Bladder senna: varieties, care & toxicity

Looking for an insect-friendly ornamental plant for the garden that defies climate change and can also get by with few nutrients? Then the bladder senna is the plant for you.

It is not only the inflated legumes that make bladder senna (Colutea) special plants. The long flowering period, drought tolerance and the ability to fix nitrogen as fertiliser from the air thanks to a symbiosis with bacteria are all among the benefits of this special woody plant. In this article you will learn everything you need to know for growing bladder senna in the garden: From finding a suitable location, pruning and care, to answering the question of whether Colutea arborescens is poisonous.

Bladder senna: origin and properties

Colutea form a genus of about 26 species within the legume family (Fabaceae). Like most legumes, bladder senna form a symbiotic relationship in their root zone with bacteria that can fix nitrogen from the air. Therefore, they do not need to be fertilised with nitrogen.

In our country, the most common Colutea species is the bladder senna (Colutea arborescens). It originates from the Mediterranean region and North Africa.
The growth of bladder sennas is rather slow and sprawling. They grow to a maximum of 2 to 4 m high and the same in width. Their root system is rather shallow. In autumn, bladder sennas lose their leaves, which are pinnate, with elliptical leaflets about 3.5 cm long. From May until August, the bladder senna is in bloom. This means the typical butterfly flowers, which are yellow-orange in colour in the common bladder senna, can be admired for a relatively long period of time. The fruits of the bladder senna also have a high ornamental value, being inflated pods that are 3 to 6 cm long. Inside are 20 to 30 black, lentil-like seeds.

Is the bladder senna bee friendly? Bladder sennas are very insect-friendly plants, especially thanks to their long flowering period. However, it’s not particularly easy for many insects to get the pollen. Therefore, larger wild bees are more likely to be found on bladder sennas, such as bumblebees (Bombus) or wood bees (Xylocopa). One insect that requires the bladder senna for survival is the endangered iolas blue butterfly (Iolana iolas) – a butterfly whose caterpillars develop in the pods of bladder sennas.

The most beautiful Colutea types

Besides the common bladder senna, only a few other species and varieties are found in our country. With the help of this short list, you can choose the right shrub for your garden.

Colutea arborescens: The bladder senna is the flagship species of the genus Colutea. It grows 3 to 4 m high, tolerates temperatures down to -23 °C and features impressive yellow flowers, as well as a long flowering period.

Colutea orientalis: Colutea orientalis is also known as red bladder senna, although its flowers are more orange-yellow. With a maximum height of about 2 m, it is the smallest of the bladder sennas shown here. Similar to the common bladder senna, Colutea orientalis is also hardy to about –20 °C.

Colutea x media ‘Copper Beauty’: Bladder senna cultivar ‘Copper Beauty’ is a hybrid, that is, a cross between two different species – in the case of Colutea x media between Colutea arborescens and Colutea orientalis. Its yellow-orange to copper flowers are special. Unfortunately, ‘Copper Beauty’ is somewhat less hardy, tolerating temperatures only to about -17 °C.

Planting: location and procedure

Bladder sennas do well in places where many other plants have problems. An optimal site for bladder sennas should be sunny and have a well-drained, lean, rather calcareous soil. Even wind and salt do little harm to bladder sennas.

The optimal time for planting bladder sennas is in autumn, regardless of whether they are containerised or bare-root plants. In particular, plants from pots can be put in the ground from April. You can have bladder sennas as a solitary eye-catcher in the garden if you have enough space, but you can also integrate it into insect-friendly hedges. In that case, make sure that the planting distance is at least 50 cm.

Once you have selected a suitable location, you can plant your bladder senna.

1. Dig the planting hole. It should be deep enough so that the plant later sits in the ground as deep as it was before in the pot.
2. Especially for humus-rich, loamy soils, it is recommended to mix the excavated soil with expanded clay and some garden lime. If the soil is too fertile, the plant will not grow well.
3. Place the bladder senna in the planting hole and refill it with the excavated soil.
4. Press the soil around the bush and water it well. Especially in the early days, when the root system is not yet sufficiently developed, bladder sennas need a little more water. By creating a watering ring from soil, you can prevent the water from simply running away.
5. In any case, in the first year, continue to make sure that the shrub does not get too cold. For example, protection can be provided by a layer of mulch on the root ball or a jute bag placed over the shrub.

Caring for the bladder senna

Few plants in the garden need as little care as a bladder senna. The watering can should only come out during extremely dry periods. Since the plant originates from the Mediterranean region, it copes very well with little water and, conversely, reacts with stunted growth to excessively moist soils.
You do not even really need to fertilise your bladder senna. Thanks to the symbiosis with nodule bacteria, the shrub itself provides an adequate supply of nitrogen. Additional fertilisation would stimulate growth, but at the same time make the shrub more susceptible to frost – because fresh, young tissue is usually killed by frost first.

Pruning bladder sennas is not essential. But it can be done again and again, if you want to keep the bush young. To do this, every few years in the spring, some of the old shoots can be cut at the base. But this should be no more than one-third of all shoots. Annual shoots can also be shortened in the spring before budbreak.

The soils here are usually rather acidic. Regular liming is therefore part of the care of the bladder senna. We recommend carbonic acid lime for this purpose as it has a longer-lasting effect. The easiest way to determine when it’s time for the next liming is to determine the pH of the soil. This should not be less than 6.5.

Care at a glance:

• Water only during periods of extreme drought.
• Fertiliser not necessary.
• In the spring you can cut the bladder senna, if you want to influence its growth.
• Liming should be done when the pH of the soil drops below 6.5.

Is the bladder senna hardy?

In Germany, the bladder senna is very hardy, because it can withstand temperatures down to -23 °C. However, since this varies slightly from shrub to shrub, very cold winter nights can sometimes affect some bladder sennas. Therefore, there is no harm in protecting the rootstock with a layer of mulch, such as foliage.

Propagation

The easiest way to propagate the bladder senna is by seeds or cuttings.

Sowing seeds of bladder senna can be done either directly on the spot, or in small pots. In the autumn, collect the black seeds found in fallen pods and store them in a dark, dry and relatively cool place during the winter.

1. February is the time for sowing. To do this, first soak the seeds in water for about 10 hours.
2. Fill small pots of about 8 – 10 cm in diameter with moistened growing soil for example, our Plantura Herb & Seeding Compost.
3. Stick 2 to 3 seeds of the bladder senna into the soil about 1 cm deep.
4. Place the pots in a bright place at a temperature of about 20 °C.
5. Over the next few days, make sure that the substrate does not dry out. After 10-20 days, the seeds should germinate.
6. If more than one seed has germinated per pot, at a height of about 3 cm, you should remove the weaker seedlings and leave only the strongest.
7. Once the plants are about 10 cm high, they can be planted out in their final location.

Propagation via softwood cuttings is also possible. To do this, at the end of spring cut off about 10 cm long of the shoot tips without flowers from the bladder senna. Remove the lower leaves from the cutting, leaving them only on the top 5 cm. Then put the cuttings in small pots filled with moist growing soil. If the leaflets are then hanging down to the ground, shorten them to a more practical length. During the next period it is especially important that the substrate is constantly moist. Also, the cuttings should be in a bright and warm, but not hot place. Drying out can be prevented by putting plastic bags over the pots and fixing them with a rubber band. However, it is important to air under the bags at least twice a week. When the cuttings are rooted, they are planted individually in pots about 9 cm. The fresh growth is later pruned to achieve bushy branching.

Is the bladder senna poisonous?

It is not only the yellow bladder senna that is slightly poisonous. Accordingly, consumption of plant parts of the other species of Colutea may cause mild symptoms of poisoning, such as nausea and vomiting. Bladder sennas are also toxic to horses, dogs and cats, resulting in gastrointestinal distress and diarrhoea, as in humans.

Are you looking for more ornamental plants for dry, lean sites? Then you might be interested in, for example, the common rue (Ruta graveolens).