What do I do about tree root holes?
Q: We moved to a property where there are many shallow, large roots spreading out from a mature tree in the lawn, and the roots have holes in a few places. Should we fill them in to prevent rot?
A:
I’ve seen this very phenomenon many times – shallow tree roots (which are normal in many situations) that rise into a lawn and which are repeatedly run-over with a lawn mower. Just like with branches and trunks, wounding can remove bark and allow decay into the live root wood underneath, killing tissues and potentially causing plant stress or canopy dieback.
This is an excellent example of why tree root zones should be mulched or planted with groundcovers (so nothing needs mowing) to avoid untreatable damage and trip hazards. Besides, when trees cohabitate with lawns, the tree usually wins if it remains vigorous, so you’d have to eventually replace that lawn area anyway as it thins out.
There is nothing you can do at this point, other than avoiding new damage by converting any lawn areas with this surface root issue into a lawn alternative, such as a medley of native perennial groundcovers.
Numerous shallow tree roots might indicate poor drainage or soil compaction, both of which reduce the roots’ access to oxygen and force growth to seek more air near the surface. While shallow, surface-visible roots are naturally more common among some tree species (birch, maple, magnolia, and beech being typical examples), aerating the soil to improve drainage and alleviate compaction may keep the situation from worsening.
Filling the existing holes will not prevent or halt decay, which already started since the roots have hollowed-out in places. Trees do have ways of internally walling-off damaged tissues to avoid the spread of decay into healthy wood, so the damage might be limited despite how severe it may look. You can have any tree whose health is questionable evaluated by a certified arborist or licensed tree expert, though waiting until the canopy is leafed-out may be best so they can assess its fullness.
The root damage pictured here is the worst I have ever seen (it was over a more extensive area than the photo shows), which I suppose someone could turn into a creative course for some ultra-compact mini golf.
Q: I want to use horticultural oil to treat some scale insects so I can avoid the use of more risky pesticides. I have read that you shouldn’t spray when it’s too chilly, but I don’t understand why … the oil isn’t going to freeze in the sprayer, is it?
A:
Not likely, though it may become more viscous (thicker and harder to pour and spray) and therefore may not coat what you are spraying as evenly as it needs to work well. Instead, the main reason for avoiding cold temperatures is a reduction in how effective the oil will be at smothering the pest.
Insects don’t have lungs like those of mammals, and they also don’t breathe through their mouths – they have pores, called spiracles, that line their bodies and can open and close as needed to essentially inhale and exhale. If spiracles are closed, for example, they are effectively holding their breath. Insects respire (breathe) at a rate that can change based on temperature, as they are cold-blooded and their growth and metabolism slows considerably as it gets colder.
In order for the horticultural oil to block breathing pores long enough to help suffocate them before the oil evaporates, they need to be respiring enough when treated. If you spray when it’s too cold, the insects are not using enough oxygen to be bothered by the impacts of the oil, so its pest-killing ability is reduced. If you wait until mild weather to make an application, however, their respiration rate will increase and an oil spray can suppress them more effectively.
Dormant oil is what you would be using in this case – it’s simply horticultural oil used at a slightly higher concentration when diluted for spraying. Dormant oil can be useful in helping to knock-down scale insect outbreaks – especially those residing on bark, like crapemyrtle bark scale that many local gardeners are encountering – at a time when the beneficial insect predators of the scale are not active and unlikely to be exposed to the pesticide.
What temperature is ideal? Always refer to the product label for directions, but as a generalization, temperatures during a dormant oil application should be 50 to 60 degrees or above for several consecutive days, with nightly lows above freezing. You can learn more about horticultural oil on our pesticide profile page.