GARDEN GURU

I have a blue hydrangea. Q What soil conditions does it need to remain this colour? Pam Hammond

One of the peculiarities of A some hydrangeas is that the flowers change colour depending on the pH of the soil. Blue varieties will go pink if planted in alkaline soil, and mauve in more neutral conditions. Those with pink flowers will turn blue in acidic soil. Your plant needs acidic soil to stay blue. If your soil is more alkaline, work sulphur soil pellets into the surface (the effect will last two years). Or grow in a pot filled with ericaceous compost.

Our ajugas attract sawflies. Q We’ve tried spraying with pesticide and household fly spray, but they keep returning. Linda Ashman

Sawflies are a widespread A pest whose caterpillars like to munch on ornamental plants. Rather than tackling the orange-bodied, black-headed flies, focus on their larvae. Flies produce several generations over the course of the growing season, so check plants regularly and remove. You could also spray vulnerable plants with Bug Clear Ultra. Rather than killing on contact, the active ingredient remains in the plant.

Plants have appeared in my Q garden that I can’t identify. They have pink flowers that look like miniature foxgloves. Roger Williams

Your photo is of Physostegia virginiana, a herbaceous perennial from North America. Its pretty blooms move with the sun during the day – hence its common name, obedient plant.