Before you call the Manitoba Maple (Acer negundo) a "weed", I want you to remember what our winters are like in Northwestern Ontario. It gets really cold and stays really cold for a long time! Typical champion trees that are common in North America like white oak, hickory or sycamore just won't survive the punishment of persistent sub zero tempreatures.
For whatever reason, the Manitoba Maple tree survives our winters exceedingly well and flourishes to become a significant portion of our urban forest canopy. In fact across our region we have countless large, mature and safe maples that return services to us to improve city living conditions by providing shade and cleaning our air. So until climate change alters our winters enough that we can replace Manitoba Maples with White Oaks, stop calling them weeds!
This past year's climate has been so very different from any before. Our winter was much warmer and our spring saw some heat waves that allowed me to go ice fishing wearing shorts and no shirt. We also saw some wild fluctuations in temperatures and an inconsistent amount of precipitation.
Our Manitoba Maples have suffered this year and many of you have likely noticed. Some of the maples are very thin, having lost most of their leaves. Newer growth on some has been stunted to the point where young leaves are barely viable. Added to this, excessive spring rain has increased leaf loss due to fungal diseases.
Not every Manitoba Maple is affected oddly enough. Many are thriving but just as many are showing symptoms that could easily lead to rapid tree death.
On severely affected trees I have not noticed a profusion of a single typical attacker like cankerworms, anthracnose or aphids which leads me to believe that climate is the culprit. When we had warmer temperatures this spring, many trees began to emerge from dormancy and were drawing water from the soil into the branches. Buds started to swell on many species and then we were hit with a week of night time temperature lows of -15 Celcius. This had an effect of freezing water in the twigs and branches which would have surely ruptured many cells. This would have the effect of severely limiting water and nutrient flow from the roots to the leaves.
We saw this effect show up earlier this spring in many coniferous trees from Geraldton to Atikokan.
For Manitoba Maples, the damaged cell problem became apparent only after our season changed from a wet spring to a hot and dry summer. With less moisture in the soil, Maples with a damaged vascular system couldn't pull enough water out of the soil so they started to drop leaves and abort twig growth.
Since we have never experienced a spring quite like this one, I do not know if the affected maples will survive or not. What is best for them right now (and for any other tree species) is to water them at least a few times throughout the summer. Apply water for a few hours near the dripline of the tree so the water soaks into the top one or two feet of soil.