UNITED STATES—It is not science fiction. It involves neither ninjas nor turtles. Cultivars really are mutant plants that can only be propagated by cloning. The word ‘cultivar’ is a portmanteau (two words combined into a single word) of ‘cultivated’ and ‘variety’. Unlike other varieties of plants that can be perpetuated by seed, cultivars must be cultivated by unnatural techniques to maintain their genetic distinction.
For example, ‘Alamo Fire’ is a variety of Texas bluebonnets with maroon flowers. The original seed were collected from a few naturally occurring variants with maroon flowers, and grown into more plants with maroon flowers, which provided more seed. No seed was collected from those that bloomed blue. By repeating this process of selection a few times, the variety was developed.
The variety ‘Alamo Fire’ is now sufficiently genetically stable to perpetuate itself, which means that subsequent generations will also bloom with maroon flowers. However, a few blue flowers might bloom in any generation; and unless they are weeded out before producing seed, they will eventually dominate until the entire colony reverts from maroon back to the more genetically stable blue.
‘Meyer’ lemon is an example of a cultivar. It must be propagated vegetatively by cuttings, or perhaps grafted onto understock. In other words, it must be cloned. It is a genetically unstable hybrid of a lemon and an orange, so plants grown from their seed would be very different from the parent. Many hybrids are so genetically unstable that they are sterile, and unable to produce viable seed.
Many variegated or dwarf cultivars of all sorts of plants are not hybrids, but are mutants. It is common for some arborvitaes to produce ‘sports,’ which are simply mutant growth that is somehow different from the original growth. If a sport has a desirable characteristic, such as densely compact growth, variegation, or golden foliage, it can be cloned as a cultivar. Just like ‘Meyer’ lemon, a dwarf golden arborvitae is very unlikely to produce genetically similar seedlings.
Highlight: ‘Emerald Green’ arborvitae
The many cultivars of arborvitaes of home gardening have been so extensively bred and selected that only the foliar texture resembles that of their ancestors. ‘Emerald Green’ which is also known by its Danish name ‘Smaragd’, is a cultivar of arborvitae that was developed from the white cedar, Thuja occidentalis, which grows wild from Minnesota to New Brunswick, as a 40 foot tall tree!
‘Emerald Green’ grows quite fast while young, but should get no taller than about fifteen feet, and no wider than about four feet. It is one of the best columnar arborvitaes for tall hedging. Although they can be shorn, they are so dense and uniform that they are at their best if only occasionally trimmed of stray stems, or to keep taller specimens from getting too much taller than shorter ones.
The tiny evergreen scale leaves are tightly arranged on vertically arranged flat foliar sprays. Foliage is quite dense, and softer than that of most other conifers. A bit of shade is tolerable, but too much compromises foliar density. Bloom is barely noticeable, and seeded cones are not much to look at. The shaggy ruddy brown bark is handsome but seldom seen on well foliated specimens.
Horticulturist Tony Tomeo can be contacted at tonytomeo.wordpress.com.