What is a Tree Cultivar?

Apple Rootstock Graft. sruumpyboy/Flickr.com

Creating a Tree Cultivar:

All existing tree species were initially conceived in a forest somewhere on Earth. In a forest environment, evolving trees grow under their own particular comfortable conditions including clean air, optimal soil conditions and sufficient moisture.

Nurseries are continually looking for and developing trees that will grow in that gap between the forest and the city. By manipulating a tree's special genetic characteristics, tree nurseries develop, select and grow variations on these natural trees that show distinct characteristics including pest resistance, flower and fruit production, drought tolerance and desirable form.

These trees are often called cultivars and a product of man's manipulation.

Tree Cultivar Vs Tree Variety:

Tree "varieties" occur in nature and the seedlings grown from seed of a variety will naturally replicate the same unique characteristics of the parent tree. If you were to germinate seed from a variety, certain characteristics almost always replicate themselves and is done exclusively through the natural processes - that's a variety.

On the other hand, cultivars are not necessarily true to type and do not have the ability to sexually reproduce consistently. In other words, cultivar means "cultivated variety" and trees have been selected and cultivated by tree lovers, botanists and nurseries for centuries - that's a cultivar, an artificial label of love.

Problems With Developing a Cultivar Sexually:

Perennial plants including trees can be propagated either sexually or vegetatively. When a male germ cell of the same species sexually unites with a female germ cell, fertilization occurs and seeds are developed.

These seeds inherit characteristics of both its parents but will not express the variety of either parent. This leads to unpredictable but unique combinations of tree characteristics.

To actually propagate accurate clones, many cultivars must be propagated vegetatively through cuttings, grafting, and even tissue culture.

While propagation by seed is unpredictable and usually results in something different than the parent tree, cultivars break the natural code and allow diversity.

Developing a Vegatative Cultivar:

There are actually several methods used to vegetatively grow a tree clone or cultivar. Using each of these methods depends on the species' tolerance for the procedure and on a successful outcome.

"Cutting" is the most common of vegetative propagation methods. A part of the "parent" plant is cut off and placed into a misted soil medium conducive to root sprouting. Adventitious roots are generated from stem and leaf cuttings and certain species of trees can do this extremely well (willow, fig, yellow poplar).

"Grafting" or budding is a very effective form of asexual propagation and a nursery standard. A stem piece or a single bud called the scion is joined on to rootstock. This rootstock is a hardy, easily produced tree, usually of the same species as the desired cultivar.

The Cultivar Business:

The major business of a tree nursery is developing and providing a perfect tree cultivar that can offer consistent and repeatable features. That tree has to perform well on a given site including problem yard and city landscapes and near highway and city streets.

There is a staged process in the production of tree cultivars and three types of nurseries may be involved - the propagator, the primary grower and the larger caliper tree grower. Some nurseries do all of these stages of production, other specialize and limit themselves to one or two stages.

Types of Tree Nurseries:

A propagation nursery grows seeds, roots and cuttings or use tree "cultures". This type nursery grows seedlings to a size of 18” height and l/4” stem diameter which takes about two years.

The primary grower takes the tree growing process a bit further and produces a tree with a root ball ready for planting. They also do the secondary propagation when grafting or budding a cultivar onto a seedling rootstock is necessary. These trees range from unbranched 4’ “whips” to 15’ tall branched plants.

The caliper grower takes trees grown by a primary grower and produces a larger sized tree to a 2” to 8” trunk diameter. These larger trees are usually planted on city and large commercial landscape projects and take 2 to 8 or more years to develop.