Traditionally, landscape conifers have been field-grown and sold balled and burlapped (B&B). Soil loss resulting from harvesting field-grown trees can be nearly 100 tons per acre for a 5-year rotation and harvest can only take place in the spring and fall (Pollock and Mathers, 2002). If diseases such as phytophthora, especially common in firs, become established in soils, they can result in extensive mortality and in some cases have caused nurseries to be abandoned (Kuhlman et al., 1989). Moreover, consumer preference of container material has been steadily increasing (Halcomb and Fare, 1995). Container production has been increasing relative to B&B and now accounts for nearly 30% of the coniferous evergreen sales in the upper Midwest (NASS, 2007).
Pot-in-pot (PIP) production is an increasingly popular component of the overall container production trend. Because PIP plants are grown in containers, they are lightweight, easy to harvest, and root systems are not disturbed by digging and transplanting (Ruter, 1997). However, unlike above-ground container (AGC) production, the PIP containers are placed into socket pots, which are sunk in the ground providing stability and protection of the root zone from extreme air temperatures. In ornamental nurseries in the southern United States, PIP production results in moderated root zone temperatures, especially during the hot summer months, and improves growth compared with field- or AGC-produced plants (Roberts, 1993; Ruter 1993, 1995, 1998, 1999). The system has also been adopted as a method of providing winter protection in northern climates (Neal, 2004).
Pot-in-pot production is also suitable for developing niche markets such as living Christmas trees. Living Christmas trees appeal to consumers who would otherwise choose artificial trees as a result of environmental concerns (Genovese, 2007). Other consumers desire a second or third Christmas tree for their home (Behe et al., 2005). Some growers have found that Christmas trees as large as 6 feet tall can be grown in 37.9-L pots using a lightweight substrate and still be manageable for a consumer to take into their home (Genovese, 2007). Small, dense trees such as Black Hills spruce [Picea glauca (Meonch) Voss var. densta Bailey] or Serbian spruce [P. omorika (Panèiæ) Purkyne] grown in a 1-gallon (3.7-L) container could be placed on a tabletop and decorated and are desirable options for a small apartment or as an additional tree (Behe et al., 2005). Living Christmas trees produced in a PIP system and displayed indoors for up to 20 d during the holiday season perform better in the landscape after transplanting compared with field-dug trees (Nzokou et al., 2007).
Landscape conifer growers converting from field production to PIP face several key challenges. Among these are selection of appropriate container substrate and nutrition management. Growing conditions in PIP systems are not the same as either field or AGC production. Lightweight organic substrate is used rather than field soil and root zone temperatures are more stable than in AGC plants (Young and Bachman, 1996). Selection of an appropriate container substrate is critical because container substrates can impact plant nutrient and water relations.
In many commercial growing operations, fertilization regimes are often based on visual ratings (Parent et al., 2005), which can miss symptoms of “hidden hunger” or “luxury consumption” (Landis and van Steenis, 2004). Suboptimal nutrition can result in impaired physiological function, particularly as reduced photosynthetic rates that can reduce growth (Gough et al., 2004; Samuelson, 2000). Applying fertilizer in excess of a tree's physiological needs, in contrast, can result in leaching of nitrate and other nutrients resulting in environmental degradation (Juntunen et al., 2002, 2003). Moreover, nursery producers are increasingly interested in optimizing nutrient additions as a result of recent volatility in the price of nitrogen-based fertilizers.
The overall goal of this project was to develop fertilizer and substrate recommendations to optimize growth of containerized conifers, reduce potential environmental impacts, and reduce costs for growers using the PIP production system. Specific objectives were to: 1) determine the growth and physiological response of conifers to increasing fertilizer levels; 2) determine the effect of substrate on growth and physiology of PIP-grown conifers; and 3) determine the relationships among plant nutrition, physiological responses, and growth.
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