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:

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.

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.

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.

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.