Visit the gardens of the Italian Lakes with Helen next May
I was given a blue hydrangea in June but it has since turned into a scruffy pink. I was told that Potassium permanganate would restore the colour, but not how to apply it or where to buy it. FRANCES JARVIS, VIA EMAIL You have the right idea – that the scruffy pink of your formerly blue hydrangea has something to do with the chemical composition of the soil – but have identified the wrong chemical to change it. For blue hydrangeas to stay that way, grow them in tubs of ericaceous compost (commercially available compost suitable for limehating plants) and only water them with rain water. Or, having improved your garden soil with naturally pH-neutral leaf mould, you can “doctor” it annually with a special tonic that will stop the colour change.
Hydrangea Colourant (from Vitax) contains iron and aluminium sulphate. The iron affects the pH of the soil around the roots of the plant, making it more acid. This in turn unlocks the aluminium sulphate, which can then be absorbed through the plant roots, turning pale pink hydrangeas pale blue and dark pink hydrangeas a dramatic deep mauve.
Emailer Jenny MacArthur complains that she can only find double-flowered hollyhocks for sale. While her husband likes them, she thinks they look like gaudy crêpe paper decorations and have none of the charm of the traditional single ones.
I do so agree with her. Luckily, my assorted singles seed themselves around (and I give seed away to friends). But from various chats with growers and garden centre people, it seems that the lack of young plants of singles is all about rust, to which they are considered to be even more susceptible than doubles. In this country our common hollyhocks ( Alcea
rosea) are bedevilled by a rust, Puccinia malvacearum, specific to many plants in the mallow family and spread by airborne spores produced by orange pustules on the back of leaves. The disease spreads rapidly from ground level, splashed upwards by rain, and so is particularly bad in wet summers.
I find that a pre-emptive strike with a systemic fungicide in early spring, followed by vigilance and prompt lower leaf removal, is worth the bother.
There are, it is worth reminding readers, singleflowered varieties of Alcea ficifolia that are more resistant to rust. You can of course grow your own from seed (Thompson & Morgan sells seeds of A. ficifolia ‘Antwerp Mixed’). The closely related creamflowered Russian hollyhock Alcea rugosa is also rust resistant.
Having moved a large clump of Acanthus spinosus because it was in the wrong place, I now have a plague of small plants appearing where it used to be. How can I get rid of them? BRIGITTE MELROSE, VIA EMAIL There are some deep and brittle-rooted herbaceous plants that just won’t go quietly, and acanthus is one such. Other notorious offenders are oriental poppies, Japanese anemones, alkanet (the big hairy forget-me-not/ borage relation), and of course the snap-off corms of ordinary montbretia. You think you have got rid of them with diligent digging, but you haven’t. You will get in a bit of a pickle if you impatiently replant. You have to either go on digging every time the little beastie pops up next year, or curb your impatience to be rid of it and allow the leaves to gain sufficient size (around the middle of next summer) to be nobbled with a glyphosate weedkiller (e.g. Roundup). Even then it may take two attempts.