Depending on the species, this plant is a deciduous shrub or deciduous tree. It can reach heights of growth of up to 25 m and a trunk diameter of up to 1.50 m. The white flowers, which depending on the species can also be yellowish or pink, appear from May to June and hang in dense clusters on the plant . The robinia forms so-called legumes, in which the seeds are located.
Table of Contents
growth and plants
Black locust trees are fast-growing and relatively hardy trees, which is why they are used not only for gardens and parks, but also for greening rather difficult areas. Under certain circumstances, however, this tree can also become a threat to the natural biodiversity of the region concerned.
If you want to plant a robinia, you should definitely make sure that there is sufficient distance to the neighboring property. With the exception of the spherical locust, depending on the region, this can be half the future diameter of the crown. For example, if the crown reaches a final crown diameter of around 10 m, the distance to the neighboring property must be 5 m, which could be problematic in smaller gardens.
The robinia can be planted in autumn and spring. Container plants can also be planted year-round, depending on the weather, provided the soil is frost-free. Planting in spring has the advantage that the wood is already well established by the start of winter and is therefore more resistant.
The soil should be loosened up well and a planting hole dug that is about twice as large and wide as the root ball. The robinia is then planted in such a way that the base of the trunk is above the ground. Then the planting hole is filled with soil and then the whole thing is well watered. It is advisable to fix the wood with appropriate stakes for the first two years to make it easier for the plant to grow and to give it a better hold.
location and soil
Robinia are very undemanding and heat-loving woody plants that thrive in almost any location and in almost any soil. Above all, the location should be very bright and sheltered from the wind and the soil should be loose, fresh and rich in nutrients. However, nutrient-poor, dry sandy and gravel soils as well as poor soils are also suitable. Unsuitable are boggy, very wet and impermeable soils. This is because the roots of the robinia need a lot of air and therefore prefer oxygen-rich, loose soil.
watering and fertilizing
- Water requirements of these typical spring bloomers are moderate
- Water the robinia thoroughly immediately after planting
- Water regularly for the first time after planting
- Later, water regularly only in summer and only on dry soil
- fertilize after the leaves have sprout until the beginning of September
- Apply liquid fertilizer about every two weeks
- In general, fertilization is not absolutely necessary
To cut
A pruning is usually not absolutely necessary with the exception of the spherical locust. Despite everything, the robinia is very tolerant of pruning, so that damaged or cross-grown branches can be removed at any time. These should then be cut off immediately at the base so that no stubs remain on the wood. The best time for this is late winter.
Larger storm damage may also make a radical pruning necessary. Otherwise, the cut should always be kept as small as possible, since the robinia then tends to form numerous runners. With the spherical robinia, however, regular thinning of the crown is advisable, but not during budding. If necessary, the cut surfaces should then be treated with an appropriate wound closure agent.
hibernate
The robinia is basically hardy. However, especially young robinias in unfavorable locations are very sensitive to both late and early frosts in autumn. If planted outdoors, it usually survives the winter best, winter protection is not necessary. Specimens that have been cultivated in tubs, however, need winter protection to prevent the root ball from freezing. Overwintering in a cold house is also possible. Then even temperatures of zero degrees are no problem.
propagation
Black locust naturally spreads via seeds and root runners, and its seeds remain germinable for a very long time. It is able to cover open sites in a very short time and also has a strong tendency to overgrow. An exception here is the spherical robinia, as it does not reproduce itself. Of course, you can also propagate this tree by hand by sowing seeds or by root cuttings.
sowing
The seeds are in the pods, which are picked from the tree and the seeds removed in November or when they have turned brown. These are then kept cool and dry over the winter, for example in a garage or in the basement. The storage location should also be safe from mice if possible.
You can then sow seeds in March/April. Since the seeds of the black locust have a relatively thick, waterproof shell, it is advisable to scratch or sand them slightly with a file or sandpaper, for example. Then the seeds are placed in a small container and briefly scalded with hot water for about 3 seconds. Now let the seeds swell in lukewarm water for about 12-24 hours until they have swollen to about twice their original size.
Now you should immediately sow the swollen seeds about 5 mm deep in appropriate seed soil. A mixture of two thirds of commercially available potting soil and one third of sharp sand is suitable as a substrate. The whole thing must then be kept evenly moist, but not wet. The seed pot comes in a bright place with temperatures of at least 15 degrees. Direct sun should be avoided. After about 8-9 days the seeds germinate and as soon as the seedlings are large and strong enough, they can be planted outdoors.
Sowing outdoors is also possible from the end of April to mid-May, provided that late frosts are no longer to be expected. Here, too, the seeds must be scratched and poured over with boiling water in order to overcome the germination inhibition.
root cuttings
For this form of propagation, root cuttings with a thickness of 0.5 to 2.5 cm and a length of 5 to 10 cm are cut in autumn or spring. These are then placed in pots filled with soil so that they protrude slightly from the ground. Now you moisten the substrate and put the whole thing in a warm place. If the cuttings begin to sprout, they can be watered a little more. It is also possible to put the root cuttings in the ground in a cold frame in spring, but this is less promising.
pests and diseases
Lice
When infested with lice, the leaves turn yellow, wilt and eventually fall off. To combat lice, there are various preparations available from the specialist trade, such as extracts from the neem tree and, depending on the type of louse, spraying with home-made plant broths from nettles or male ferns.
Robinia
leaf miners The robinia leaf miners and their larvae also ensure that the leaves of the robinia turn yellow and fall off. As a rule, the robinia can deal with this pest on its own. But you can also use different types of parasitic wasps such as braconid wasps or chalcids to combat them. Chemical agents are also available on the market, but these should be avoided if possible.
Phloespora leaf spot
This leaf spot is caused by a fungus and is identified by light brown irregular spots with a dark border. The leaves as well as the petioles and shoots can be affected. Corkscrew locust (Tortuosa) and spherical locust (Umbraculifera) are particularly susceptible to this disease.
In the event of an infestation, infested foliage should be removed and if the shoots are infected, the crown should be cut back. If necessary, you can administer appropriate fungicides in the spring.
toxicity
- With the exception of the flowers, the robinia is highly poisonous in all parts of the plant.
- Bark and fruits are the most poisonous.
- Toxic proteins, so-called toxalbumins, are responsible for this.
- The toxicity applies to both humans and animals.
- Endangered animals are dogs, cats, birds, rodents, cattle and horses.
- Therefore, you should think carefully about planting in the garden.
particularities
The planting of a robinia should not only be well considered because of its high toxicity, but also because of another special feature. The robinia, with around 20 species, is a so-called neophyte. These are plants that, through human intervention, spread outside their natural areas of origin and displace native species.
This is reinforced by bacteria that are contained in the root nodules of the robinia and bind the nitrogen in the soil, which means that the soil conditions change over the long term and the surrounding original vegetation also changes significantly. On the other hand, this property makes the robinia a good soil improver, so that it can also spread well on particularly poor soils, but due to the aggressive spread it results in the displacement of native wild species. As a result, it is also on the ‘Black List for Invasive Exotics’.
special species
Corkscrew Locust pseudoacacia ‘Tortuosa’
This medium-sized tree impresses with its corkscrew-like branches. It reaches heights of growth between 10 and 12 m, a crown width of 5-8 m and produces white flowers, but only relatively rarely.
Dwarf corkscrew locust pseudoacacia ‘Twisty Baby’
This type of robinia is still a relatively new variety and is particularly suitable for smaller gardens. It grows up to 3 m tall and wide, has an umbrella-like crown and corkscrew-like branches. It flowers only rarely and then produces white flower clusters.
Black locust pseudoacacia ‘Umbraculifera’
The thornless black locust is a small tree that grows tall and has a crown width of 4-6 m. It forms a densely branched, spherical crown, but no flowers. The leaves are bluish green on the upper side and light green on the underside of the leaves.
Black locust pseudoacacia ‘Frisia’
The black locust is a small tree that grows to a height of 6-10 m and a crown width of 5-8 m. The crown is initially ovoid and later arched and loosely branched. The shoots are bright yellow and the tree has reddish thorns. The flowers of the robinia are white.
Conclusion
Robinias are very beautiful trees. However, due to their strong spread and not least their toxicity, one should think twice about planting a robinia. It may be very difficult or even impossible to get rid of them later. If you still don’t want to do without a robinia, you can opt for a spherical robinia, because it does not reproduce itself.
