Amateur Gardening – 13 July 2019

36 AMATEUR GARDENING 13 JULY 2019

Ask John Negus

John has been answering

reader queries for 49 years

Q

Why does my healthy hydrangea

produce pink and blue flowers?

Chris Anderson, Chippenham, Wilts

A

THE colour is influenced by the

pH of the soil they are growing

in, which affects aluminium availability.

Those with blue or pink flowers tend to

be blue in acid soil with high available

aluminium levels, mauve in acid/neutral

soil and pink in alkaline conditions.

I wonder whether the natural pH of

your soil is quite varied, even in the

localised region around the roots of your

hydrangea. I would hazard a guess it is

acidic to neutral with pockets of greater

acidity – hence the mix of flower colours.

If it is supposed to be a blue

hydrangea, then you can keep the

Why is my hydrangea multi-coloured?

Not such a ‘dwarf’

Q

I bought this ‘Jazz’ buddleja as a

‘dwarf’ but it grows to more than

6ft (1.8m). Was it mis-sold to me?

Ian Mitchell, via email

A

I think the buddleja, which could

be ‘Royal Red’, is a normal-height

variety that will grow to 6ft (1.8m) or

more. Dwarf ‘Buzz’ and other naturally

small varieties seldom grow larger than

4ft (1.2m) high and across.

Like their taller cousins, they are

pruned in March, shortening all stems

to within 12in (30cm) from the ground.

The colour of hydrangea fl owers is infl uenced

by the pH of the soil in which they grow

Buddlejas are best pruned in March to

keep them neat and fl owering well

flowers blue by acidifying the soil. 

Mulch with organic matter, as all bulky

organic matter (apart from mushroom

compost) tends to have an acidic pH.

Mulching your hydrangea in late winter

may lower the pH around the roots so

the flowers are more reliably blue.

However, this will be short-lived and

annual mulching will be needed.

Sulphur is the common acidifying

material, although it can take a long

time to produce a colour change.

Aluminium sulphate is used in

hydrangea ‘blueing agents’ where the

soil conditions are not sufficiently acid

to give blue flowers naturally. Blueing

compounds are designed for this

purpose and should be used according to

the manufacturer’s instructions.

All photography TI Media, unless otherwise credited

Q

Please can you advise on pruning

my misshaped rhododendron?

Margery Milligan, Clackmannan,

Central Lowlands

A

The best time to prune a

rhododendron is immediately

after it has finished flowering.

Initially, because yours is slightly

misshapen, cut back ill-placed stems

to create a pleasing rounded head and

remove dead or dying shoots in the

centre of the bush.

Then carefully prune out faded

blooms, taking care not to damage

new shoots growing close to them.

If the soil is dry, water

liberally before

feeding with an

ericaceous

granular

fertiliser.

Hoe or

water it in,

then mulch

with compost

or manure.

Rhododendron

pruning query

Desperate about my delphiniums

Q

I am finding it almost impossible

to grow delphiniums these days.

They used to flourish in my garden, but

now they either snap in the wind or the

slugs get them. What can I do?

Emma Beale, Stockport, Greater

Manchester

A

You are obviously good at

growing delphiniums and I am

sorry they were smashed by the wind.

Sadly, hollow stems are vulnerable

to wind damage, and canes – one

supporting each stem – should be tall

enough to reach the top of the bloom.

If they are not, the unsupported part of

the stem at the top may snap off.

You could try protecting them from

slugs using beer traps or organic slug

pellets. Another method is to place the

peel of half an orange upside down on

the soil. Slugs and snails crawl in there

overnight and they are easy to pick up

and dispose of.

Delphiniums need supporting and

protecting from pests

If the soil is dry, water

liberally before

feeding with an

with compost

Prune rhododendrons

after fl owering