Cyclic Irrigation Does Not Overcome Low Water Holding Capacity of Juniperus virginiana-based Substrates for Greenhouse Production of Rudbeckia

Nursery and greenhouse studies evaluating wood-based alternative substrates have consistently cited poor physical properties, specifically low water holding capacity and high air space, as reasons for reduced growth in plants produced in a high percentage of wood materials. In the Great Plains region of the United States, Eastern redcedar (Juniperus virginiana; ERC), is a locally available, weedy tree species. Several studies have indicated that ERC is suitable as a substrate amendment, but not as a replacement for pine bark (PB). When ERC is used up to 40% (by vol.), plant growth is often similar to PB-based substrates, but when blended at volumes greater than 40%, reduced growth is evident. The objective of this study was to determine if cyclic irrigation (same volume of water distributed throughout the day as opposed to one irrigation event) can help overcome low water holding capacity and improve plant growth. Black-eyed susan (Rudbeckia fulgida ‘Goldstrum’) liners were planted into three substrate treatments: 1) 80% PB:20% sand;  2) 40% ERC:40% PB:20% sand; and 3) 80% ERC:20% sand (by volume) on February 29, 2012. Container size was 1-gallon (3.8 L) and substrates were pre-plant incorporated with Osmocote 18N–5P–13K + micronutrients (3–4 month release, The Scotts Co., Maryville, OH). Plants were placed in a glass greenhouse maintained at 70 °F (21 °C). An automated irrigation controller was used to apply the following cyclic irrigation treatments: 1x per day (0800 hr), 2x (1100, 1500 hr), 3x (0900, 1200, 1500 hr), and 6x (0800, 1000, 1200, 1400, 1600, 1800 hr). Total volume of water applied was 350 mL per day. Plants were arranged in a randomized complete-block design with a 3x4 factorial (substrate by irrigation frequency) with 6 reps. There were no interactions between substrate and irrigation for any data. Black-eyed susan had the greatest growth index and shoot dry weight when grown in 80% PB:20% sand, though plants grown in the PB:ERC mix were marketable. Plants grown in 80% ERC : 20% sand were smaller than plants in the other two treatments and increasing irrigation frequency in this substrate resulted in the smallest plants in the study. Among all substrate treatments, plants irrigated 6x had less shoot dry weight than those irrigated 1x and 2x per day. Cyclic irrigation did not overcome low water holding capacity in the 80% ERC:20% sand substrate treatment for black-eyed susan. However, plants grown in PB:ERC grew to a marketable size.