Thorny Prob­lems

Visit the gar­dens of the Ital­ian Lakes with He­len next May

The Daily Telegraph - Gardening - - Front Page - He­len Yemm

Wrong chem­i­cal

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.

Hol­ly­hock shocker

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.

Peren­nial plague

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.

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