Jerusalem Artichoke (Helianthus tuberosus)

Jerusalem Artichoke: Delicious Tuber with an Urge for Freedom

Jerusalem artichoke (Helianthus tuberosus) is a crop plant with a long tradition from North America. Below ground it forms edible tubers that can be harvested in autumn and winter. But it is precisely these tubers that are the reason why Jerusalem artichoke sometimes hangs on in the garden more stubbornly than planned.

Even small tubers overlooked at harvest can sprout again the following year. Once established, a stand can therefore persist for years and gradually spread.

Outside gardens, too, Jerusalem artichoke now occurs permanently in Germany. Larger stands can develop especially on riverbanks and on disturbed ground. In nature conservation assessments, the species is therefore not simply regarded as a harmless cultivated plant.

In the garden, Jerusalem artichoke can still be put to good use, provided the position is chosen deliberately and the tubers can be harvested regularly and as completely as possible.

The lesson of the Jerusalem artichoke:
What we want to harvest can at the same time be a plant’s most important means of spreading.

Red Valerian (Centranthus ruber)

Red Valerian: Adaptable, and Carried by the Wind

Red valerian (Centranthus ruber) at first seems like an uncomplicated garden plant. It copes well with dry, warm and nutrient-poor sites, grows in cracks in walls, on gravel areas and on sunny slopes, and can hold its own there astonishingly successfully.

Particularly interesting is the combination of two characteristics: it is very adaptable and assertive, and its seeds are dispersed by the wind.

It is precisely this combination that can enable it to reach new sites comparatively easily. While tubers or runners can be at least partly controlled with containers, root barriers or careful harvesting, spread by the wind is much harder to limit.

If you want to prevent heavy self-seeding, you would have to remove faded flower heads in good time, before the seeds ripen. With larger stands or wildlife-friendly plantings, however, this can hardly be fully controlled.

In Germany, red valerian is a neophyte and is not to be equated with the invasive species that are particularly problematic there. Under other climatic conditions, however, the same plant can be assessed differently. In the Canary Islands, it is considered an invasive non-native species and is subject to legal restrictions there.

This example in particular makes it clear that invasiveness is not a fixed characteristic that attaches to a plant equally everywhere.

The lesson of red valerian:
Plants can become especially successful when high adaptability and an effective dispersal strategy come together.

Cherry Laurel (Prunus laurocerasus)

Cherry Laurel: Familiar Hedge with Ecological Downsides

Hardly any garden plant seems as commonplace as cherry laurel (Prunus laurocerasus). Its evergreen leaves, fast growth and good tolerance of pruning have made it a popular hedging plant for decades.

But cherry laurel does not always stay behind the garden fence. Birds eat its fruits and can carry the seeds into woodland, scrub and other near-natural areas. There the plant can become established and, under favourable conditions, form dense evergreen stands.

A common criticism is that cherry laurel is fundamentally worthless for native insects. It is not quite that simple, though. Its flowers can certainly be visited by bees and other pollinators.

The bigger ecological problem lies elsewhere: ecologically, a cherry laurel hedge cannot be equated with a diverse hedge of native woody plants. Native shrubs have been linked with numerous animal species over a long period. They serve not only as a nectar source but also, for example, as food for specialised caterpillars and other herbivores, and they offer fruits, hiding places and varied structures.

At the same time, cherry laurel can impair natural vegetation outside the garden. In woodland close to settlements, dense stands can change the amount of light reaching the ground and make natural regeneration of other woody plants more difficult.

The difference in assessment becomes particularly clear when you look at Switzerland: there, cherry laurel is classified as an invasive neophyte and, since 2024, may no longer be placed on the market for direct use in the environment.

This does not mean that every existing cherry laurel hedge in Germany suddenly becomes an ecological disaster. But it does show that even a garden plant that has been familiar for decades can be reassessed when its spread outside gardens increases.

The lesson of the cherry laurel:
A popular garden plant can indeed offer food for some insects, and still cause ecological problems outside the garden.

Nasturtium (Tropaeolum majus)

Nasturtium: Invasive? Hard to Imagine Here

And then there are plants for which the label ‘invasive’ at first seems almost absurd in Germany.

For many people, the garden nasturtium (Tropaeolum majus) belongs to the summer garden just like tomatoes and marigolds. It grows quickly, forms lush trailing stems, its flowers and leaves are edible, and by the time of the first hard frost at the latest, its season in Central Europe is usually over.

How could this of all plants become invasive?

The answer lies above all in the climate.

In regions without our cold winters, nasturtium can behave quite differently. There it lacks the natural limit set by frost. Under favourable conditions it can survive permanently, naturalise and form dense stands.

In various mild and subtropical regions of the world, Tropaeolum majus is therefore observed as a naturalised or invasive plant.

For gardeners in Germany, this does not mean that nasturtium suddenly has to be regarded as problematic. But that is exactly why it is a particularly valuable example: our personal experience with a plant does not automatically apply to other regions of the world.

Frost, rainfall, altitude, competition from other plants and available habitats all help decide whether a species disappears after one season, or gains a permanent foothold.

The lesson of the nasturtium:
A plant that is reliably kept in check by winter here can develop completely different dynamics in a frost-free climate.

Is My Plant Rampant, or Is It Invasive?

A mint that takes over half the herb garden can be extremely annoying. Even so, that does not make it an invasive species in the nature conservation sense.

‘Invasive’ does not simply mean ‘grows vigorously’.

What matters, rather, is whether a non-native species can establish itself and spread outside cultivation, and whether negative effects on biodiversity arise or are to be expected as a result.

For us as gardeners, this gives rise to a very simple but helpful question:

What happens if this plant leaves the place intended for it?