Understock Compatibilities

Grafting is a process of plant propagation that involves taking a small branch or shoot (scion) from the plant being propagated (usually a cultivar) and attaching it to a seedling (understock). This shoot then grows into a replica of the cultivar. One method of grafting is described, with pictures, at this location on my web site.

The choice of understock is relatively simple, but there are a number of considerations that need to be taken into account. The species of the cultivar must be related in some way to the species of the seedling. The process of grafting can have various effects on the resultant plant.

Cultivars of Abies (true firs) are compatible to any of the various species of understock. The actual selections used are restricted to a few species mainly due to availability considerations.

For example, there are three species of fir most commonly used in North America. Abies balsamea is possibly the most popular species used. The plants created cannot be grown in warmer climates as the roots cannot withstand persistent high temperatures and hot sun. In the northwest Abies fraseri is preferred since greenhouses are exceptionally humid during the grafting season and winters are mild and wet. This species appears to tolerate these conditions better than balsamea. Cultivars propagated on this species can be grown in a wider range of climates.

Abies concolor has always been a popular understock in the northeast but it performs poorly throughout the northwest, especially when grown in containers.

I like to use Abies procera as an understock for Abies procera, concolor, and lasiocarpa. They perform well and do not develop a large burl of callous tissue at the graft union, which can occasionally happen with other species understocks.

Abies koreana is a popular understock throughout much of Europe. It would probably be more extensively used in America but the seedlings are not widely grown in this country.

Abies firma is being tested as an understock in the southeast where the climate prevents the growth of most firs. I believe it is its tolerance for the poorer soils of the South that gives it an edge on survivability.

Cedrus atlantica, brevifolia, and libani are all grafted onto Cedrus deodara, the least hardy of the three. There are several reasons for this choice. The hardiness is not a factor since the deodara root system in the ground is at least as hardy as any Cedrus cultivar above ground. Secondly, the root system is more fibrous and better developed than in the other species making it a much better choice for nursery use.

Chamaecyparis species are relatively easy to propagate from cuttings so grafting is seldom used as a propagation method. Chamaecyparis obtusa (hinoki cypress) is compatible with Chamaecyparis lawsoniana and Thuja occidentalis. Since Chamaecyparis lawsoniana has disease problems it is not used in North America as an understock. In Europe it works well but eventually creates a deformed graft union when it develops a much larger diameter than the cultivar.

Chamaecyparis obtusa is grafted for two reasons. First of all it will grow faster than a rooted plant and become saleable in a shorter period of time. Eventually the growth rate will return to normal. Secondly, when it is grafted onto Thuja occidentalis it can be grown in clay soils.

Chamaecyparis lawsoniana has limited use in America due to disease entering the root systems. Grafting could prevent this problem and allow wide use of hundreds of cultivars throughout this country. However, any understock used is overgrown by the cultivar.

Cupressus cultivars can occasionally be rooted but are most commonly propagated by grafting. Some propagators contend that they can be grafted onto Juniperus or Thuja occidentalis. However, there is a strong tendency for Cupressus propagated in this manner to overgrow the understock and be very susceptible to blow over. Cupressus x-leylandii has been used in Australia for many years and I also use it for my understock. The compatibility is excellent with no overgrow of the understock and a good root system supports the growing tree.

Juniperus cultivars are compatible on any of the species but few are grafted since most cultivars are relatively easy to root. Juniperus virginiana cultivars are difficult to root and are generally grafted. The understocks most commonly used are Juniperus x-pfitzeriana ‘Hetzii’ or Juniperus scopulorum ‘Moonglow’ or ‘Skyrocket’. They all root easily and rapidly grow into a graftable size.

Larix cultivars can be grafted onto any of the species. If a dwarf selection is grafted as a standard, the understock may not develop the necessary caliper to support the plant without falling over. This is especially true of the Newport series of dwarf selections. The higher the graft, the more severe the problem. It can be alleviated to some extent by leaving branches along the trunk for a number of years. In fact, with any grafted standard the growth can be accelerated in this manner.

As a side note: I noticed that many of the dwarfest cultivars show accelerated growth when grafted at about 1’ but will slow down if grafted higher than that.

Spruce (Picea) cultivars may be grafted onto any species of spruce understock. Picea abies is the most abundant understock grown from seed and is most commonly used. Picea pungens is also used by many growers in the northwest because it develops a more fibrous root system on plants that are field grown in the nursery.

Pine cultivars are best grafted onto seedlings that have the same number of needles per fascicle as the cultivar.

Pinus strobus is the most common understock used throughout the world for cultivars of species with 5 needles per fascicle. Pinus peuce is used in parts of Europe and Pinus parviflora is occasionally used as understock for Pinus parviflora. I have seen large specimens of Pinus parviflora grafted onto Pinus strobus that were quite healthy but the trunk of the parviflora had a 12" diameter and the understock had an 18" diameter. It can be unsightly on older plants with a high graft union, but Pinus parviflora seedlings are hard to find in any quantities and are expensive.

Pinus thunbergiana is used by some people as understock for Pinus parviflora since it gives a higher salt tolerance to the plant. Bonsai growers especially like this combination and it even works with other five needled species of pine. However, there is a lower grafting success rate and some of the plants do die each year due to delayed scion/understock incompatibility. I heard a story about a bonsai master in Japan who was showing his colletion to a visitor. The visitor touched an old specimen of parviflora and it broke at the gtraft union. The Master told him not to worry, it was an effect of using thunbergiana understock on parviflora.

Pinus bungeana is a three needle pine and only grafts successfully onto five needle understock. I use Pinus strobus with great success.

The two needle pines are fairly well intercompatible but different grafters have different preferences. For example, Pinus sylvestris is a universal understock used throughout the world, with the Spanish strain preferred due to its superior root system. Pinus nigra is used by many nurseries for all two needle pines with good success. I have found that Pinus contorta v. latifolia (lodgepole) is a good universal understock for two needle pines and is by far the root hardiest of them all. I have had good luck with ‘Frisian Gold’ and ‘Chief Joseph’ using this understock when other species have failed completely. Pinus thunbergiana is a universal understock but is the least hardy of the group.

Some grafters like to put Pinus leucodermis, mugo, nigra, and thunbergiana onto nigra and the others onto sylvestris. However, using too many different kinds of understocks increases expenses and impacts production schedules, so most grafters use the fewest kinds possible.

Pseudotsuga menziesii (Douglas fir) is usually grafted onto hardy strains of the species that are grown from seed harvested from the mountains of the western United States. The cultivars don’t show many compatibility problems since most of the cultivars originated from the hardier varieties of the species.

Tsuga canadensis should never be propagated by grafting. There is always a delayed incompatibility problem with this species. If 100 plants are grafted, count on several deaths every year until 10-20 attain a mature size before eventually dieing. Since cultivars from this species do root. That is how they all need to be propagated.

Taxus (yews) root from cuttings with a few exceptions among the Taxus baccata that have need to be grafted. Taxus media ‘Hicksii’ is a common understock since it is compatible and strikes easily from cuttings.

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