Stacking substrate-filled containers influence root and shoot growth of bedding plants

Substrate compaction occurs when substrate-filled containers are nested within each other. Our objective was to quantify the effects of substrate compaction on shoot and root growth and development of several annual bedding plants. Containers (11.4-cm diameter; 600-mL) were filled to the rim with a commercial soilless substrate comprised of (by vol.) 65% peat, 20% perlite, and 15% vermiculite and placed in shuttle trays with a carrying capacity of 15 containers per tray. The weight of a single tray and the weight of each subsequent tray nested on top of another was determined and recorded until a total of 9 trays were stacked. Stacked trays were covered and placed on a greenhouse bench for 7 d. Substrate compaction was measured from the substrate surface to the rim of the container. African marigold (Tagetes erecta ‘Moonsong Deep Orange’), pansy (Viola × wittrockiana ‘Matrix Yellow’), and petunia (Petunia × hybrida ‘Wave Purple Classic’) seedlings were transplanted into five containers from each shuttle tray. Plants were grown in a glass-glazed greenhouse for 28 d under a 16 h photoperiod and an average greenhouse air temperature of 21°C. In general, substrate compaction increased from the top to the bottom layer of shuttle trays. Shuttle tray weight increased by 8-fold from the second (4.4 kg) to the bottom layer (19.6 kg). Root dry mass of pansy and petunia, but not African marigold, was negatively affected by substrate compaction. For example, root dry mass of petunia was 44% less from the top to 5^th shuttle tray layer (0 to 10.8 kg). The 8.7 kg of additional weight from shuttle trays one to five negatively affected root growth. Vegetative growth did not appear to be affected. Visually, plant size was less as substrate compaction increased. Overall, stacking shuttle trays up to five layers will not affect growth of the three bedding plant species investigated.