"We have a snowball bush about 40 years old, and for the past three to four years we have been fighting a black worm on the leaves. We have tried Sticky Foot around the trunk and numerous sprayings of insecticidal soap, and nothing helps. Can you help us save this old bush?"
Keith, via email
Your problem sounds like the viburnum leaf beetle. Sticky material around the trunk won't help, because the adults of the larvae that eat the leaves can fly to the bushes to lay their eggs on the twigs.
You can start controlling the beetle by examining the twigs for tiny black spots - clusters of beetle eggs which are laid in tiny cavities and sealed with a mix of chewed wood and beetle feces. They are usually on the tiny fragile twigs at the outside of the bush.
Pruning those twigs and garbaging them will remove a lot of eggs. Each black spot holds about eight. But the larvae hatch in May, so probably too late for pruning this year. Start pruning in fall or over winter. The more eggs you can throw out, the fewer worms next year.
In June, the larvae will drop to the soil and go into a chrysalis stage. Starting in early July, if you have bare soil under your viburnums, you could try removing the top inch of earth and burying it in a deep hole or sealing it in a garbage bag for a few months. In fall, it should be safe to dump it somewhere in your garden.
That should remove quite a few of the chrysalises, which hatch into beetles in late July. Any you miss will turn into beetles that can lay several hundred eggs over the next two and a half months.
Some eggs and chrysalises will get away from you, or beetles can fly in. But it's worth doing, because viburnums can die after years of this beetle's work.
"I haven't removed my dahlias from the ground since planting them last year, and they have started sprouting. Is it too late to dig them up and divide them? Do you need to divide them every year?"
Dahlias don't have to be divided every year. Some coastal gardeners in well-drained situations deliberately leave them in the soil over the winter. In mild winters, they can come back.
But, like other clump-forming plants, they need to be divided sometimes.
Plants usually show when they need division by producing fewer flowers, smaller leaves, and spindly stems.
The timing will depend on how nutritious your soil is and how well the dahlias are responding to feeding.
If you don't want to wait until your dahlias complain, dividing every two years is safest.
"Can I grow garlic in planters? How many cloves do I plant in a 12" to 15" container?"
Linda, via email
For a 12-inch container, about eight plants is the absolute maximum. In a 15-inch container, a maximum of a dozen or so is best. In the garden, the best row spacing is six to eight inches (15 to 20cm). The closer they are together, the smaller the cloves you'll get.