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Helen Yemm: easy, colourful perennials, a spotty olive and diseased rhododendron

Pink diascia in Helen's garden - Helen Yemm
Pink diascia in Helen's garden - Helen Yemm

BEST VALUE PERENNIALS FOR COLOUR?

To try to make life easier, I want to grow fewer annuals and invest in more perennials. Which in your view will give me the best run for my money? Sheila Wingrove, via email

This is difficult since you give no information about your location and what kind of garden you have, but I am guessing that if you are used to a spread of blazing annuals you will appreciate those perennials that are not just eye-catching but will go on and on until the frosts.

A lot depends on growing conditions, but these are a few that work for me in my garden, most in slightly less than full sun, in soil just on the acid side of neutral that tends to be heavy but has been lightened/improved over the years:

  • Diascia personata – a tall, bushy perennial that produces a stunning cloud of coral-pink flowers from June to November. Short-lived, but I propagate it from stems stuffed  in a jar of water.

  • Geranium Rozanne and ‘Anne Folkard’ (sky-blue and vivid magenta respectively) – they go on, and on, scrambling through everything else.

  • Astrantia – mine is a hotchpotch of self-sowns, but have one astounding plant that produces a succession of tall-stemmed, large, deep red flowers non-stop from June to mid-October.

  • Persicaria amplexicaulis – I favour a particularly good red one (‘Taurus’), but all form impressive clumps that flower for weeks, especially given a bit of shade and soil that’s not too dry.

 

A tip for winter
A tip for winter

SPOTTY POTTED OLIVE

Can you tell me what is wrong with my olive tree? It has been growing in a pot for several years, the leaves are becoming sparse and several have got white “things”’on them.  Lizzie Warren, via email

Your series of pictures told a big story. They showed a lanky tree standing about 8ft (2.5m) high on a rather exposed balcony in a pot that would appear to be nearly 2ft (70cm) tall, with a neck that is narrower than its waist. 

The lower branches of the olive have very few leaves on them, and the upper spotty-leafy ones have “gone feral” i.e. lost any formal shape that they presumably had when you first acquired it.

The spots on the leaves appear to be fungal, since each spot has a characteristic black outer circle. The leaf drop is the result of disease, therefore, but is also down, I strongly suspect, to winter exposure – possibly also to uneven watering in a restrictive pot containing tired compost, all causing general root malaise. 

Olive tree in a green pot - Credit: WLADIMIR BULGAR/SCIENCE PHOTO LIBRARY
Small olive tree Credit: WLADIMIR BULGAR/SCIENCE PHOTO LIBRARY

Giving this beleaguered tree a more sheltered site for the winter is important. And a good prune next spring (involving the removal of dead branches and general re-shaping) would, by reducing its size, make it easier to spray with systemic fungicide that may just possibly control the leafspot.  

Also helpful would be the regular clearing up of all future fallen diseased leaves. Recovery might be boosted by the topping up or replacement of some of the old compost and the application of a fistful of blood, fish and bone. The alternative, getting the tree out of this pot and transferring it to a new one, would be well-nigh impossible. 

If at the end of next summer the tree shows no sign of improvement and still looks, how shall I put it, just as hideous, I would, well… must I spell it out?

HELP WITH DIAGNOSIS 

Nerissa Evans has (or more correctly, had) a rhododendron called ‘Christmas Cheer’ that grew and flowered contentedly in a tub for four or five years. Then suddenly last summer its leaves started to wilt, turn yellow then go brown and tubular. It is now as dead as a dodo, still in its tub.

She suspects, from everything she has read, that the cause was Phytophthora ramorum, a certifiable fungal disease. How can she dispose of the plant safely? As mentioned elsewhere on this page, shrubs and trees grown in containers get into trouble for other reasons (including simply a lack of water), and Nerissa cannot be sure that this nasty disease was the cause of death in this case.

This form of phytophthora is most often associated either with newly bought and already infected plants or with conditions of severe and prolonged waterlogging. The best way to get a definitive answer is to join the RHS, avail herself of their free-to-members garden advisory service, and specifically to send samples of the roots/foliage of the plant for diagnosis in their labs. 

Information as to how to go about this (how and from which parts of the plant to take samples etc), and indeed on the possible causes of the death of woody plants, can be found by searching rhs.org.uk.