July 15, 2007 Oaks

Good morning Gardeners. It’s another lovely summer day in Manitoba, a cool day, but good for working in the garden, dealing with the weeds that are prolific this year.

Yesterday at our house, Glenn and I finally tackled the job of cleaning the garage, which doubles as my potting shed and his workshop – the car has not see the inside of this space for many years. It’s a very satisfying thing to see order come from chaos.

This is true of the garden too, which is very overgrown this year. Some things need cutting back to let in light for others, and have you ever seen such huge hostas? They really loved the warm wet nights of June.

But some plants are not as happy. For example, have you wondered about what is happening to the oak trees of Winnipeg? After being there seemingly forever, sturdy and enduring, many of them are fading away.

Well, it seems that this is not a new phenomenon. Many of our Winnipeg oaks were already in place when the houses were built between the turn of the century and the forties. As far back as 1940, scientists noticed significant changes in growing patterns in the wood of dead oaks. Growth rings often narrowed considerably as conditions changed. Oaks are extremely sensitive to even small changes in their growing environment and the growth of the city has created quite significant changes.

Extreme wet weather followed by drought in the past few years has caused severe stress among the oak population. This has left the trees with few defences against the two-lined chestnut borer, a slender, dark winged beetle which is about one-half to three-quarters of an inch long and has two yellow strips on its back.

In late May and through to July, the adults emerge from the bark to feed on oak leaves. Females lay eggs in bark crevices and the young larvae hatch two weeks later, boring a hole through the bark and into the cambium and the phloem where they build winding tunnels. This is where the damage is done as the tunnels block water and nutrient flow from the roots to the leaves. The first sign of trouble usually begins at the top with leaf wilt which travels downward. Leaves turn brown and stay on the branches after dieback.

Although healthy oak can withstand an attack of the two-lined chestnut borer by constructing new channels for water conduction, heavy infestations can kill a weakened tree in one to four years. Sadly, there are no approved chemical controls to deal with this bug.

So your best defence is a healthy tree, one that is well-watered during drought – and for oaks , like tomatoes, that means in a consistent fashion. The sweet sap of the oak is favoured by leaf sucking insects. In dry years, sufficient watering can actually dilute the sap, making the leaves less attractive and protecting your oak from further weakening.

Deep-root fertilization is a good idea and any deadwood should be promptly pruned back one metre into the healthy wood to get rid of the borer. Be sure to bury prunings in landfills or otherwise destroy them.

A note of caution: Don’t interfere with the oak soil around an oak. Even the addition of top soil can upset the air to soil moisture content they thrive in.

Under the right conditions, bur oaks live to 300 years, but they are difficult to transplant, so may of the large stands we see are natural stands and, as I mentioned earlier, houses were built between the trees.

Oaks are one of those few trees with a tap root that can grow down several feet in the first year of growth. This, and the fact that their roots systems are so sensitive to change, make them very hard to successfully transplant. So if you have lost an oak tree or two, think about growing a replacement from an acorn and take good care of it for your children and their children.