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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.

Preparation of Recommended Materials

Container Selection

One of the first and most important decisions involves selecting the type of container to use (Landis and others 1990b). Experience has shown that a "plug"-type container, where the root system is extracted with the medium intact before planting, should be used (fig. 5). The ideal individual container cavity should have a volume of about 6 cubic inches, a minimum depth of 4.5 inches, and a seedling density of <50 per square foot. Smaller containers that improve the economics of container production can be used if cultural practices are carefully controlled. Only one type and size of container should be used within a growing area because cultural techniques, especially irrigation, differ among areas and growth phases. If container types are mixed, each type or size should be kept under separate watering systems. Examples of types of containers are listed below:

Container types


Depth (in)

Number/ ft2

RL Stubbies




Styroblock 6




Styroblock 8




Multipot 3/96




Multipot 4/96




Multipot 2-67








Figure 5—Longleaf pine "pIug" seedling extracted from a Multipot container.

Excellent quality seedlings can be produced in these, or similar containers, although some have properties more favorable to a particular species, nursery operation, or outplanting site.

  • RL Stubbies are probably the best containers for growing small quantities, such as progeny tests, because empty individual cavities or cells can be removed and remaining full cells can be reorganized (fig. 6A). The cells extend 1 .0 inch above the top of the tray that holds them together, and media falls between the cells when they are being filled. The excess media should be removed to allow good air circulation between seedlings, a process that adds time to the filling operation. Care must also be used when handling trays, because individual cells may fall from the tray if it is tipped.
  • Styroblocks are relatively inexpensive, but they are easily damaged during handling. Moreover, if seedlings are held for extended periods, roots will begin to penetrate the styrofoam, making plug extraction difficult (fig. 6B).
  • Multipots are suitable for large-scale operations, because they are durable and easy to handle (fig. 6C). However, they are relatively expensive and lack ridges or other supports on the bottom that allow air circulation needed for root pruning. Air movement beneath the containers must be provided.


Figure 6—Containers frequently used to produce longleaf pine
seedlings include (A) RL Stubbies, (B) Styroblocks. and (C) Multipots.

Media Preparation

Selecting media—Materials such as peat, composted organic material, sawdust, bark, vermiculite, topsoil, and perlite have been evaluated for use as container-growing media (Landis and others 1990b). A 1-to-I mixture of sphagnum peat moss and medium-grade (#2) horticultural vermiculite has been a consistently good product (fig. 7). This mixture has physical, chemical, and biological properties that result in good water-holding capacity and aeration and high cation exchange capacity. The quality of the peat and vermiculite varies among sources. The peat should be screened (free of large sticks, etc.), and the vermiculite a course grade. Fine grades of vermiculite result in a medium that compacts in the container and, thus, reduces aeration and restricts drainage.

Figure 7 – A 1-to-1 mixture of peat and vermiculite is a commonly
used growing medium for southern pine container stock.

Good quality seedlings can be grown using other blends of media, especially those that include perlite. A small proportion of perlite can improve drainage and aeration in wetter spring months when lower transpiration occurs. The grower must recognize that changing the blend of the media can drastically change cultural practices such as irrigation and fertilization.

The nursery manager may purchase a commercial medium or blend it at the nursery. Many commercial blends are designed for horticultural use and the pH is too high (about 6.0) for conifer use. If commercial products are used, the grower should specify the components and the pH of the media. The pH should be adjusted to about 4.5 to 5.0. When higher pH water is used, the pH of the media increases to the optimum of 5.0 to 5.5. This level also restricts pathogen development.

Mixing media—If large amounts of media are needed, on-site blending is encouraged because the chance of crushing or compacting the vermiculite particles is reduced. Such crushing frequently occurs during bagging, stacking, and transporting. Although blending can be done with hand tools or equipment such as concrete mixers, equipment specifically designed for blending is recommended for large operations (fig. 8). This blending equipment is recommended for three important reasons. First, it mixes the media thoroughly in a short amount of time. (To avoid crushing the vermiculite, blending time should not exceed 2 minutes.) Second, it facilitates incorporation of amendments, such as lime needed for pH adjustment. Third, it allows water to be added during mixing. Adding water reduces dust and moistens the medium for better filling of containers. The medium should hold its form without dripping water when squeezed in the hand. If too wet, the medium will become too compacted during the blending and filling process. If too dry, it will not compact properly and will be too porous, making irrigation difficult.

Figure 8—A mechanized blender is recommended for larger
commercial operations where on-site mixing of media is desired.

Amending media—Although adding a surfactant or wetting agent would increase the uniformity and rate at which moisture spreads through hydrophobic peat moss, many of these products reduce germination of southern pine seeds when added at their recommended rates (Barnett 1977). Adding surfactants in the blending process is not recommended without preliminary evaluation.

Growers may add lime to the medium to adjust pH to the recommended levels of 5.0 to 5.5. The amount of lime added depends on the initial pH of the media (Landis and others 1990b).

Normally, water-soluble fertilizers are supplied through the irrigation system. However, because longleaf pine seedlings have little stem elongation, incorporating a slow-release fertilizer, such as Osmocote 18-6-12 NPK, into the media can be very helpful, especially for an inexperienced grower (Landis and others 1989). The recommended rate for Osmocote 1 8-6- I 2 is 6 to 10 pounds per cubic yard of media. Some growers use about half the recommended rate to maintain more flexibility in fertilization. This treatment supplies most of the NPK needed, but additional nutrients will he necessary. Using slow-release fertilizers is particularly valuable when frequent rains leach water-soluble nutrients and frequent irrigation is infeasible. Incorporating slow-release fertilizers will also reduce the time spent applying nutrients through the irrigation system.

Fungus gnats (Bradysia spp.) are small, dark, mosquito-like insects that can damage roots and spread fungi and disease from one container to another (James and others 1995). The larvae are small and maggot-like and thrive on organic matter in high-moisture-content growing media. Using a well-drained medium and allowing it to dry between irrigation impedes fungus gnat development. However, some nursery managers incorporate Pratt-Oxamyl 10-percent granular insecticide into the medium at 2 to 3 ounces per cubic yard to control fungus gnats. Nursery personnel should wear gloves and respirators when mixing this material into the media.

Although seedlings are naturally inoculated with ectomycorrhizae by wind-borne spores, inoculation with a specific mycorrhizal fungus such as Pisolitus tincorius (Pt) is feasible and may be desired by some growers (Landis and others 1990a). Inoculation can be accomplished by incorporating specially produced vegetative mycelium into the growing medium.  The high cost and limited availability of this vegetative mycelium usually makes this option prohibitive. The most practical approach is to inoculate with a spore suspension of Pt. Pisolitus tinctorius spores can be obtained from local-source fruiting bodies or from a reputable commercial source. Inoculating with a water suspension of spores, the dosage rate should be 5 grams of double-sifted spores per 3 gallons of water per 10,000 seedlings. The spore suspension can be applied to the medium surface after seed germination using a diaphragm-type backpack sprayer. A mycorrhizae specialist should be consulted about the specific techniques and methods associated with the collection. extraction, storage, and application of Pt spores.

Container Filling

During container tilling, either by hand or mechanical equipment (fig. 9), the medium should be slightly packed by vibrating or bouncing. The amount needed to fill containers should include an additional 20 percent to allow for compaction. For example, a 6-cubic-inch container cavity will need 7.2 cubic inches of medium for proper tilling. The containers should be filled completely and the excess should be brushed off, leaving the medium approximately one-half inch below the top of the containers.  This depression keeps the seed in place and facilitates watering.   The depression will become deeper as irrigation and rainfall continue to pack the medium.

Seed Preparation

Selecting seeds—High-quality seeds should be used in container nurseries. Seeds should have a minimum viability of 75 percent (fig. 10). Longleaf pine seed collection, processing, storage, and treatment requires exceptional care to maintain quality equal to that of other well-processed southern pine seeds (Barnett and Pesacreta 1993). Although stratification of longleaf seeds will increase speed of germination by I to 2 days, it may reduce total germination and is not recommended without preliminary evaluations.

Figure 9—Mechanical equipment is needed in large
operations to fill containers with growing medium.

Figure 10—Longleaf pine seeds are large, thin coated, and
sensitive to damage during collection, processing, and storage.

Treating seeds—Longleaf seed coats commonly have significant populations of pathogenic fungi that may cause damping-off of germinants. If seeds have low viability or vigor, treating them with hydrogen peroxide or a fungicide may minimize disease losses. Soaking seeds in a 30-percent peroxide solution for 30 to 60 minutes and rinsing thoroughly in water removes most seed-coat pathogens and generally improves germination of low-viability lots (Barnett 1976).

Personnel at the North Carolina Claridge State Nursery routinely use a hydrogen peroxide treatment on their longleaf pine seeds. They soak 20 to 25 pounds of seeds in special nylon bags. After soaking for 55 minutes in a 30-percent hydrogen peroxide solution at a temperature of 75 oF or less, the bags are drained and drenched in three separate containers of clean water. The seeds are then removed from the bags and allowed to surface dry.

A more practical but less effective alternative involves lightly coating or drenching seeds with a fungicide. such as thiram or benomyl, just before sowing. A benomyl soak prevents disease development from seed-coat organisms of longleaf pine. A 3- to 5-minute soak (2 tablespoons per gallon of water) is used at some nurseries. Germination is typically improved by about 5 percentage points. However, this use of benomyl is not currently registered.

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