November 15, 2008
There is an alternative to green ash
By Jen Llewellyn,
OMAFRA Nursery Crops Specialist
It’s getting a little tricky for nursery growers and landscapers to decide what shade trees they should plant. Every time we turn around there is a new invasive alien species being found associated with ornamental or forest trees. Or, it’s announced that there’s a need to expand the quarantine zone for a pest like emerald ash borer (EAB). The larvae of borer feed in the phloem, in the cambial tissue under the bark. Their galleries disrupt the flow of water and nutrients in the plant. Most trees are killed within three years of the first symptoms of the infestation.
A research team at the University of Guelph has graduate student Shelley Stewart looking at ash species in Essex County that are resistant to EAB. The pest has been reported to attack and kill all five ash species in eastern North America. Green ash (Fraxinus pennsylvanica) is highly susceptible to attack. In contrast, blue ash (F. quadrangulata), a species of special concern in Canada, is the least preferred species.
Point Pelee National Park (PPNP) presents a unique situation with its natural populations of blue ash and green ash occurring in very close proximity. A case study was conducted in 2008 at Northwest Beach in PPNP to compare the effects of EAB on blue and green ash trees. All green ash trees at this site exhibited symptoms of EAB infestation and reduced health; most were either dead or dying from EAB damage. In contrast, we found no evidence that EAB had infested the blue ash at this site; they were all in excellent health and no EAB exit holes were observed. Resistance to the EAB, coupled with excellent drought and salt tolerance, make blue ash a suitable tree species for urban areas in southern Ontario to replace other ash trees killed by EAB.
Blue ash is not a species commonly available in a nursery catalogue. It is slower growing, somewhat difficult to propagate and often it has a slightly irregular crown of spreading branches. After all, when there was green ash available, which grows fast and is easy to propagate, why choose a difficult species like blue ash? Now EAB has changed all the rules, making blue ash look pretty good. Unlike green ash, blue ash has gorgeous, glossy, dark green foliage and is very well suited to our calcareous soils. The stems are four-sided with corky wings that make identification easy. Personally, I think it is a beautiful tree.
Even if the propagation challenges are worked out, all Fraxinus species are regulated for EAB and the number of regulated areas is growing. This means that more nurseries are getting caught in EAB regulated areas. Since shade trees can take three to seven years before they become saleable, nurseries that plant blue ash today may not be able to ship their blue ash by the time it’s ready to harvest. These are the same nurseries that have already had to destroy their pre-existing green and white ash inventory and absorb all of those costs due to a collapse in the market.
Jen Llewellyn can be reached at (519) 824-4120, ext. 52671 or by e-mail at jennifer.llewellyn@ontario.ca
See her Nursery-Landscape Report at http://apps.omafra.gov.on.ca/scripts/english/crops/agriphone/index.asp.
OMAFRA Nursery Crops Specialist
It’s getting a little tricky for nursery growers and landscapers to decide what shade trees they should plant. Every time we turn around there is a new invasive alien species being found associated with ornamental or forest trees. Or, it’s announced that there’s a need to expand the quarantine zone for a pest like emerald ash borer (EAB). The larvae of borer feed in the phloem, in the cambial tissue under the bark. Their galleries disrupt the flow of water and nutrients in the plant. Most trees are killed within three years of the first symptoms of the infestation.A research team at the University of Guelph has graduate student Shelley Stewart looking at ash species in Essex County that are resistant to EAB. The pest has been reported to attack and kill all five ash species in eastern North America. Green ash (Fraxinus pennsylvanica) is highly susceptible to attack. In contrast, blue ash (F. quadrangulata), a species of special concern in Canada, is the least preferred species.
Point Pelee National Park (PPNP) presents a unique situation with its natural populations of blue ash and green ash occurring in very close proximity. A case study was conducted in 2008 at Northwest Beach in PPNP to compare the effects of EAB on blue and green ash trees. All green ash trees at this site exhibited symptoms of EAB infestation and reduced health; most were either dead or dying from EAB damage. In contrast, we found no evidence that EAB had infested the blue ash at this site; they were all in excellent health and no EAB exit holes were observed. Resistance to the EAB, coupled with excellent drought and salt tolerance, make blue ash a suitable tree species for urban areas in southern Ontario to replace other ash trees killed by EAB.
Blue ash is not a species commonly available in a nursery catalogue. It is slower growing, somewhat difficult to propagate and often it has a slightly irregular crown of spreading branches. After all, when there was green ash available, which grows fast and is easy to propagate, why choose a difficult species like blue ash? Now EAB has changed all the rules, making blue ash look pretty good. Unlike green ash, blue ash has gorgeous, glossy, dark green foliage and is very well suited to our calcareous soils. The stems are four-sided with corky wings that make identification easy. Personally, I think it is a beautiful tree.
Even if the propagation challenges are worked out, all Fraxinus species are regulated for EAB and the number of regulated areas is growing. This means that more nurseries are getting caught in EAB regulated areas. Since shade trees can take three to seven years before they become saleable, nurseries that plant blue ash today may not be able to ship their blue ash by the time it’s ready to harvest. These are the same nurseries that have already had to destroy their pre-existing green and white ash inventory and absorb all of those costs due to a collapse in the market.
Jen Llewellyn can be reached at (519) 824-4120, ext. 52671 or by e-mail at jennifer.llewellyn@ontario.ca
See her Nursery-Landscape Report at http://apps.omafra.gov.on.ca/scripts/english/crops/agriphone/index.asp.