Picea pungens 'Hoopsii' straddles size categories: Dwarf in youth it will eventually cross into the Large category
Conifers and Size Classification
When one sees the word “dwarf” attached to a conifer one would logically think, “This evergreen will remain small.” Not so quick! One must know the eventual size of the straight species from which this dwarf plant descends. If, as in the case of Dawn Redwood which may grow to a staggering 100 feet, a dwarf growing version may achieve 60 or more feet over time! This is certainly not the conjured image the word "dwarf" would suggest in the minds of the uninitiated... So, let me please help initiate you.
Chamaecyparis thyoides, commonly called White Cedar, offers many worthy forms which are genetic variants. Some are small and others large. 'Shiva' which is a large and fast growing upright green pyramid easily growing 12 to 15 feet high in ten years. Comparatively, Chamaecyparis thyoides 'Little Jamie' grows into a fat 4-foot gumdrop of a shrub in 10 years. Very different in look is Chamaecyparis thyoides 'Variegata' which displays almost thread-like dark green foliage splattered and speckled in cream-yellow is a large-grower like 'Shiva' and will stretch up to 15 feet tall in a 10-year span. Very different in look and size, growing 15 to 25 feet in 10 or more years but, nevertheless, quite elegant is Chamaecyparis thyoides 'Glauca Pendula'. This version of White Cedar grows into an irregular upright displaying great character at an early age with its striking affect in an amazing powder blue color. 'Glauca Pendula' is also an elegant large growing White Cedar and may reach 25 feet in good conditions. Comparatively different sizes, forms and colors naturally occur and are sometimes helped along or outright created by the hand of man.
The American Conifer Society has developed a system representing four growth rates. This system cannot determine for you the eventual size of a plant but it will indicate the rate of expansion. Growth rate is a very useful tool in determining how quickly a plant may increase in size and whether it may work well into your design. The classification system is not science-based. Rather, it is merely a framework to help gardeners understand the amazing variations of size found in the world of conifers.
The four size classes are as follows. A “miniature” grows 1 inch or less each year; growing this slowly a miniature form will never become large. These are often represented by "buns" so popular in rock gardening circles. In the best of all possible scenarios a mini which grows as much as 1 inch each year will achieve only about 1 foot in size over the course of ten years but not more.
“Dwarf” conifers are those that grow from just over 1 inch to as many as 6 inches each season, just over 1 foot to as many as 6 feet in ten years. “Intermediate” growers will achieve 6 to 12 inches of new wood every year, 6 to 12 feet over a ten year period. And finally there is the “Large” classification. Each of these plants will grow 1 foot or more each growing season, reaching heights of 12 feet or more in ten years time, the named forms growth rates much more in line with their straight species counterparts.
Miniature = 1 inch or less per year / 12 inches or less in 10 years
Dwarf = 1 to 6 inches per year / 1 to 6 feet in 10 years
Intermediate = 6 to 12 inches per year
Large = 12 or more inches per year
Each growth rate category can be divided into two halves. The first half spells out just how much a given plant may be expected to grow in one year, the second indicating the rate of growth over the course of a ten year span.
As mentioned earlier though a helpful tool growth rate categories are hardly exact science - imperfect because some conifers straddle growth rate categories. For instance, an intermediate grower may achieve the high end of the dwarf category – let’s say 5.5 inches in one year depending upon cultural conditions such as drought, the lack of water minimizing growth potential. But the next year conditions may be sublime and its growth rate may cross the line into its sanctioned intermediate category, growing more than the expected 6 inches.
Also, many conifers are very slow in youth but pick up speed over time. We, at Quackin’ Grass have a beautiful powder blue Colorado Spruce, Picea pungens ‘Hoopsii’, one of three beautiful very blue upright pyramidal forms. ‘Hoopsii’ was painfully slow for a period of about twelve to fifteen years. When initially planted it was a two-year graft - very small. During its early years it was essentially dwarf adding growth commensurate with the low to middle end of this range. But it reached an age and a size when the critical mass of it began to grow exponentially - through the dwarf growth rate category. It picked up even more speed placing it in the intermediate range, 6 to 12 inches per year. In some recent years it has grown 12 or more inches annually placing it in the large category. It now vacillates between the Intermediate and Large growth rate categories.
Another consideration is that some intermediate growers, for example, will eventually become quite large plants, some as grand as their large-growing counterparts. But because of a slower, more restrained growth rate it may take approximately a span of time one-third longer to achieve that mature size. As always with conifers knowing the eventual adult size of the species will be helpful.
Remember: growth rate categories as invented by the American Conifer Society are but a guide. Attempts to place living creatures into strictly defined categories can be tricky as there are frequent exceptions. Though many of us strive to comprehend the world in simplistic black and white terms very little in life is black and white. Rather, we live in a world comprised of varying levels, most oftentimes represented in tints and shades of gray. I do hope, however, that this article has been helpful in deciphering what these labels actually mean and it is hoped that my narrative has not further confused readers. But once you’re done scratching your heads, break out those shovels. Dig in. Have fun!