Biocontainer Type and Paclobutrazol Drench Concentration Affect Growth of Angelonia and Petunia

Biocontainers are manufactured from materials that differ in physical and chemical properties and the effect of plant growth retardant drenches applied to plants may vary with container type. The objectives of this experiment were to quantify the growth and development of herbaceous annuals grown in different biocontainer types treated with paclobutrazol substrate drenches. Seedlings of ‘Serena White’ angelonia (Angelonia angustifolia Benth.) and ‘Wave Purple Improved Prostrate’ petunia (Petunia × hybrida Vilm.-Andr.) were transplanted into eight different types of containers containing 590 mL of soilless substrate comprised of (by vol.) 75% sphagnum peat moss and 25% perlite. Containers included: bioplastic #1 [90% polylactic acid (PLA) and 10% lignin], bioplastic #2 (60% PLA and 40% soy protein polymer), coconut coir (coir), biopolyurethane-coated paper fiber, uncoated paper fiber, peat, rice hull, and petroleum plastic (control). Ten d after transplanting seedlings, 70-mL aliquots of solutions containing 0, 1, 2, 5, 10, 20, or 40 mg∙L^–1 paclobutrazol were applied to the substrate surface. The date the first flower opened was recorded. Six weeks after transplant, data were collected, including widest and perpendicular width, and height (angelonia only). Shoots were harvested and dry mass (SDM) was recorded. Growth Index (GI; angelonia) and diameter (petunia), and time to flower were calculated. Container type and paclobutrazol concentration interacted to affect size and SDM of angelonia and petunia. For example, GI of angelonia treated with 0 mg∙L^–1 paclobutrazol grown in coir and peat containers were 19–29% and 29–38% smaller than plants in other container types, respectively. Diameter of untreated petunia grown in peat containers was similar to coir and uncoated paper fiber, but was smaller (10.9–13.5 cm) than other container types. SDM of petunia grown in coir containers were the same for 0 to 20 mg∙L^–1 treatments, while plants in rice hull containers were only similar for 0 to 2 mg∙L^–1 treatments. Container type or paclobutrazol concentration affected time to flower for both species. Angelonia treated with 0 mg∙L^–1 and 40 mg∙L^–1 paclobutrazol flowered in 34.2 and 39.7 d, respectively. Alternatively, angelonia grown in PLA/lignin containers flowered in 35.5 d, compared to 38.1 d for peat. Our results indicate that growth suppression of angelonia and petunia grown in biocontainers using paclobutrazol drenches varies by type of biocontainer being used. Producers should reduce paclobutrazol drench concentrations to produce appropriately-sized plants if substituting coir or peat biocontainers for traditional petroleum plastics.