In the last issue I discussed the easy broadleaf evergreens for Colorado and this time we will go into the difficult ones. Many people have killed or had poor success with rhododendrons, hollies, azaleas and daphnes etc. so it is good to understand these plants further. Unfortunately in my sunny, low-water gardens, I have had little experience with them so I went to two people who have plenty of experience, Don Zaun and Allan Taylor.
Don Zaun came to Boulder eight years ago from northern Illinois where he lived on the edge of an oak forest. There he grew 200 evergreen rhododendrons and some 300 deciduous azaleas and rhododendrons. He belonged to both the Rhododendron Society and the Holly Society and was quite successful with his results. Now in his 80’s, Don is gardening less ambitiously but he still loves rhododendrons so he has adjusted his methods to fit Colorado. He says the main problem with growing rhododendrons here is not the cold temperatures, because northern Illinois was colder than it usually is here; the main problems are dry winter soil, alkaline soils and lack of snow cover. Because our soils do not naturally support acid-loving plants, Don doesn’t recommend planting large areas of them. However for his own smaller plantings, here is how he prepared the ground. He mixed in half Canadian peat, half redwood bark, some quarter inch granite chips, a sprinkling of iron sulfate and up to half a cup per plant of horticultural sulfur. He let this sit for up to four weeks because planting immediately might have burned the plants. These beds were sited where they would get very little direct sun in the winter. He recommends planting on the north side of a building or with eastern exposure with the protection of evergreens from southern sun.
The varieties Don recommends for here are PJM, Olga, Algo, Purple Gem and Rhamapo. In general, he believes the smaller-leafed varieties are easier here, but that the large-leafed ones can be grown if protected from direct sunlight and drying winds and if they are winter watered. He has tested many evergreen azaleas in Illinois and does not recommend them for Colorado.
Near his sheltered front door, Don is growing Gaultheria procumbens, Hardy Wintergreen, which is an uncommon broadleaf evergreen with white, bell-shaped flowers in May followed by red berries. It requires acid soil and moderate moisture. When I asked him about growing evergreen hollies in Colorado, he was quick to say that he would be growing some if he had the room, because it is less crucial to give them acid soil and they can be hardy. Ilex opaca has some hardy varieties, but he would probably plant one of the Meserve Hollies; Blue Prince and Blue Princess are good but Blue Maid is perhaps hardier with better color. These breakthrough successes for cold climates were bred by a woman amateur in her New York kitchen where she crossed the European Holly, Ilex aquifolium with Ilex rugosa which is evergreen in Siberia.
On The Hill near Baseline Road in Boulder is a remarkable and wonderful garden belonging to Allan Taylor. Allan has long been in the linguistics department at CU, but even longer he has had a love of plants. In particular, he has given much of his life to pushing the envelope of plant hardiness and adaptability. Because of his successes with evergreen rhododendrons, I asked him how he did it. His six keys to success are: 1) pick cultivars that are hardy to –35 F and are –25 F bud hardy. In general these would be varieties of the American native R. catawba, the Asian R. meternichii and even better, R. yakushimanum from a Japanese island. He said, yes, PJM is hardy but blooms too early and is less attractive. 2) choose the right site with winter shade for five months and no direct sunlight even in the growing season, but good indirect light. 3) right soil prep means removing a large well of soil and back-filling if the original soil is too alkaline; or incorporating Canadian sphagnum peat, fine bark of pine or cedar and coarse sand. Leaves, especially oak leaves are also good to include. Aim for good drainage and lots of organic matter. 4) use a heavy mulch 2”-3” deep of bark or pine needles to hold moisture and to keep the ground from freezing 5) don’t let the soil dry out summer or winter. If the leaves hang or the flowers droop, they need water. In the winter if the ground is not frozen and we’ve had some hot, dry weather, water. 6) give them wind protection; this usually means from the drying west winds. Alan recommends three mail-order sources for rhododendrons: Greer Gardens in Eugene, Oregon; Kelly Green in Drain, Oregon; and Roslyn Nursery in Dix Hills, NY.
Allan agreed with Don about the Meserve Hollies being good for Colorado, and added that good varieties of Ilex opace are ‘Thanksgiving’ and ‘Canary.’ He suggests the same treatment as for rhododendrons.
Another broadleaf evergreen that Allan has many specimens of is Arctostaphylos. Many people are familiar with Kinnikinnick, A. uva ursi, the groundcover form, but few people are familiar with the taller varieties such as A. nevadensis and A. patula. These are natives of Colorado, Utah, Nevada, Arizona etc. Every spring Alan goes out to search for excellent natural crosses and selections which he marks and then returns in the fall to take cuttings. Consequently he has numerous enviable varieties in his garden that are unavailable from nurseries. These native broadleaf evergreens are far more tolerant of sun and Colorado soils than the eastern natives like rhododendron and holly, though protection from late afternoon sun and the addition of decomposed granite to help acidify are preferred.
Allan also has specimens of the broadleaf evergreens Pieris and Kalmia which are uncommon but can be seen at Denver Botanic Gardens. Then he surprised me with broadleafs I had never even heard of before: Quercus vaccinifolia, an evergreen oak; Berberis trifoliolata with desert mahonia-like blue leaves; and Cistus laurifolius, from Bosnia and the only hardy species.
Another broadleaf evergreen requiring proper selection and siting is boxwood. Many varieties are tender or burn too badly in our winters, but some are successful. I have an 18” diameter specimen of Buxus microphylla ‘koreana’ on the east side of the house that gets little direct sun in winter. It survives well on my low watering regimen. It is supposed to have brownish leaves in the winter, but mine is usually a good green. Another koreana varieties is ‘Winter Gem’ ; and B. koreana crossed with B. sempervirens are ‘Green Gem’,’Green Mountain’ and ‘Green Velvet.’ B. sempervirens ‘Vardar Valley’ is doing fine in a sheltered spot at DBG but reportedly is only hardy to –15 F. ‘Julia Jane’ is one I know to be hardy but I don’t know its origins.
One might well wonder why I would include Pyracantha with the difficult broadleaf evergreens since they grow well in our soils, even with little water, sometimes putting on 18”-24” of growth in a single season; and decking the walls with copious sprays of orange berries. But I have my reasons. To begin with, “Pyra” means fire and “ cantha” means thorn; just keeping them in bounds by pruning requires gauntlet gloves and goggles. Never plant one near a walk or doorway. In addition, pyracantha is both susceptible to fireblight and only marginally hardy here, so it can die back severely. Then get out the chain saw and loppers and prepare to learn why roses only have “prickles” and pyracantha has firethorns. It is true that from this hacked and stubby appearance they can recover quickly and proper siting can reduce dieback. Unfortunately the promise of a hardy variety is questionable. Dirr says that none of the P. coccinea varieties will tolerate cold below –15F and regarding ‘Yukon Belle’ which is supposed to be Z4 hardy, Dirr says “but I doubt it.” However we see them thriving all over. Still I think they are difficult. Now if you want a barrier or are trying to discourage thieves from getting close to your windows, go for it. The painters will hate you, but they are beautiful plants, best sited out of late afternoon sun and west wind.
Other difficult but possible broadleaf evergreens for Colorado are Daphne burkwoodii ‘Carol Mackie’, D. burkwoodii ‘Somerset’, Daphne tangutica and Myrica pennsylvanica, the Bayberry. I’m sure there must be others. In Colorado the difficult broadleaf evergreens must be screened for the right varieties. Many need soil modifications to match their forest origins.In general they need protection from winter sun and drying winds and must have moderate watering and mulching. I suspect that the use of anti-transpirants like Wilt-Pruf or Anti-Stress 2000 would help a lot to protect the foliage in winter. I favor well-adapted (well-adjusted) plants and usually think “why bother” with the rest, but Don Zaun and Allan Taylor have taught me a valuable lesson. Like the human infant that requires care and culturing for quite some time before it is ready to be self-sufficient, there are plants that need more care to get established. Some of these can later become more self-sustaining like the great Tulip Tree at The Briar Rose on Arapahoe in Boulder. Others will never “leave home” and will always require some care, but their value in the garden makes these children worth it.