By John A. Wott, Director Emeritus
It is said that humans “have a habit of growing.” We grow tall in our formative years, and most of us also grow wider in the later years. We could also say that trees have a habit of growing. Tree species grow to specific heights and widths. Some smaller trees obtain their normal mature size in a few years, while the larger species may grow for years and years. In fact some large forest trees may continue to grow for hundreds of years.
In our urban sites, native conifers are capable of continuing their growth for hundreds of years. Any time the temperatures are in the 40s or above (which happens just about every day of the year here), the chlorophyll molecules are busy manufacturing sugars.
When we visit a park on a regular basis, we are very unaware that the trees we see are growing larger every day. I remember someone once saying, “I visit the Arboretum every year, and the plants have not changed at all over the last 25 years.” Rubbish! The Arboretum changes daily due to this continuous tree growth. The conifers gradually grow larger and larger and suddenly, their size can “ambush” us. I am sure most of us have had the experience of suddenly realizing that the cute little evergreen we planted 20 years ago is now overpowering the house.
This series of pictures shows such a progression of growth. The first picture above, taken by Fred Leissler, Assistant Park Director in 1935, shows seedling trees of western hemlock (Tsuga heterophylla) planted along Section C of the Nursery and Seedling Beds as a screen and windbreak. The picture notes indicate that thousands of plants had been started there in those sunny beds. Note that Arboretum Drive E. is a wide lane.
Pictures 2 and 3, below, show the same trees on January 15, 1950, just 15 years later, and already making a sizeable screen.
Along Arboretum Drive, other species of conifers were planted in rows during this time and into the late 1950s. These were mostly native species, such as the Western redcedars (Thuja plicata) in pictures 4 and 5 below.
However, one of the final plantings of this type were of the newly introduced Leyland cypress (× Cuprocyparis leylandii), which is a cross between the Monterey Cypress (Cupressus macrocarpa) and the Alaska yellow cedar (Xanthocyparis nootkatensis). These were sent to us from Hillier Nurseries in England via the Institute of Forest Genetics in Placerville, CA. They were planted at the north end of Arboretum Drive E., just to the west of the Greenhouses (and current Plant Donations area).
These trees, while extremely fast growing, have proved to be inferior landscape trees. They have weak, soft wood, and are prone to wind damage. They are best used as a tall sheared hedge and kept under 20 feet. Our trees were planted out in the late 1950s and grew rapidly. One large specimen, shown below in picture 6, toppled on December 10, 2015, probably due to root removal by the lowering of Arboretum Drive for the construction of the Graham Visitors Center in 1985 (west side), and the recent heavy rains. This is an excellent pictorial example of continuous tree growth and how conifers grow and grow and grow. It is also an example of the need for continual evaluation and management of trees and their appropriate placement in the landscape.