Using Retractable Roof Greenhouses and Pot in Pot System for the Development of a Double Crop Tree Liner Production

In recent years, increased interest in retractable roof greenhouses (RRG's) and pot-in-pot (PIP) systems to accelerate tree liner/caliper production has occurred. PIP and RRG's as production systems to manipulate the growing environment have been shown to increase root mass and improve plant adaptation to stress versus conventional container or bare-root grown plants. Root dormancy as a physiological process to manipulate plant growth is unexplored in ornamentals. Media Geohumus amendments that increase plant water use efficiency during production and could potential carry through production to increase transplant survival in landscapes has not been explored. The objectives of this study are: evaluate root growth of landscape trees from cell (plugs) to 3-, 7- or 15-gallon black rounded pot when grown double cropped (6-month) versus a twelve- month-cycle in a RRG; 2) explore root dormancy and GEOHumus media amendments at up-shifting as means of manipulating plant growth. Three landscapes tree species, Red Maple (Acer rubrum ‘October Glory®'), Avondale Redbud (Cercis chinensis) and Littleleaf Linden (Tilia cordata ‘Greenspire®') were planted from tissue culture into 3 gallon containers (black rounded pots) with a soilless mix. Planting occurred in October 2008 with and out-planting to 7 and 15 gallon PIP installations in June. Treatments consisted in Geohumus amendment in the media (1% by volume and no Geohumus), and Bottom heat mat treatments from December to March, at 42°F (with and without bottom heat). All plants were irrigated using cyclic-micro-irrigation two times/day, applying 500 ml of water total. Plants were fertilized using a control release (CR) fertilizer (40g of Osmocote 19-5-8) applied at potting. A sample of plants was held in the 3 gallon pots in June to compare in September with the 7 and 15 gallon PIP. Plants were harvested for growth measures in April, June and September 2009. Measurements consisted of height, caliper (taken at 2.4 cm), leaf area, and shoot and root dry weights. The measures will be analyzed in ANOVA using PROC GLM at a least significant differences with α = 0.05 using SAS software (SAS Institute, Inc., Cary, NC). The bottom heat in treatment improve the growth of the plants during the various stages and was carried out after a year. Geohumus treatment was only significantly different in root dry weight on June harvest, and did not improve growth after a year.