<em>Paulownia</em>, imperial tree: planting, cultivation and care

Description and botany

Previously included in the large family Scrophulariaceae, Paulownias now make up their own botanical family: Paulowniaceae. The genus includes six species: Paulownia catalpifolia, Paulownia kawakamii, Paulownia elongata, Paulownia × taiwaniana, Paulownia fortunei and the best-known Paulownia tomentosa (formerly called Paulownia imperialis), nicknamed “Imperial tree”.

All are very similar, but only Paulownia tomentosa and Paulownia fortunei are commonly cultivated; the latter differs from its cousin by flowering about three weeks earlier.

All these species originate from Asia, more precisely from mountains of China and Korea. They are pioneer trees that establish first on derelict or disturbed ground to “prepare the ground” for other plants. This pioneering habit has led Paulownia tomentosa to become an invasive species in some parts of North America. Later, in its natural environment, thanks to its soil-preparing activity, other tree species establish quickly and create too much shade, which can prove fatal.

It was introduced long ago to Japan and is cultivated worldwide, notably for its tolerance of pruning and pollution.

Paulownia tomentosa (syn. imperialis) – botanical illustration

Capable of reaching twenty metres in its native range, Paulownia rarely exceeds twelve metres in our gardens. Which is already not bad… It is a majestic tree with a straight trunk and a spreading dome-shaped crown, ideal for providing shade in summer.

Bark is grey, smooth and slightly striate. Small orange vesicles can be seen on young trees.

Shoots are pubescent, that is covered with hairs, and bear on long petioles large deciduous opposite leaves about 25 cm, heart-shaped with silky underside. This explains the species name of P. tomentosa: “tomentosa” meaning “silky”. Older shoots become hollow. So do not be surprised when pruning them!

Foliage turns yellow in autumn before falling quickly. From autumn onwards, flower buds of a reddish colour can be seen on the shoots.

Flowering occurs in spring, generally before leaves appear. The hermaphrodite flowers, arranged in erect paniculate panicles at shoot tips, are violet and scented. The resulting fruits are oval, pointed capsules that remain decorative throughout winter. They open at ripeness to release hundreds of winged, cotton-like seeds.

Paulownia reaches sexual maturity at about twenty-five years, but the tree only lives about a hundred years. Paulownia is a fast-growing species; it can grow 4 to 7 m in just three years.

The tree has a fairly dense root system that helps reduce soil erosion and stabilise banks.

Paulownia tomentosa: flower buds, flowers, fruits and huge leaf

Main species and varieties

Paulownia tomentosa - Foxglove Tree

Paulownia tomentosa is a species that becomes a large tree, easily reaching 12 m in height, but by pruning it into coppice shoots it can be kept at bush size. Leaves will then reach impressive proportions but, sadly, you will lose flowering...

• Flowering time June, July
• Height at maturity 12 m

See the product sheet

Paulownia fortunei Fast Blue Minfast - Foxglove Tree

Paulownia fortunei has a very slender habit, almost conical. It will therefore eventually take up less space than its cousin P. tomentosa while retaining generous foliage and magnificent spring flowering.

• Flowering time June, July
• Height at maturity 8 m

See the product sheet

Planting Paulownia

Where to plant?

Paulownia thrives in rich, humus-bearing, fresh, well-drained and deep soil. An exposure in full sun suits it perfectly, but it can also succeed in partial shade. To flower well, however, it needs long, warm summers, but not too dry.

As this tree already produces its flower buds in autumn and flowers early in season, it is wise to protect it from cold winds and severe frosts that can destroy flowering.

It is indifferent to soil pH.

When to plant?

Paulownia is preferably planted in autumn or early spring. Planting outside these periods is possible, but watering must be monitored. Do not plant during heatwaves or frosts!

How to plant?

• Sink the pot of your new Paulownia in a bucket of water for a few minutes to re-wet the rootball
• Dig a hole twice as deep and twice as wide as the rootball
• Add one or two handfuls of well-rotted compost to the bottom of the hole
• If your soil is heavy, you can add some gravel to improve drainage
• Begin to “break up” the rootball slightly to free the roots. Do this gently by hand or with a small fork to avoid damaging roots
• Place remaining rootball in bottom of hole, spreading out roots
• Refill hole with excavated soil that has been loosened
• Firm soil around plant gently with your hands (not with your feet!)
• Give a 10-litre watering to reduce risk of air pockets between roots and soil
• Apply a mulch to protect young tree from drought or plant directly at base a few small groundcover plants (creeping bugle, sweet woodruff, Glechoma, Geranium macrorrhizum, …)

→ Find out more in our tutorial How to plant a Paulownia?

Maintenance, pruning and care

Protection against cold

Young branches and flower buds can be damaged by late frosts or cold, dry wind. Best is to find a sheltered spot in garden or place a winter fleece during tree’s early years.

Pruning

Just before growth resumes, between February and late March, prune diseased or dead wood and some branches that spoil tree’s structure and habit. Bear in mind that pruning large-diameter branches weakens trees. So be light-handed with saw and pruning shear!

At same time, if you wish to keep your Paulownia to modest bush size, you can prune all branches leaving only 2 or 3 buds at their base. Tree will then have a bushy habit with even larger leaves, but, unfortunately, you will not enjoy flowering.

→ En savoir plus sur la Paulownia pruning dans notre tutoriel !

Possible diseases

Paulownia is highly resistant to pollution and parasitic organisms, but in very humid conditions, especially in summer, it may show some signs of disease: cankers, root rot, foliar spots, but above all powdery mildew are to be watched for, though still relatively rare. In very poorly drained soil, attacks by honey-coloured Armillaria, a wood-eating fungus that kills trees may also appear.

→ En savoir plus sur les Paulownia diseases and parasitic organisms dans notre fiche conseil !

Propagation

By sowing

Paulownia, with its pioneering character, reproduces easily by sowing. Seeds need a period of cold to germinate. Therefore, either sow seeds outdoors in autumn, or place them in a seed tray in the refrigerator (5°C) for a few weeks before sowing in spring. You can sow them one per pot or in rows direct into the ground, then transplant the young seedlings into pots later (keep the sturdiest specimens!) and only plant them permanently in the ground the following autumn.

Can’t find Paulownia seeds? We thought of you with Paulownia tomentosa seeds available to order online.

Separating suckers

Sometimes the tree produces suckers. Simply dig carefully at the base of the tree to free the sucker. Cut the root that still attaches it to the mother plant, then pot it up for planting out in the ground in autumn.

→ To learn all about suckering in your favourite bushes, read My bush is suckering: why? What should I do?

By cuttings

Propagation by cuttings of Paulownia is done in autumn by root cuttings. Ideally, when digging at the base of the tree, find a root with rootlets. Place this piece of root on a mixture of sand and potting compost and keep warm (20°C) and in the dark. At the very start of spring, first leaves will appear; you can then pot up and wait until the following autumn to plant out in the ground.

You can also take a cutting in summer from a young shoot about ten centimetres long into a light substrate (potting compost + sand) and keep it covered. Place the shoot, having first shortened two-thirds of its leaves, into the compost. Cover with a plastic bag or the bottom of a plastic bottle to retain heat and humidity. New leaves should appear quickly, a sign of successful rooting. Plant out only in the following spring or autumn.

Some people successfully root any cutting in water! So why not try: place a young shoot in a vase filled with water with a piece of charcoal (to prevent rotting; otherwise change the water every two days). As soon as the first small roots appear, you can pot the seedling into a pot with a light substrate (potting compost + sand). Do not let the roots grow too long, they will be too fragile to survive transplanting later. You can plant your tree in the ground in the following spring or autumn.

→ Learn more in Alexandra’s tutorial : How to propagate Paulownia?

Companion planting with Paulownia in the garden

Grove of bushes with spring flowering

Paulownia tomentosa produces purple flowers in April. Ideally, pair it with a shorter bush in a tone that answers it, such as the uncommon Syringa x persica var. laciniata with lilac flowers and attractive laciniate foliage. To contrast with the purple, try yellow and orange flowers, for example the charming but, unfortunately, rarely planted Weigela middendorffiana, a small weigela with pale yellow to amber blooms. Pair it with a Forsythia koreana ‘Kumsun’, a very hardy compact forsythia that flowers bright yellow in spring, its leaves taking on a speckled appearance thanks to fine cream-white veins. Complete the group with a Cornus mas ‘Jolico’ which, as well as giving a lovely yellow flowering in spring, will produce large edible red cornels in autumn — a colour that will perfectly echo Paulownia’s floral buds.

An example of a grove association: Paulownia tomentosa, Forsythia koreana ‘Kumsun’, Syringa persica ‘Laciniata’, Cornus mas ‘Jolico’ and Weigela middendorffiana

Pair your Paulownia with a beautiful climbing plant

If your Paulownia fortunei ‘Fast Blue’ is already tall and sturdy, you could echo the colour with a wisteria that flowers at the same time and in the same tone, such as Wisteria venusta ‘Okoyama’, a light, delicate wisteria that will not overwhelm your Paulownia.

You can also play with strong contrast by training a liana with yellow or even orange flowers. In that case, and if your Paulownia is in a spot sheltered from frost, try a striking trumpet vine with orange and yellow blooms such as Campsis capreolata ‘Tangerine Beauty’.

An example of an association with a climbing plant such as trumpet vine ‘Tangerine Beauty’

A bed of giant foliage in rich, moist soil

If you like exuberant, lush foliage, you can cut back to coppice shoots your Paulownia tomentosa every year. It will then produce enormous leaves! Pair it with Catalpa bignonioides ‘Aurea’, also cut back to coppice shoots, with magnificent golden-yellow foliage whose leaf shape will echo Paulownia’s. Add a handsome, imposing Astilboides tabularis with bold foliage and airy flowering, as well as Fatsia japonica, a timeless classic for lush gardens. If you have a little space at the edge, plant a few Hosta ‘Empress Wu’, giant hostas with slightly bluish foliage that will contrast beautifully with the catalpa.

→ Discover 5 other great ideas for pairing with Paulownia in our advice sheet!

An example of lush foliage association: Paulownia tomentosa, Fatsia japonica, Catalpa bignonioides ‘Aurea’, Astilboides tabularis and Hosta ‘Empress Wu’

Did you know?

• The name was given in honour of Anna Pavlovna, daughter of Tsar Paul I.
• In former times in China, a Paulownia was planted at the birth of a daughter. The tree grew and was eventually felled at her wedding so the wood could serve as dowry.
• From the 11th century, Paulownia, in association with the phoenix, has been used as a symbol on garments of the Chinese Imperial household.
• Viscount de Cussy brought the first Paulownia seeds to the Jardin des Plantes in Paris in 1834. The tree flowered for the first time in 1842 and died in 1956… to be quickly replaced by a young specimen of the same species.
• Even today in Japan, Paulownia leaves form an emblem that honours deserving figures, as well as the Japanese Prime Minister and his government.
• Paulownia is still used for its wood in China for cabinetmaking and luthiery.
• It is widely used in agroforestry because the leaves are rich in nitrogen and can therefore be used as forage or as mulch. However, they compost rather poorly.