Citrus Nursery Production

Julian W. Sauls, Ph. D.
Professor and Extension Horticulturist
Texas AgriLife Extension

January, 2008

Field production of citrus nursery trees to be balled and burlapped for transplanting has been standard practice in Texas. However, advantages offered by container production have resulted in a significant increase in the number of container-grown trees produced, especially of trees grown for the retail trade.

Major advantages of container production include:

Smaller land area required, no need to change sites

Growing in greenhouses/shadehouses provides climate control

Sterile growing medium eliminates soil-borne diseases, insects, nematodes and weeds

Production time of 15 to 18 months from seed, as compared to 24 to 30 months common to field nurseries

Reduction of transplant shock because of transplanting entire plant with undisturbed root system

Major disadvantages of container production include:

High initial capital investment for growing structures and climate control facilities and equipment

Intensive management requirements

Smaller caliper trees--3/8- to 1/2-inch rather than 5/8- to 3/4-inch

Partial removal of medium necessary at transplanting

More frequent irrigation during orchard establishment

No significant differences have been reported between container-grown and field-grown trees in terms of time to come into production or in total yields, fruit size or quality. However, because of their younger age, the smaller container-grown trees tend to remain smaller than field-grown trees for several seasons after orchard establishment.

Texas Citrus Budwood Certification Program

Citrus can be adversely affected by a number of virus diseases, many of which can be transmitted during budding. Many old-line selections of citrus in Texas have exocortis, xyloporosis and possibly other viruses. Moreover, a number of citrus trees in urban areas, especially in southeast Texas, have been illegally imported from other states. Random testing over the last several years has identified a significant incidence of citrus tristeza virus among these trees.

Because of the threat of accidental introduction of citrus viruses and other diseases and pests from other citrus-producing areas, the Texas citrus industry, in cooperation with the Texas Department of Agriculture and the Texas A & M University System, initiated a citrus budwood certification program in 1993. Major objectives of this program are 1) to establish a foundation block of disease-free trees of all major commercial and non-commercial varieties of citrus grown in Texas, 2) to maintain a rigorous program of testing and retesting of foundation trees to assure continued freedom from disease, 3) to evaluate the horticultural characteristics of foundation trees to assure trueness-to-type and 4) to develop and maintain increase blocks for the production and sale of certified budwood for the production of disease-free citrus nursery trees.

A very important aspect of this program is expected to be implemented in 2003, i.e., that any citrus nursery tree produced for sale in Texas must be propagated only from certified budwood, the only source of which currently is the Texas Citrus Foundation. This requirement, in combination with existing quarantines against the importation into Texas of citrus trees, buds or other plant materials except fruit, will ultimately assure a disease-free citrus industry--both commercially and in the all-important home gardening sector.

This program is maintained at the Texas A & M University-Kingsville Citrus Center in Weslaco, TX, under agreement with the Texas Citrus Foundation. Its operation is overseen by the Foundation, with assistance from both an industry Technical Committee and an Advisory Committee of the Texas Department of Agriculture. Its operations are coordinated by Mr. Craig Kahlke of the Citrus Center (jwatson@ag.tamu.edu).

Certified budwood of numerous varieties of citrus is currently available in limited quantities. Budwood availability for most varieties in any given time period will be determined primarily on the basis of orders placed sufficiently in advance that the desired numbers can be produced in the increase blocks.

Sanitation

The use of virus-free propagation material requires sterilization of propagation tools. Sterilization of pruning shears and budding knives can be easily accomplished by cleaning the tools throughly with warm, soapy water, then spraying them with a 10-percent solution of chlorine bleach (one part bleach to nine parts water). A small spray bottle of the bleach solution is especially handy for periodic re-treatment of propagation tools during nursery operations.

Budding tools should be re-sterilized anytime the budder changes varieties. Too, pruning tools used to remove sprouts or to trim nursery trees should be periodically re-sterilized during operation, most particularly when moving from one variety to another.

Because the bleach solution is caustic to most metals, sterilized tools should be rinsed in tap water, dried thoroughly and then given a light coating of protective oil at the end of the day.

Rootstock Production

As a general rule of thumb, most nurseries plant about twice as many rootstock seed as the ultimate number of budded trees that are expected to be produced, as total losses through the entire production process can account for about half. Initial losses are those seed which fail to germinate, as well as those that die before reaching liner size. The major loss occurs in the culling process of liner selection. Other losses include liners that die, budding failures and budling death or budlings which simply do not grow normally.

Seed. It is common practice in Texas to collect rootstock seed from fruit produced on root sprouts in existing orchards. However, it is recommended that seeds be purchased from certified sources or that nurserymen establish and maintain trees for rootstock seed production to assure uniformity of rootstocks year after year.

Rootstock seed are extracted from fruit normally harvested in the fall. The simplest means of extraction is to cut a horizontal ring into the fruit, just deeply enough to avoid cutting the seeds, then twist the two halves apart. An electric juicer can be used to collect the seeds, after which they should be separated from the accompanying pulp by repeated washing. Commercial seed operations commonly crush the fruit in an ice crusher, followed by treatment with pectinase enzymes at controlled temperatures, with constant stirring. The enzyme treatment essentially digests the pulp, rag and peel, leaving the seeds intact for easy separation.

Washed seed should be surface-sterilized in hot water maintained at 125o F for 10 minutes, then dipped in one percent 8-hydroxyquinoline sulfate, air dried and packaged for storage or planting. Properly treated and packaged in sealed plastic bags, seed can be stored in a refrigerator for several months with little loss in viability.

Just-planted seed trays.

Pre-plant soaking in aerated water maintained at 85o F for 24 hours will increase total germination, uniformity of germination and will shorten germination time. Seeding in outdoor seedbeds is normally delayed until February or March, but greenhouse seeding in sterile media in seed trays, flats, tubes or cells can be done anytime.

Germination and initial seedling growth in greenhouses can be further enhanced by using seedboxes designed and built to provide supplemental bottom heat to maintain constant 85o F temperature of the medium, supplemental lighting to provide 16 hours of light daily and use of polyethylene covering to maintain high relative humidity.

Seedlings in seedboxes. Liners ready for transplanting.

Seedlings. Seedlings for field nurseries are lined out in nursery rows in 4 to 6 months, at a spacing of about 12 inches. Row width is dependent upon mechanical equipment used in the nursery. Greenhouse-grown liners can be potted in sterile media in growing containers after 2 to 3 months. A number of containers are in use, including plastic bags and reusable plastic pots of various dimensions. Minimum dimensions of the various containers used locally are 4 to 6 inches wide and up to 14 inches deep. Standard 3-gallon and 4-gallon plastic nursery containers are also used.

Liners planted in 6-inch citripots.

Seedlings should be graded critically at transplanting. Those few that are obviously much larger than average are probably of hybrid origin and therefore should be discarded as not true-to-type. Obviously-stunted seedlings should be discarded as they will probably never catch up to normal seedlings. Those having curved or crooked lower stems at or below the soil line should be discarded, as such stocks may take years to outgrow such deformities. Albino seedlings should also be discarded.

Remaining seedlings may then be graded into two or three sizes and transplanted by size groups to establish blocks of uniform seedlings, both in field and in container operations. Thus, all seedlings within a block should mature together, thereby receiving uniform treatment, requiring less labor and producing a higher percentage of saleable trees from a given block at the same time.

Propagation

Budwood. While it has been common practice in Texas to randomly cut budwood from existing trees, this practice will become completely unacceptable once the mandatory provisions of the Texas Certified Citrus Budwood Program are implemented. The enabling legislation already exists, awaiting only the recommendation of the Texas Department of Agriculture's Citrus Budwood Advisory Committee to be implemented. Under this legislation, citrus trees produced for sale in Texas must be propagated only from buds obtained from a certified source. The Texas Citrus Foundation is currently the only certified source. Thus, the budwood will be cut, labeled and bagged by employees of the Foundation.

Budding. Budding height on the stock should be 6 to 8 inches, which normally provides ample height of the union to reduce the incidence of Phytophthora foot rot. Budders use the inverted-T method for inserting the bud into the stock. For a complete, well-illustrated discussion of the budding process, see http://aggie-horticulture.tamu.edu/citrus/budding/budding.htm.

Microbudding is a technique developed by Dr. Mani Skaria of the Texas A & M University-Kingsville Citrus Center. Its promise is the quicker and less costly development of citrus trees, either for orchard establishment or for retail use. Basically, the technique involves germinating rootstock seeds in test tubes, then decapitating them at a couple of months of age to insert a tiny bud into a small, vertical incision into the decapitated stem. Within another couple of months, the newly budded plant will have achieved typical liner size for transplanting into the field or into a container to be grown on to acceptable size for ultimate use.

While the technique has been amply demonstrated by Dr. Skaria in cooperation with a local nursery, it has not been commercialized at this point. The relatively low insertion of the microbud resulted in a somewhat higher incidence of Phytophthora infections in field planting, but that should not be the case in container production. Fruit production occurred on microbudded trees in the second year post planting, which would certainly be advantageous for retail trees. Contact Dr. Skaria (m-skaria@ag.tamu.edu) for details of this procedure.


Rootstocks cleaned up for budding.

Most field nurseries are budded in the spring to summer of the year following lining out, although earlier budding is possible. Container nurseries normally are budded 3 to 6 months following transplanting into containers.

Aftercare. Rootstock sprouts below the bud should be removed periodically to avoid competition with the budling. Emerging buds can be easily rubbed off or broken off.

When the initial growth flush of the budling has hardened off, a stake is placed next to it for protection and support. Stakes may be metal, plastic, wood or bamboo--lengths of 2 to 3 feet are commonly used. At 4 to 6 inches of growth, the budling should be pulled to the stake and tied to it, with additional ties every 4 to 6 inches as the budling grows. All lateral buds which grow from the forming trunk should be removed regularly to direct all growth into a single, upright stem. At 18 to 20 inches of growth, the budling top is cut off to stimulate lateral branching (heading). Depending upon the operation and forcing method used, the rootstock top may be removed anytime up to the time of heading.

Finishing. Prior to movement from the nursery, container-grown trees may benefit from a short period of hardening-off. This entails placing the trees in a holding area where they are exposed to the natural climatic elements for 2 to 3 weeks.

Field-grown trees are cut back to the scaffold branches in preparation for digging. Trees cut back and dug immediately prior to a growth flush generally respond better upon transplanting than do those cut back and dug during a growth flush.

Spring-budded field nursery trees should be ready for digging in the fall, although selective digging may be necessary. Thus, the field nursery tree is finished about 24 months after seed planting. Container nurseries produce a smaller caliper finished tree in 12 to 18 months after seeding. Both systems can take another 6 months to finish, primarily because of inadequate culling and grading of liners, percentage of rebudding required, slow growth resulting from sub-optimal care and, in the case of field nurseries, slower growth because of climatic conditions.

Cultural Practices

Irrigation. Optimal irrigation should be provided to both field and container nurseries to maintain adequate soil moisture without waterlogging. While flood irrigation is still practiced, many field nurseries use drip irrigation almost exclusively. Container nurseries are normally established with automatic watering systems, using individual drip irrigation emitters in each container. Such systems are generally designed for daily operation.

Nutrition. Some container nurseries routinely incorporate a slow release fertilizer into the growing medium, though most do not. In either case, water-soluble fertilizers are applied during irrigation, either during each irrigation or weekly. Constant fertilization should provide rates of 75 to 100 ppm total nitrogen, whereas weekly fertilization of 200 to 400 ppm nitrogen is acceptable.

Generally, field nurseries should receive 150 to 200 pounds of nitrogen per acre annually in multiple split-applications at intervals of approximately 30 to 45 days from late winter through summer. The actual fertilizer program will vary with soil type, plant response and grower experience. Nurseries using drip irrigation typically inject liquid nitrogen into the irrigation system periodically.

Weed Control. Weeds in container operations are rarely a problem, but weed growth should be eliminated where appropriate. Weed competition in field nurseries should be eliminated by the use of appropriate pre-emergence and post-emergence herbicides and/or by mechanical means.

Pests. Typical orchard pests can affect nurseries. Nurseries can also experience damage from caterpillars, cutworms, slugs, snails and rodents. Good pest control is essential to the production of high quality nursery trees.

Producing Trees for Retail

The demand for citrus trees for urban planting has increased tremendously in recent years. To meet that demand locally, some field nurseries dig trees as usual, but without the burlap wrap, and set the tree into a 2-gallon or 3-gallon standard plastic nursery container. They usually hold the trees for a couple of weeks to harden before delivery. For local delivery, where freight rates are not a factor, this procedure is adequate.

Individual hand-watering of such trees is problematic, as the soil tends to wash down and out of the pot through the drainage holes, which also means that the moisture in the root ball is not adequately replenished. The use of overhead sprinklers, small spray heads or individual drip emitters usually precludes this problem--both in the wholesale nursery and in the retail outlet.

Generally, however, citrus trees for urban use should be container-grown in a suitable, soilless medium, as most retailers are not receptive to soil-grown plants, to say nothing of the higher freight costs associated with soil-filled containers. Neither are they receptive to classic balled-and-burlapped trees.

Container nursery under shadecloth. Field nursery trees headed and staked.

Aside from the field nursery versus container nursery dilemma, the urban market requires far more diverse types of citrus trees than the usual four or five varieties grown for commercial orchards. While some numbers of trees of the commercial varieties can be sold in the urban market, the greater demand is for lemons, limes, kumquats, tangerines, tangelos, satsuma mandarins and other, lesser known, varieties.

Although a select few people, mostly in southeast Texas, prefer trifoliate orange rootstock, especially Flying Dragon, the vast majority of potential consumers in Texas would be better served with trees on sour orange rootstock. In some situations, such as with limes and kumquats, rooted cuttings are also acceptable, as would air layered (marcott) plants.

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