Rhododendron johnstoneanum 'Double Diamond'

Rhododendron johnstoneanum 'Double Diamond'

Item: TJOHNDD

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Flowering Month:
May
Flower Colour:
Cream
Hardiness:
To -10 °C
Height After 10 Years:
75-100cm
Scent:
Scented
Interesting Foliage:
No

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(H4) Scented cream-coloured double flowers with yellow blotch in May. Bristly foliage. Height 80-100cm in 10 years. johnstoneanum Double Diamond is a plant for very sheltered gardens, or cool conservatories. Needs well drained compost. Hard prune after flowering.

Good to know

Tender (maddenia) Rhododendrons are suitable for the cold greenhouse and mild areas such as Cornwall where they can be grown outdoors. Elsewhere in sheltered gardens they are worth trying against a South facing wall if temperatures are unlikely to go below -5°C. They give a magnificent display in spring, and the pinks and whites often have gorgeous scent to fill a conservatory. Many of these rhododendrons are epiphytic, meaning that they naturally grow in the boughs of forest trees and rock crevasses. Hence they often grow and flower better when their roots are constrained in a relatively small pot, and they need a very free draining (orchid type) compost, with just a teaspoon of slow release fertilizer.

When grown in a conservatory, these plants grow vigorously, so prune hard straight after flowering to encourage bushiness, leaving only about 10cm of the previous year's growth. They much prefer to be grown outside during summer months where there is fresh air and rainfall, so try to move out in May and back indoors in October. Heating is not needed in the UK, and plants can be easily scorched if placed too close to radiators or heaters.


Please note:Watch out for sooty mould on plants grown under protection (see advice centre for further information).


For further advice, For further advice, see here

The Basics

Ideal soil

Acidic soil, good organic content, pH 4.5-6.0. Inkarho range of rhododendrons will tolerate soils up to pH7.5

Sun or Shade

Light dappled shade is best for most varieties.

Shelter

Refer to hardiness rating. Give young plants protection.

Site Selection

Avoid close to trees, roots, invasive weeds, walls, hot patios, dry banks and waterlogged soils. Do not use weed matting or stone mulch

Plant spacing

Use the height shown in 10 years as a guide to the distance between each plant. Allow room for plant to fill out. If planting closer for instant impact, be prepared to move plants after a few years.

Compost

  • 3 litre pot, dig in 10-20 litres of ericaceous compost.
  • 7.5 litre pot, dig in 20-30 litres of ericaceous compost.
  • 70-80cm specimen, dig in 60 litres of ericaceous compost.
  • 100-120cm specimen, dig in 120 litres of ericaceous compost.

Planting depth

Plant high in the ground, with the top of the rootball visible.

Feeding

Slow-release ericaceous feed recommended in March and straight after flowering.

Mulch

Recommended every few years.

Water

The key ingredient! Keep moist all season, especially the critical time at end of June for flower bud initiation. Tap water is better than no water. Heavy dose at least once per week in dry weather.

Drainage

Ensure good drainage in winter, especially with yellow varieties. Avoid waterlogged sites.

Pruning

Not normally required. Tidy wayward shoots after flowering. Evergreen azaleas and Bloombux can be clipped into a low hedge.

Deadheading

Remove old flower-heads, particularly on young or weak plants.


For further advice see here

Size Guide

Size guide

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