QBARS - v18n1 The Influence of Mulching Practices on Bud Production and Plant Size

The Influence of Mulching Practices on Bud Production and Plant Size
Dr. R. L. Ticknor
North Willamette Experiment Station, Aurora, Oregon

What is the proper use of organic matter for the best growth of rhododendrons is a question often asked. To determine whether it was better to use organic matter as a surface mulch or whether it was better to incorporate it into the soil, a small trial was conducted at the North Willamette Experiment Station of Oregon State University.
An elepidote rhododendron, R. catawbiense , 'Roseum Elegans', and a lepidote rhododendron, R. carolinianum album seedlings, as well as many evergreen azalea seedlings were used in this trial. Cuttings of 'Roseum Elegans' were rooted during the fall of 1959 and were planted in the lath house in the spring of 1960. In January 1962, the plants were planted in the trial area. R. carolinianum album was started from seed in the spring of 1959. The seedlings were planted in the lath house in the spring of 1960 and in the trial area in January 1962.
Three treatments were used: three inches of ground bark, containing sawdust, was disked into the soil in one area; the same amount of ground bark was used as a surface mulch, after planting, in another area; and no organic matter was used in a third area. In the spring of 1962, ammonium sulfate to supply 80 pounds of actual nitrogen per acre was applied. Ammonium nitrate at a rate of 100 pounds of actual nitrogen per acre was applied in the spring of 1963. This rate of nitrogen application is lower than suggested for use with the same amount of sawdust. An application of one pound of ammonium sulfate to each 100 sq. ft. of ground area covered one inch deep is suggested with sawdust. This suggested application is approximately 260 pounds of actual nitrogen per acre for a three inch layer. The nitrogen level where the bark was incorporated into the soil has not been high enough to maintain the desired green color in the leaves.
Several effects of the use of organic matter other than nitrogen starvation have been observed. Marked differences in bud production were noted from the different treatments. All rain or sprinkler irrigation water is absorbed into the mulched plot while considerable runoff occurs from the bare soil plot. Some runoff does occur on the incorporated plot. All plots are on a slight south-facing slope. A four or five day delay in bud opening has been observed in early azaleas in the mulched plot as compared to the bare soil plot while no delay has been noted in the incorporated plot.
Average plant size and bud production at the end of the 1962 and 1963 growing seasons are presented in tables 1 and 2.
The best bud production and plant size of 'Roseum Elegans' occurred where the surface mulch was used. Bud production was reduced but plant size was not altered appreciably in the incorporated treatment as compared to the no organic matter treatment. Plant quality in color and amount of branching was poorest in the incorporated treatment, intermediate in the no organic matter treatment, and best in the mulch treatment.
With R. carolinianum album seedlings the best growth and bud production occurred in the mulched plot. Plant size and bud production were quite similar in the incorporated and no organic matter plots. Plant quality in color and branching was better in the no organic matter plots as compared to the incorporated treatment but the differences were not as great with R. carolinianum album as with 'Roseum Elegans'.