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Why some plants prefer tough treatment and when to snip seed heads, by garden expert Helen Yemm

iris pallida 'Variegata'
iris pallida 'Variegata' likes stony soil Credit: mauritius images GmbH / Alamy

Every week, Telegraph gardening expert Helen Yemm gives tips and advice on all your gardening problems whether at home or on the allotment. If you have a question, see below for how to contact her.

Why Mediterranean plants like mean gardeners

We have all done it at one time or another, I am sure – hankered after a certain plant, thought we had found a perfect spot for it, hunted it down (sometimes involving a lengthy search) and planted it following all the so-called rules about improving soil, feeding, watering and general nurturing – then the blessed thing underperforms or worse, turns up its toes.

Two such Thorny Problems readers whose perplexed emails landed with me this week were Peter Hodges and Camilla Woodhouse, 
who lovingly planted rhizomatous Iris pallida and summer-flowering shrub Cistus x purpureus respectively.

Peter’s iris, planted in a suitable hot spot in well-drained soil, has produced a healthy flush of handsomely variegated leaves but utterly refuses to flower. The border where it is planted is well mulched and fed every year, and everything else there performs well. Camilla’s cistus, planted in a not-quite-fully-sunny space in good soil much enriched with “soil improver”, has developed worryingly yellow leaves and appears to be “on its way out”.

I think the answer here lies in the origins of these two plants – both of them from the stony-soiled hot hills of southern Europe and needing, as well as sun, absolutely perfectly draining soil conditions. Peter’s iris rhizomes need a thorough direct all-year sunbathe, impossible if covered in mulch. Similarly Camilla’s cistus would fare better in soil that had not been pre-plumped with soil conditioner, which may have made it too moisture-retentive and rich.

Moral of the tale: when hankering, look up the origins of the object of your desire (so easy using the internet). That Mediterranean plants need a lot of sun is well understood. But soil conditions really matter, too. In this case, perfect drainage (if in doubt, adding coarse sand when planting helps) and the sparse use of mulches and fertilisers are just as important.

Problems with a young rowan 

How can we encourage our eight-year-old rowan ‘Olympic Flame’ to spread out and provide the lawn shade we had hoped for? Would pruning help to open it up a bit?

Ann Charles – via email

The slim shape of your tree – referred to generally as “columnar” – is fairly typical of this beautiful rowan, and it may never become wide and spreading. However, on the evidence of your picture it would seem that, rather than thinning it out, perhaps it is time to remove some lower branches as the tree is beginning to make a clearly defined “head”.

Prune while the tree is leafless. Using a slim pruning saw, cut each branch cleanly, right back to the trunk, making a little nick out of the bark on the underside of each first, to avoid ripping the bark on the trunk itself. It is no longer seen as necessary to use a pruning compound to seal the wounds.

By removing lower branches, you will encourage more growth in the “head” which, as it gets heavier, may slacken outwards a bit to form a slightly wider canopy, creating more shade. Finally, it would be beneficial to feed this tree (with a general fertiliser in spring) after removing a circle of the competing grass that is growing right up to its trunk.

Agapanthus seed heads

I have grown hardy agapanthus in a pot for the first time. Should I have removed the old flower heads once they were over? As I didn’t do it earlier, will it not flower as well next year?

Beryl Woodgate – via email

Agapanthus in full flower Credit: Debu55y / Alamy 

Whether or not you remove agapanthus seed heads makes not a jot of difference to their future flowering. Logic tells me that by the time the flower heads have started to look really shabby (when the Tidy Brigade get all itchy-fingered and start their decapitation ritual), the seed heads are already fully formed, the plant having already put masses of energy into reproducing itself, so there is no point in removing them.

Personally, I love the ripening seed heads and only remove those old tawny stems that get bent and battered by the wind. Hardy (deciduous) agapanthus are tougher than they look and can winter outdoors in most areas whether potted or in the ground. With any luck, if you leave the heads to drop their seed around you will find (as I do) little easily recognisable seedlings coming up here and there.

Those that happen to be in “right” places and survive undisturbed will leaf up the following year and start to flower in two or three years’ time. Some can be lifted and potted up, too, though this may set back their flower production by a year.