I had been sent to assist in corrective pruning of this specimen. As usual, I became overindulgent with my explanation of the reliably unappealing fruit. My colleague, arborist Paul Swagerty, patiently listened and then suggested I write about it because there may be others who, unlike him, might be interested.

I will start at the beginning, in the good old days, when tree surgery was still a respectable career. Of course, this was before my time, but I am told that tree surgeons performed the arboricultural procedures that contemporary arborists now perform. Furthermore, tree surgeons were necessary for maintenance of the vast orchards throughout the Santa Clara Valley. At that time, tree surgeons also performed some of what is now done by growers (my kind of people) by "assembling" fruit trees in home gardens as they did in the orchards.

A tree surgeon would plant an understock (seedling or rooted cutting) for the desired fruit tree where the finished tree was desired. After the understock had grown for a year, the tree surgeon would return in season with a scion to graft onto the established rootstock. Of course, this is now done in nursery production, and finished fruit trees as well as a broad range of other grafted material are commonly available from retail nurseries. Regardless of how a tree is grafted, the understock roots are genetically different from the scion that forms the tree above the soil.

Over the years, some of these old grafted trees have been cut down or have (almost) died. If stem growth emerges from roots of the understock, it is not genetically similar to what the scion once was, but is only more understock. Among dwarf citrus, the understock is shattuck, which produces wicked thorns and very large, insipid lemons. Among most standard citrus, understock is "sour orange," which resembles sweet orange in most regards, with the exception of sour flavor and nastier thorns. (Meyer lemon and Seville sour orange are not usually grafted but grow on their own roots.)

Unhappy sweet orange trees may produce unappealing fruit for many years but can be corrected with cultural modification. However, no amount of pampering or fertilizer will cause a sour orange tree to produce sweet fruit. A graft union should be visible at the base of the trunk of a sweet orange tree that is producing sour fruit, but an understock tree lacks any graft union. (A graft union of most citrus cultivars can be seen as a uniform constriction and variation of bark texture where the understock meets the scion.)

A "sucker" is a stem that originates below the graft of a viable tree. Because many grafted cultivars are less vigorous than their own understock, they may easily be overwhelmed by suckers. For example, dwarf kumquats grow rather slowly, but their shattuck understock grows considerably more rapidly. If suckers emerge and are not removed, they grow above the kumquat scion and shade it. The shattuck eventually blooms and sets fruit. The garden enthusiast who initially planted the kumquat then telephones me to determine why his kumquat tree is producing five-pound kumquats!


Perennial of the Week: Burro tail

The very limber and pendulous stems of burro tail, or donkey tail, Sedum morganianum, can be striking hanging over the edge of a suspended container or from an elevated planter. Growth that cannot hang sprawls slowly over a surface without becoming more than eight inches deep. Container culture is easily tolerated if the soil contains considerable organic material and is very well-drained. Regular irrigation and periodic applications of fertilizer through the summer are preferred.

Unlike most succulents, burro tail does not tolerate desiccation of soil well. Protection from wind and bright or reflective solar exposure will minimize the potential for foliar damage. The thick, gray-green leaves are so densely arranged that the thin stem cannot be seen unless partly defoliated. Stems only grow about six inches each year, but may eventually grow longer than four feet. The small pink or red flowers are rarely seen. Protection from frost may be necessary during the coolest of winter weather, particularly if containers are exposed.

Horticulturist Tony Tomeo can be contacted at 408-358-2574 or at LGHORTICULTURE@aol.com.