This document summarizes a study on the effects of different proportions of Douglas fir bark, peat moss, and pumice in soilless substrates on plant growth and water retention. Key findings include: 1) Increasing the proportion of peat moss in the substrate linearly increased plant growth and water retention for both Weigela florida and Azalea plants. 2) Substrates with a higher proportion of bark resulted in lower plant growth and water use efficiency than those with more peat moss or pumice. 3) Over time, substrates with fresh bark had larger increases in air space and water holding capacity compared to substrates with aged bark.

1. Understanding Bark, Peat, and Pumice in Your Soilless Substrate J.S. Owen Jr., M. Zazirska-Gabriel, and D. Sullivan Oregon State University, Aurora, OR J.E. Altland Application Technology Research Unit, USDA-ARS, Wooster, OH J.P. Albano US Horticultural Res. Lab, USDA-ARS, Fort Pierce, FL

2. Douglas Fir Bark AGED FRESH Altland

5. Year 1 Percent volume Fresh Aged Time (months) 46%»29% 59%»50% Air Space (AS)

6. Percent volume Fresh Aged Time (months) 42%»58% 27%»34% Container Capacity (CC) Year 1

7. Year 2 Percent volume Fresh Aged Time (months) 47%»59% 34%»47% Container Capacity (CC)

8. Year 2 Percent volume Fresh Aged Time (months) Up to 80% unavailable water Container Capacity (CC) Up to 60% unavailable water

9. Corresponding Research

13. Initial physical properties

14. Initial physical properties

15. Initial physical properties

16. Initial physical properties

17. Weigelia crop response Peat

18. Weigelia crop response Peat Peat Pumice Percent addition (by vol.) Dry weight (g) Significant (p < 0.05 ) linear and quadratic response for peat and pumice, respectively peat vs pumice ** bark vs peat ** bark vs pumice NS

19. 45% Control (100% DFB) Percent addition (by vol.) 30% 15% Pumice Peat

20. Water applied to Weigelia (0.2 LF) Significant (p < 0.05 ) linear response for peat. Peat Pumice Percent addition (by vol.) Water applied (L) peat vs pumice ** bark vs peat ** bark vs pumice NS

21. Weigelia water status as ƒ(leaf length) *Significant (p < 0.05 ) linear and quadratic response for peat and pumice, respectively Peat Pumice Percent addition (by vol.) peat vs pumice NS bark vs peat ** bark vs pumice **

22. Azalea crop response

23. Azalea crop response Peat Peat Pumice Percent addition (by vol.) Dry weight (g) peat vs pumice NS bark vs peat ** bark vs pumice NS Significant (p < 0.05 ) linear response for peat.

24. Azalea water use efficiency (WUE) Peat Pumice Percent addition (by vol.) WUE (g mL -1 ) peat vs pumice NS bark vs peat ** bark vs pumice NS Significant (p < 0.05 ) linear response for peat.

25. 45% Control (100% DFB) Percent addition (by vol.) 30% 15% Pumice Peat

27. Jim Owen: 503.678.1264 or jim.owen@oregonstate.edu