Rhododendron arizelum

Attractive large campanulate flowers in shades of yellow, pink or white, with or without a dark crimson blotch. April or May flowering. This plant makes a wonderful small foliage tree with dark green obovate to oblanceolate leaves up to 22cm long. The upper surface of the leaves are shiny with semi persistent indumentum, with a thick fawn or cinnamon velvety indumentum on the underside. Height 150-175cm in 10 years, eventually growing to around 8 metres.

Big leaf Rhododendron species have great architectural foliage which looks stunning in a woodland glade or city courtyard garden. However, the lovely large leaves do need protection from winds, especially cold winds, to prevent them being marked or damaged. Strong winds can even snap off these big leaves at the petiole (leaf stalk) or midrib and ruin the plant. Carefully choose a sheltered, moist but well drained position, and you'll grow a fantastic foliage plant with the bonus of wonderful flowers after 6 to 10 years.

Epithet: Notable.

• Recommended for outstanding foliage.
• Plantsman range
• Ideal position: Sheltered woodland or shaded courtyards, avoiding cold winds.
• Habit: tree-like.
• Group: Species Rhododendron.
• Subsection: Falconera.
• Introduction date: pre 1950 (1917).
• Species distribution: Arunachal Pradesh, Yunnan, NE Upper Burma, Assam, SE Tibet, in conifer or rhododendron forest.
• Approximate altitude: 2,500 to 4,400m.
• Ideal soil: pH 4.5 to 6.5.
• RHS Hardiness Rating: H4.
• How we usually propagate this plant: Seed and cutting.

Good to know

Wonderful architectural plants with huge leaves up to 60cm long! These are choice woodland Rhododendrons needing the very best sheltered positions to allow them to grow to their full stature of up to 5 metres. As a bonus, flowering will typically start after 6 years, but can take up to 10 years. Big leaved Rhododendrons are among the more tender of our range, but are well worth trying if you have the correct conditions. They are best suited to sheltered woodland in the milder regions of the UK, though they can also be used as feature plants in sheltered courtyard gardens. Winter protection is recommended for young plants.

Rhododendrons like moist acidic soil, with good drainage, and plenty of organic matter such as leaf-mould and added ericaceous compost. Big-leaved species should be given the best planting positions, which are neither too wet nor too dry. For a guide to plant spacing, use the height we give in 10 years as a guide to the distance between each plant, or approximately 1 plant per 3m². Plant no deeper than the top of the rootball, and dig in plenty of good ericaceous compost around the sides. A teaspoon of slow release feed is sufficient for a 3 litre plant, rising to a small handful for a mature plant. Plants can be grown as single stemmed trees, or the leader can be pruned as a young plant to encourage a more bushy habit. You may need to give protection from deer rubbing their antlers on the stems

Please note: Big leaved Rhododendrons need to be sheltered from wind to prevent the large leaves being broken off at the petioles (stalks) during gales. Moisture control is critical to prevent moisture stress and misshapen leaves, so we do not recommend them being grown in pots.


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Size Guide

Propagation

We use different methods of propagation such as seed, cuttings, grafts and micro-propagation.

Seed

Many of our Species Rhododendrons are sown from wild collected seed. Where this is the case you may see a seed number in the plant name or description. This number records who collected the seed, and can be cross-referenced to the collectors' notes. New species are introduced by this method. We are always excited when seed collectors send us their precious seeds that they have collected from the remotest parts of the world. Alongside the seed number that they assign to their seeds, nowadays they list the GPS location of where the seed was collected from, (and yes they do write descriptions like “next to the second rock on the right”) and the date that they collected it. The number is preceded by their initials, thus identifying themselves.

Of course seeds can be collected from Rhododendrons that flower in your garden, and while these will not come true to their parent, it can be an exciting exercise. Rhododendrons are fairly promiscuous and they can be crossed pollinated by insects flying from one plant to another. To grow species or to propagate new hybrids, one has to carry out controlled pollination to avoid insect pollination. This process is best left to the experts! Seedling variation means that there can be differences from one batch of plants to another.

Seeds are collected in the autumn, dried breifly to open the seed pod, seived clean and then stored in brown envelopes in the fridge until we are ready to sow them. We sow onto moist spongey compost at 15C in December, and cover with polythene to help humidity. Germination usually takes 2 to 3 weeks in our Propagation glasshouse and then we give extra lighting to encourage growth before the light levels improve in spring. The whole process takes 3 years until your plant is ready in a 3 litre pot.

Cuttings

All our Evergreen Azaleas and Dwarf Rhododendrons are propagated by cuttings. Some of the tall hybrid Rhododendrons and the Deciduous Azaleas are much harder to root by this method. Timing of taking cuttings is crucial, and ranges from June to January to get the best rooting results. Most of our cuttings come from our own garden and trials area but we do occasionally get material from other large Rhododendron gardens, particularly if we want to introduce a new or different variety to our range. We are busy taking cuttings when the new plant growth has hardened but is still flexible. We have modern Propagation facilities with the latest automatic mist, humidity, heating and ventilation controls. You can try cuttings in a pot on a North-facing windowsill with a polybag over the top if you would like to have a go!

Grafts

As mentioned earlier some varieties of Rhododendrons are particularly difficult to propagate from cuttings and we get better results by grafting. The process is similar in that we collect cuttings but instead of planting them in compost, we graft them onto the rootstock of another Rhododendron plant during the autumn. In simple terms, we root Cunningham's White cuttings, and then make some accurate cuts to the base of that and match up the variety we want to graft on to it. By tying the two together with rubber bands, the 2 parts will fuse together forming a choice plant growing on the roots of Cunningham's White. We use Rhododendron Cunningham’s White as it is a reliable, disease resistant plant which rarely produces suckers from below ground. In the past grafted plants did have a bad reputation for throwing out suckers because R. ponticum was used as a rootstock, and in time some branches escaped and the mauve flowers became the dominant part of the plant.

Micropropagation

In other words, test tube Rhododendrons! Since about 1985, this method of propagation has proven to be successful, particularly for bulking up new varieties. Tiny pieces of cutting are rooted on agar gel in a laboratory, before being weaned into small cells of compost. Again, this is a useful method to propagate the varieties that are hard to root by conventional methods.

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 flowering varieties. Avoid waterlogged sites.

Pruning

Rhododendrons and Camellias: Not normally required. Tidy wayward shoots after flowering.

Evergreen azaleas and Bloombux can be clipped into a low hedge.

Magnolias and Acers: Formative pruning when young to shape into a tree or bush.

Deadheading

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

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