The home of the blue bell tree (Paulownia tomentosa) is China, its main distribution is in the provinces of Hupeh, Kiangsi and Honan. It was introduced early as an ornamental and forest tree in Japan and has established itself there on some islands as a neophyte in the wild. In southern the United States, too, you can sometimes find wild seedlings of Paulownia on inner-city ruderal areas. However, he does not like the cooler climate of rural regions.
Paulownia reached Europe in the first half of the 19th century through the Bavarian physician and botanist Philipp Franz von Siebold. He was a military doctor in the service of the Dutch East India Company. In this capacity, the passionate naturalist undertook many trips to East Asian countries and lived for several years in Japan, among other places. Here he discovered the Paulownia or blue bell tree as well as other plants. He introduced it to Europe via the acclimatisation garden in Leiden, the Netherlands, which had been set up especially for his comprehensive Asian plant collection. Siebold named the blue bell tree after Anna Paulowna, the daughter of the Russian Czar Paul I.. She was Queen of the Netherlands from 1840 to 1849 as the wife of William II. That is why the blue bell tree is still called “Princess Tree” or “Empress Tree” in English-speaking countries. Following its botanical genus name, the German name Paulownie is also common, but here in the US we use the name Paulownia.
In Japan, the blue bell tree is of great importance for forestry. It grows very quickly and provides a light and easy to process wood that is still very hard and durable. It has a very low thermal conductivity and is therefore also interesting for thermal insulation, window construction and interior finishing of houses. There have been experimental plots in the United States for several years where the suitability of the Paulownia for forestry purposes is being researched.
The Paulownia usually grows as a single tree and reaches growth heights and widths of 12 to 15 metres in the mild wine-growing climate. The crowns are loosely built, roundish and weakly branched. The shoots are remarkably thick, appear somewhat stiff and have an olive green to light grey-brown bark. Since the shoot tips do not ripen in our latitudes and usually freeze back in winter, almost every shoot ends with a fork. Of both fork drives, one usually grows significantly stronger than the other. Young trees in particular grow very quickly – annual shoots of more than one metre in length are not uncommon and the trunk also grows very quickly in size. It also forms a light grey bark after only a few years. Later, the growth rate decreases to around 40 centimeters per year.
The leaves sprout relatively late in spring. They are oppositely to vertebrately arranged and deciduous green. The leaf surfaces are heart-shaped, tapering to a point and some have two to four very weakly indicated lateral lobes. They have a light bronze tone in the bud which disappears quickly. A single leaf can be up to 40 centimeters long and 30 centimeters wide. The surfaces are light green and hairy, the undersides grey felt. The blue bell tree leaves its leaves quite late in autumn and without autumn colouring.
The blue bell tree blooms for the first time after six to ten years. It forms its flower buds in the previous year and opens them from the end of April to the end of May. The blue-violet flower bells stand in upright panicles up to 30 centimeters long and are reminiscent of the flowers of the foxglove. They exude a light scent. Reliable flowering is only possible in the mild wine-growing climate. In cooler regions with more severe winters and late frosts, the flower buds often freeze to death.
The flowers produce grey-brown walnut-sized capsule fruits, which are filled with countless dust-fine seeds. Even after opening, some of the fruit shells remain attached to the tree well into the winter.
Paulownia needs a sunny and warm place in the garden. Pay particular attention to good protection against cold easterly winds.
Paulownia does not make high demands on the floor. The lime-loving tree grows on all moderately dry to fresh, slightly acidic to alkaline soils, which should be permeable and not too moist. Nutrient-rich, moist loam soils do not allow the shoots to lignify in time.
Plant your bluebell tree or Paulownia best in spring so that it can grow well into the first winter. Heavy soils should be loosened and drained with sand and humus.
Paulownia does not need any special care. It copes quite well with dry summers after rooting in and regular fertilization is also not necessary. It often even has negative effects, because the shoots become brittle if the nitrogen supply is too good and are more sensitive to frost in winter.
If you want to raise a young blue bell tree to a high trunk, you should cut off one of the two fork shoots in time when the shoot tip is frozen, and guide the other up vertically on a long bamboo stick as a new trunk extension. Already finished high trunks do not need a regular cut, but also here it is recommended to cut out one of the two fork drives at least at the main branches of frozen shoot tips in spring.
Small, newly planted Paulownias should be protected by fir twigs and a mulch layer of leaves in climatically very unfavourable locations. For larger specimens, a trunk cuff against frost cracks is recommended in autumn.
Paulownia is a very expressive wood. It should always be planted individually to ensure that its beauty is fully appreciated. It is quite resistant to heat and dryness and is therefore also suitable as a road and parking tree for inner-city locations. Under its loose crown, even perennials in need of light grow quite well.
The blue bell tree can be propagated by sowing and cutting wood. The cut wood variant is the more suitable propagation method for hobby gardeners, as the winter cuttings grow very reliably and without any problems and only a few plants are needed for one’s own needs anyway. You also save one to two years of cultivation compared to growing from seeds. You use the annual shoots as stick wood material and cut around 20 to 30 centimeter long pieces in early winter after the leaves have fallen, which should each have a pair of eyes at the top and bottom. Then insert them vertically into well loosened garden soil so that the upper pair of eyes protrude only a few centimeters from the Soil. In the course of spring roots form and both eyes shoot out. Now remove one of the two shoots and guide the other up vertically on a bamboo stick. Next spring you can then transplant the young trees to their intended location.
Diseases and pests
Paulownia is one of the few trees that hardly ever have problems with plant diseases and pests.