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Practical Guidelines for Producing Longleaf Pine Seedlings in Containers

Barnett, James P.; McGilvrary, John M.  1997. Practical guidelines for producing longleaf pine seedlings in containers. Gen. Tech. Rep. SRS-14. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 28p.

Cultural Practices

Growing Schedule

The best growing schedule, both biologically and economically, for longleaf pine is to sow seeds in the spring, grow the seedlings through the summer, harden the seedlings naturally in the fall, and outplant them in late fall or early winter (Brissette and others 1991). This schedule eliminates the need for an elaborate greenhouse structure, provides full sun for optimal growth, and achieves natural hardening. The optimum dates in the following growing schedule for longleaf pine may vary by location of the nursery.

Production phase Approximate dates
Preparation April to mid-May
Germination May
Juvenile growth June
Exponential growth July to mid-September
Hardening Mid-September to shipping

This phase includes the activities of mixing media, filling containers, and sowing seeds. These three distinct procedures make up one operation. Batches of freshly blended growing medium should be used to fill containers. Filled containers should be seeded immediately and placed under an irrigation system. Completing these activities sequentially keeps the medium moist in the containers and makes rewetting unnecessary.

Sowing seeds—Because all cells, with or without seedlings, cost the same to carry through a growing cycle, vigorous seedlings should be grown in as many cells as possible. Seed-sowing strategies should be based on current germination test results. If the viability of the seedlot used is in the 70 to 80 percent range (typical for most lots of longleaf seeds), two seeds per cavity should be sown, then thinned to one seedling before the seed coats are shed. Regardless of the sowing strategy, oversowing and thinning is preferred to transplanting germinants (Pawuk 1982).

Preparation Phase

Situations may arise where sowing of one seed per cavity is the best option, even when seed quality is low. One such situation is an inadequate seed supply, a frequent occurrence in longleaf pine. Another possible situation occurs when the labor force is inadequate for thinning or transplanting. However, economic considerations of carrying empty cells must be evaluated against the cost of thinning.

The scale of operation determines whether the seeds are sown by hand, by simple templates (fig. 11A) or by more elaborate seeding machines (fig. 11 B). When mechanical sowers are used, the containers should be visually checked after sowing to ensure that the prescribed seeding rate has been met.   In some instances, hand seeding may be needed to complete the operation.

Covering seeds—After filling and seeding the containers, most growers cover the seeds with a light layer of media or vermiculite. Vermiculite may be the better choice, because it allows more light penetration to the seed. This covering improves the moisture relationships around the seeds and, thus, improves and hastens germination. Seeds should be covered with no more than one-eighth inch of material. Deep covering slows germination and increases the chance of damping-off and other disease problems (Barnett 1988).

The need to cover seeds varies by the type of watering system used. Germination is usually most complete and rapid when seeds remain uncovered and receive water by a misting system (Brissette and others 1991). If seeds are watered less frequently, a light seed covering facilitates germination by mulching that retains water near the seeds (fig. 12).

Figure 11—Containers can be seeded
(A) by hand, by simple templates or
(B) by more elaborate vacuum seeding machines.
Figure 12—Seed covering equipment can be
added to the end of the seeding production line
to provide a covering that will enhance
seed germination.

Germination Phase

Controlling temperature and moisture is critical during the germination phase. Longleaf pine seeds germinate better at cooler temperatures than other southern pine species, because they are ecologically adapted to fall germination (Wahlenburg 1946). Day and night germination temperatures should be near 70 oF with a permissible range of 60 to 80 oF. Therefore, crops should be started in April or early May when temperatures are usually near this range.

Containers must be watered frequently to keep the surface of the medium moist. The moisture content of the medium near the surface should remain near field capacity. However, care should be taken to avoid overwatering, which can lower germination and promote disease problems. In a typical watering schedule, seeds would be misted 30 to 40 seconds every 30 minutes. Cycles vary among different types of irrigation systems. When germination is complete, frequent misting should be stopped, any shade cloth removed, and less frequent watering begun.

Fungicide applications should begin as soon as feasible to reduce damping-off of germinants and to inhibit pathogenic fungi development. The following fungicide, rate, and application method works well:

Fungicide — benomyl 5OWP
Rate — 1 rounded teaspoon per gallon of water
Method — apply as a thorough drench
Time — apply at biweekly intervals until October 1

Heavier rates of benomyl are not detrimental to seedling growth; therefore, more frequent applications are much better than applying too little. If inoculation with a spore suspension of Pt is planned, using benomyl is appropriate because it does not adversely affect Pt development. In fact, most studies show that it promotes Pt development (Pawuk and Barnett 1981).

Juvenile Growth Phase

Because longleaf pine seedlings are sensitive to shade, they should be grown and hardened in full sunlight (Barnett 1989). During the first few weeks after germination, top growth is minimal, but root growth is extensive. Thinning or transplanting should be completed early in this period. Although not generally recommended, needed transplanting must be done before radicle elongation exceeds about 2 inches. A broken radicle slows seedling development and results in smaller seedlings that do not compete well with uninjured seedlings (Pawuk 1982). 

Figure 13—LongIeaf pine germinants thinned to one per container cavity.

Seedlings should be thinned before root branching begins and preferably before seed coats are dropped (fig. 13).

If slow-release fertilizers are not used, fertilization of seedlings should begin as soon as possible after germination and the first fungicide application. The following fertilization program has produced quality longleaf pine seedlings:

Fertilizer — Peters' 15-16-17 NPK Peat-Lite Special or equivalent
Rate — 350 p/m (based on nitrogen only)
Time — Apply as needed
Method — Apply through the watering system

The timing of fertilization depends on seedling development. Weekly application will result in maximum development of seedling diameter growth, but the timing of applications will vary with weather conditions. Lower frequencies of application reduce or eliminate the need to clip longleaf pine needles. This program can also be used to supplement slow-release fertilizers.

Seedlings should be watered thoroughly and the medium surface allowed to dry between waterings. Early morning irrigation is preferred, because the seedlings will dry before evening, reducing fungal growth. During irrigation, enough water should be applied to leach the cavities. Because the outer edges of rows or benches will dry faster than those toward the center, they will need additional watering (usually by hand). Water management is critical during this phase and requires daily observation.

During the latter part of the germination or juvenile phases, weed growth in the containers may become a problem. Weed seed may be incorporated in some grades of peat, borne by the wind, or spread through the water system if the water source is a pond or river. If weeds are spread throughout the crop, a herbicide may be applied. GOAL  1.6E  (oxyfluorfen) is a selective, pre- and post-emergent herbicide that controls a broad spectrum of grasses and broadleaf weeds in conifer seedbeds and container stock. However, because newly germinated longleaf pine seeds are sensitive to herbicide damage, the application rate should be reduced to prevent damage. Tests have shown that applying 12 milliliters of GOAL 1 .6F per gallon of water over 800 square feet does not result in damage. This reduced rate may require subsequent applications (at weekly intervals) to obtain complete weed control.

The crop must be carefully observed for other pest problems. Insect problems may include cutworms, fungus gnats, and ants. Applying diazinon at labeled rates will control most of these pests. Diseases may occur, such as rhizoctonia, and a pest control specialist should be consulted for treatment recommendations.

Exponential Growth Phase

The rapid growth during this phase is exhibited primarily by needle elongation. The initiation and length of the exponential growth phase is determined by both needle and stem development. Control of moisture is not as critical as during the previous growth phases. However, the seedlings must be monitored closely for signs of stress due to disease, nutrient deficiencies, or insufficient water.

Fertilization as outlined for the juvenile phase should continue unless observations indicate a need for change such as more frequent applications. If growth remains slow or nitrogen deficiency is observed,  a few applications of Peters' 20-10-20PL

Figure 14—Gasoline-powered hedge clippers are often used to clip seedlings and prevent lodging.

NPK will usually correct the problem. If higher fertilizer rates are used, especially with the nitrate-rich 20-10-20PL NPK formulation, the fertilizer should be washed off the foliage to prevent possible burning.

The rate of fertilization will determine whether clipping the longleaf pine needles is necessary. When the needles begin to lie over surrounding seedlings, problems in uniform growth and in pathogen development occur, and clipping or mowing is recommended (fig. 14). Care should be taken to clip only enough of the needles to reduce the problem, because excessive pruning will reduce growth. The needles should not be clipped to less than 6 inches, and all clippings should be removed to minimize fungal development. More than one clipping or mowing may be required during a growing cycle.

As the seedlings become large in the latter stage of this phase, they will use more water. This increased usage combined with the normally hot, dry weather will result in the need for more frequent and heavier periods of irrigation. Irrigation should thoroughly leach the containers.

Hardening Phase

Hardening should start when stem diameters are near the desired size or when day lengths and temperatures restrict growth. The seedlings should be stressed by reducing moisture availability. The media should be allowed to dry to near the wilting point between waterings. No nutrients need to be applied. Just before shipping, the root plug should be brought to field capacity.

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Last updated on Tuesday, July 09, 2002 at 10:16 AM
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