soil (pH4.8 to pH6.0) that is high in organic matter. 

 Certainly we have plenty of acid soil (except house 

 foundations and some Connecticut River Valley areas). 

 For this reason, use of acid-type fertilizers is unneces- 

 sary and even undesirable. In particular, aluminum sul- 

 fate (AlSOJ, or materials containing it, should never be 

 used around any rhododendron regardless of the soil 

 pH. Although AlSO^ will reduce the pH, the aluminum 

 component is deadly to all members of the genus. 



If azaleas are well-mulched, they are unlikely to 

 need much fertilizer. It is best to have soil tested to 

 determine actual needs. But since testing is so seldom 

 done, I recommend that a single light fertilization be 

 given annually. Any low-nitrogen type of fertilizer such 

 as 5-10-5 or 5-10-10 is satisfactory. It should be applied 

 in small amounts (maximum of a tablespoonful around 

 a four-foot plant) after the first fall frost and before 

 spring thaw. Late-spring or summer fertilization can in- 

 duce late-summer growth, resulting in winter kill. 



Azaleas (indeed all plants) need calcium and magne- 

 sium for photosynthesis. New Hampshire soils are noto- 

 riously deficient in both of these. Though all members 

 of the rhododendron genus are well-adapted genetically 

 to growing in mineral-deficient soils, it is not unusual to 

 see plants in New England with yellowing, chlorotic 

 leaves due to the lack of these two minerals. 



Unfortunately, the ordinary solution of using of 

 dolmitic limestone to correct the problem is not a 

 choice. Since this would increase the soil pH, it could 

 create even worse problems. The answer is to use gyp- 

 sum (CaS0^.2H^O) and Epsom Salts (MgSOJ. Not much is 

 needed. Lacking a soil test, 1 would suggest using no 

 more than half a teaspoon of each annually per four- 

 foot plant. Now a tip for extra business and customer 

 goodwill: carry some gypsum in stock and put up a 

 sign, "For your rhododendrons". There are very few cur- 

 rent sources and few people know its value. 



APRIL 8^ MAY 



997 



