Some knowledge of nursery practices is necessary to take best advantage of desirable characteristics of stock types.
This concept was pioneered with Pli. Untreated container walls allow lateral roots to turn down, resulting in the accumulation of most of the active growing tips at the base of the plug. Copper-treated container walls prevent lateral roots from turning down, effectively "pruning" them at the wall. These roots resume lateral growth upon release from the container, affecting lateral root egress higher up the plug. Lateral root egress from the upper portions of the plug increases seedling stability and access to nutrients and warmth nearer the soil surface. Studies have shown this to be beneficial, increasing initial root egress, survival and performance in general, particularly on fine-textured soils. This has led to a general increase in the recommendation and request for copper-treated Pli stock types. In addition, copper treatment may help reduce the incidence of root and foliage diseases. Copper treatment has been applied experimentally to most other species, and to 2+0 stock types in general as a means of reducing the degree of rootbinding. However, assuming that the concept will generally benefit other species in the same manner may not be correct. Although detriment has not been observed, species such as Sx and Ss, which naturally produce a lot of roots adventitiously just below the root collar, may not require it unless the crop is in danger of becoming root bound. Fdc may be a candidate for copper-treated culture if, along with altering root morphology, the incidence of root dieback is also lessened.
Also still under investigation are the anti-fungal effects of copper that may be of value when growing species that are sensitive to root diseases such as Fdc and Hw.
At the nursery, copper-treated
Air pruning containers
The concept is similar to copper treatment in that it prunes lateral roots prior to them being able to turn down at the container wall. It also encourages lateral root egress from upper plug regions after outplanting (Plate 4). The difference is that the pruning agent is air instead of copper. At this point in time it appears that air pruning provides a root morphology at least intermediate between a copper-treated and untreated container. Perhaps the question to ask is how many roots egressing from the top of the plug does a seedling need to overcome stresses requiring this ability.
Plate 4. Post-planting root egress from Pli seedling grown in a PAB 410. Note that new root growth egress is from all regions of the plug. Photo credit: Anne Johnson-Flanagan.
In order to meet specified delivery dates on summer- and fall-shipped crops, and stated morphological specifications in general (e.g., seedling height), stock may be induced to set bud prior to natural induction. This can be achieved by traditional means such as drought stressing, or by shortening photoperiod or "blacking out" (Plate 5). Photoperiod control is thought to be a less stressful budset induction tool. Exact duration and photoperiod employed varies with species, stock type, provenance, and nursery preference. For summer delivery crops, blackout treatments are usually imposed three to four weeks prior to shipment to ensure adequate bud set and sufficient hardening-off. If the time between budset induction and planting is extended beyond five weeks, stock may start to reflush, perhaps requiring a repeat of the process and delay in planting. Budset induction affects the onset of dormancy and hardiness cycles. Depending on chilling and storage schedules, earlier induction may, in this way, be able to affect the timing of budflush the next growing season. An effect of up to several days has been reported in the literature.
Stock scheduled for summer planting
Plate 5. Conifer seedlings in greenhouse with blackout curtains almost closed. Photo credit: Eric van Steenis.
A recent alternative to blackout is the use of a plant growth regulator (PGR) to induce budset for summer-shipped and spring-plant crops. In addition, it may be used to control height growth in the first year of a 2+0 crop. The only PGR currently registered for reforestation seedling stock is paclobutrazol. The chemical's mode of action is to inhibit gibberellin production resulting in a reduction in the rate of cell division and elongation. Trials have shown that paclobutrazol will reduce seedling height growth while increasing stem diameter and root weight. Also, some trials have reported increased seedling survival and enhanced drought resistance. The chemical is applied approximately two weeks prior to the time when conventional growth inhibiting practices (blackout, drought and nutrient stress, temperature) would be used. The effects on seedling morphology, physiology and long-term outplant performance are unknown and further assessment is needed to effectively evaluate the potential benefits or detriments to seedling growth.
Rooted cuttings have traditionally been used to bulk up material for which seed was in short supply or of very poor quality. An example is Yc. More recently, the process has been employed to bulk up material of superior genetic quality. Examples are black cottonwood (Act), Sx, and Ss. Because the technique starts with a sizable portion of plant material, it usually yields a robust product that easily meets minimum root collar diameter specifications. A proportion of rooted cuttings may exhibit plagiotropic growththese are culled before shipping. Plantation trials have shown growth and form of rooted cuttings to be equivalent to seedlings.
Somatic seedlings (emblings)
These are produced by removing embryos from individual seeds and vegetatively multiplying them by a process termed somatic embryogenesis. In this way each chosen seed becomes the parent of a line or clone of "emblings" or "somatic seedlings." The process is used to bulk up seed from genetically superior crosses that might otherwise be in too short a supply to generate a substantial seedling crop. At present, the cost of production is very high due to the labour requirements to transplant the embryos. In the future, encapsulating embryos into artificial seeds will greatly reduce the individual propagule cost. At this time, the cost may only warrant bulking up material with very high genetic worth (e.g., seeds generated from putatively weevil-resistant spruce parents).
Overheight stock often incurs additional grading, lifting, storage, and planting costs and may at times be completely inappropriate for the site to which it is destined. Some broadleaf deciduous species, as well as Cw, may grow too rapidly, easily exceeding contract height specifications. If anticipated early enough, stock can be top-pruned to stay within height specifications. Early top-pruning (before mid-July) also encourages more lateral branching without causing multiple tops, and prevents lower foliage from senescing. Later top-pruning may delay hardening-off, result in unsatisfactory growth form, and lead to increased incidence of storage moulds due to dense canopies and damaged stems. Top pruning is not recommended for birch or red alder, as tops die back from the cut.
The BBR and PBR (field grown) stock types are routinely root-pruned at the nursery to encourage branching of individual roots and the development of a more fibrous root system. This type of root system is essential to assure plantability and proper establishment of the stock. Root-pruning is a nursery practice and should not be required once the stock arrives in the field.
Plug transplant stock is initially grown as mini-plugs in high density blocks. Approximately 10 weeks after germination, the mini-plugs are transplanted into larger containers, (typically 512/515/615) to complete their growth. This practice generates several efficiencies for the nursery which translates into savings in the production costs of the larger container stock types.