Effect of planting date and seedling cover on seed germination of mountain ash (Fraxinus excelsior L.) and decrease of spring late cold damage

In recent years, spring late coldness has caused a lot of damage to mountain ash seedlings in the mountainous nurseries. This research aimed to reduce mountain ash seedlings damage due to spring late coldness. The research was carried out during 2007 and 2008, in Darzikola nursery (Farim Wood Co.), located 17 kilometers far from southeast of Pol-e-Sefid city of Mazandaran province of I.R. Iran. Seeds were treated by two levels of seedling cover (nylon sheet and control), and four levels of seed sowing dates (Fourth week of February (1), first week of March (2), fourth week of March (3) and second week of April (4)) under split plots design with three replications. The trial was repeated for two years. Results showed that effects of sowing date on seed germination rate were significant (p<0.01), whereas effects of cover treatments and interactions between the two treatments were not significant on seed germination rate. Effects of cover treatments on seedling height were significant (p<0.05). Seedling sowing date treatments had different effects on seedlings height. For instance, seedling height increased gradually at first treatment, whereas reached its maximum height at third treatment and then decreased. Effects of seed cover, sowing date and their interactions on mortality rate of the seedlings due to spring late coldness were significant. The mortality rates of the seedlings at covered and control treatments were 80 and 20 percent, respectively. There were not significant differences in seedling loss at different levels of sowing date treatments when they were covered by nylon sheet, whereas at control treatment, the mortality rate decreased significantly when the sowing date reached the second week of April. Due to seedlings propagation productivity and mortality rate caused by coldness, it is suggested: first to sow the seeds at 15th March to 1st April, after application of one year cold stratification treatment and second to cover the plots containing the mountain ash seedlings by nylon sheets against probable coldness.