Germination of Nonstratified Japanese Tree Lilac Seeds as Influenced by Seed Capsule Maturity and Moisture Content

Temperate-zone woody plant species generally require seed stratification to overcome embryo dormancy. Embryo dormancy is variable in japanese tree lilac (Syringa reticulata) with a recommendation of 30- to 90-days stratification at 1-5 degrees C. Cultivar propagation generally is done by grafting onto seedling rootstocks. It would be advantageous to rootstock seedling production to be able to reduce or eliminate the need for seed stratification to increase germination numbers as well as having production moved from field beds to greenhouses for quicker production of high-quality rootstock plants suitable for grafting. Research objective of this study was to determine if "green" seed could be used without the need of stratification for japanese tree lilac seedling production. Seed capsule fresh weight and seed moisture content were evaluated to determine if these factors could be used as predictors of germination percentages. Seed was randomly collected at the North Dakota State University campus in Fargo, ND, for seven consecutive weeks starting in Sept. 2011 and 2012. Germination and seed moisture tests were performed weekly. Germination percentage was highest (89.5%) at week 2 and steadily decreased to 0% at week 7. Germination percentages were 77.5%, 89.5%, 78.5%, 67%, 24.5%, 1.5%, and 0% for consecutive collection weeks 1-7, respectively. Seed moisture content was 59.0%, 52.6%, 49.8%, 51.8%, 44.5%, 27.4%, and 8.6% for collection weeks 1-7, respectively. Germination percentage was directly correlated with seed moisture content and decreased as seed capsules matured (natural drying and splitting of capsule seem to disperse seed) during the fall season. Data suggest that timing of fall seed collection from japanese tree lilac is critical and must be done before maturation of the seed capsule to avoid the stratification requirement. Seed capsules with an average fresh weight higher than 0.2 g and seed moisture content greater than 50% produced the highest germination rates without requiring stratification.