I ATTEND Rock garden programs every chance I get, hoping they will say something about woody plants. If they do, it is usually just something on daphnes. Of course, they say a lot about the small perennials, and I guess there are a lot more of those than woody plants. So I decided that if I want to read about woody plants for the rock gardens, I have to write about it myself, even though I don’t consider myself an expert. By the way, plants I discuss are growing in our garden in Iowa (Zone 4) and if you like, you can come see them, anytime. I will start with trees, then shrubs, and lastly, my favorite, conifers, which are small trees in the alpine landscape.
I grow my woody plants in a mixture of pea gravel, #2 chicken grit, sand, topsoil, and alfalfa meal. If the plant requires an acid soil, I use a mixture of granit pea gravel, peat moss, and compost.
Acer palmatum ‘Sir Happy’ is supposed to be the smallest A. palmatum out there. We shall see. We have to keep our Acer palmatum in a pot so that should help keep them small. We got most of our small maples from Randy Dykstra, a Fulton, Illinois, nurseryman. I think our Acer platanoides 'Pickens Strain’ is the smallest one yet. We won’t know for some time as we have had it just three years. We got most of our maples from Randy, but my brother Tom found one growing in our garden. We think it is about eight years old now, having moved it once. It is about six inches (15 cm) tall and it has red leaves on it in the spring, making it the most colorful one we have.
You will know why I love woody plants so much if you come to know Salix x boydii. Bill Boyd discovered this willow in Scotland in the 1870s. Only one plant was ever found so all plants now in cultivation are clones from this original discovery. The parents are thought to be Salix lanata (a compact bushy shrub to three feet (90 cm) tall) and Salix reticulata (a compact bushy shrub to five feet (1.5 m) tall).
A beautiful shrub, Arcterica nana, is extremely rare in commerce. I got mine from Bovees Nursery, but they have gone out of business. If you know of somewhere else I can get this plant, let me know. The flowers are white to pinkish and quite fragrant. It is an evergreen and the growth bronzy in winter. The shrub grows in Siberia and, unlike most rock garden plants, likes to be moderately moist.
Most heathers are, in my opinion, too big for the rock garden, but there are a few that fit right in. One of these is Calluna vulgaris 'Yellow Globe'. In the winter, you would wonder how it got the name ‘Yellow Globe’ as the foliage is a deep red. But in August, when it blooms, the foliage is a nice yellow. There are many heaths but only two I would recommend as far north as Oelwein, Iowa, where I live: Erica carnea and Erica tetralix. Erica carnea 'Pink Mist' flowers early and long from April till July. This cultivar withstood -29°F (-34°C), last winter; however, we did have a good snow cover.
Chamaedaphne calyculata is hardy from Zones 1 to 5. C. calyculata is prettiest where its stems grow thick and stout in colder climates, as it may become leggy in the southern portion of its range. It grows best with some summer shade under deciduous trees. The flowers are white with a touch of pink.
The woody plant you will most likely hear of if you go to a rock garden presentation is daphne. Except for a very few, in my opinion, most daphnes also get too big for the rock garden. Daphne velenovskyi ‘Old Port' may be one of the exceptions. D. velenovskyi is small anyway, and ‘Old Port’ seems to be very small. Trying to move a Daphne cneorum 'Eximia' that got too big I learned that daphnes do not like to be moved. I killed the plant.
Gaylussacia brachycera (Box Huckleberry) is a rare American plant, lost to American gardens for a time. It was reintroduced through the efforts of the Arnold Arboretum. Ours is about five years old and its evergreen leaves look red to me in the winter. Flowers are white or pinkish and it is an ericaceous plant requiring considerable cultural manipulation to be grown successfully. It is theorized that one colony in central Pennsylvania came from one plant and is over 1,000-years old.
Leiophyllum buxifolium (syn. Kalmia buxifolia) may be my favorite woody plant, but then I like all woody plants. It is evergreen, which I prefer. I love to be able to walk through the garden in the winter and see the foliage. The flowering is spectacular on the box sand myrtle with white flowers and pink buds that were almost red last spring in our garden. I have ours planted in very loose soil, you would almost say sand, and we have had it quite some time, about 15 years. It grows in the Appalachians, so it is a North American native plant.
The only downside to Pieris floribunda is you may have trouble finding the plant for sale. Pieris floribunda (mountain pieris), our own native plant, is not only hardier than Pieris japonica, it’s showier, more restrained in habit, and nearly pest and disease-free. Pieris japonica is bothered by the lace-bug quite a bit. I like the plant in cold weather best. The flower buds are pinkish-red and exposed all winter, which is even better than the white flowers. Our plant has been in the rock garden for about five years and is still quite small so it may be the compact selection ‘Millstream’.
Vaccinium macrocarpon ‘Hamilton’ is a slow-growing, non-vining, compact form of American Cranberry. The flowers are pink, but the thing I like most is the foliage in the winter, which is almost black. Vaccinium vitis-idaea 'Minus’ has the common name of mountain cranberry. It is hardier than Vaccinium vitis-idaea and I appreciate it for its refined habit, excellent evergreen foliage, and big berries. Both Vaccinium macrocarpon ‘Hamilton’ and Vaccinium vitis-idaea 'Minus’ have evergreen foliage, which you know by now I love.
Now it’s time for my favorite woody plants, conifers. Don’t use many dwarf conifers in the rock garden unless you want to cut or move them a lot. Choose miniature varieties growing an inch (2.5 cm) or less per year. Even if you plant all miniature conifers, you will have to move or cut even some because one inch a year is quite a bit of growth when you look at 20 years. I think Abies lasiocarpa ‘Duflon’ is the best conifer for the rock garden. I have some that are 20 years old and they are still only about eight inches (20 cm) tall. I have no trouble growing this one. Remember conifers, even firs, like it on the dry side, so they work well in the rock garden. Our Abies procera 'Glauca Prostrata' and Picea glauca 'Goldilocks' will both have to be moved or cut down. They are slow-growing and we have had them for several years but they already need some pruning. Abies procera 'Glauca Prostrata' is very blue and Picea glauca 'Goldilocks' is a nice gold; a terrific combination when grown together.
Chamaecyparis obtusa 'Butter Ball' is one of best obtusas out there. We normally have trouble growing obtusas, especially the gold ones, due to lack of cold hardiness. Also, I had one not turn gold because it was in too much shade on the north side of my neighbor’s house. I told my brother Tom I would move it to the south side of our house, but it probably would die. It didn’t and is the best gold conifer we have in the rock garden. It will eventually have to be moved but not for a long time. Normally, the chamaecyparis we use is C. pisifera as it seems to be hardier in our climate. Chamaecyparis pisifera 'Dwarf Blue' is a nice plant that looks like it could be sheared and stay in the rock garden a long time.
Larix kaempferi 'Bambino' may be the smallest larch we have. We have it in the rock garden now and I think it can stay there for many years. The larch is one of the few conifers that loses its foliage in the winter, but it gets a nice gold color before it drops its needles. Picea asperata 'Mongolei' is a plant I got from Larry Stanley, a Boring, Oregon, nurseryman, for my birthday with prodding from my traveling companion, Pam Maurer. It is almost ten years old and still small, so I think it can stay in our rock garden for many years. Picea glauca 'Spring Surprise', a witches broom found on Picea glauca 'J.W. Daisy White', stays fairly small, but I think it will have to be cut or moved in several years. Picea omorika 'Froendenberg' has been in our rock garden for 15 years. Dennis Hermsen, a Farley, Iowa, nurseryman, thinks it will be good for at least ten more years and I agree. I didn’t think there would ever be a Pinus cembra small enough for the rock garden, but Pinus cembra 'Bergkonigin' stays small enough for quite a few years. I’ve had one for almost ten years, but eventually, I think it will have to be moved.
A lot of people have trouble with Pinus flexilis but not us. I think it takes a dry place to grow it and we have a dry garden. We got Pinus flexilis 'Ginger Baby' from Randy and I think it will be in the rock garden a long, long time.