This sustainability project, organised by the Environmental Sciences student body in co-operation with School 5, aims to support and enhance biodiversity on the Wechloy campus of the University of Oldenburg. The main focus is on wild bees – not the honeybee (Apis mellifera), which is the species most commonly associated with them at first glance, but the many wild bee species that often live in seclusion and receive far less attention. Unlike the honeybee, most wild bee species are solitary, do not form colonies, and collect nectar and pollen exclusively for themselves and their brood.

In Lower Saxony, 381 species of wild bees are currently known (as of 2023), of which around two-thirds nest in various soil structures. However, ‘nesting in the ground’ is not a uniform strategy: some species prefer loose, sandy soils with sparse vegetation, whilst others prefer to dig their brood tunnels in loamy or loamy-sandy substrates. Still others use only vertical loess walls or existing cavities in the ground. Cuckoo bees and other brood parasites do not dig at all themselves, but are indirectly dependent on sandy structures because they lay their eggs in the nests of other ground-nesting species. What most of these species have in common are similar requirements regarding sunlight, soil moisture and vegetation cover: open, well-sunlit areas with little competition from dense vegetation. Among the genera that are particularly well suited to such sandy soils are the sand bees (Andrena), furrow bees (Halictus and Lasioglossum), trouser bees (Dasypoda), shaggy-haired bees (Panurgus) and silk bees (Colletes).

In addition to wild bees, other groups of ground-nesting insects also benefit from such structures. Digger wasps, which feed their larvae with paralysed prey, have similar requirements in terms of soil structure and sunlight exposure to many wild bee species. The real problem for many wild bee species is that the cultivated landscape hardly functions as a mosaic anymore. In the past, different habitats alternated within a small area: open ground, flower-rich margins, copses and fallow land. This coexistence is crucial because many species depend simultaneously on a wide range of different pollen sources and on suitable nesting habitats, both of which must be within reach. The reasons for the loss of this mosaic landscape are manifold: intensive agriculture, soil sealing and the disruption of natural nutrient cycles all play a significant role. One factor that is often underestimated is the absence of large herbivores. Megaherbivores such as the European bison, wild horse and aurochs kept landscapes open through grazing, trampling and rooting, created break lines and patches of bare ground, and prevented areas from becoming overgrown. It was not only wild bees that benefited from these disturbances, but also many animal and plant species that are now rare. With the disappearance of these animals from our landscapes, a crucial driver of habitat dynamics is missing.

When it comes to wild bee conservation, many people first think of nesting aids from DIY stores. However, these only benefit a small proportion of species – namely those that nest in existing cavities – and are often poorly constructed. Ground-nesting species, which make up the majority of the wild bee community, do not benefit from them at all. Measures that really make a difference go deeper: they focus on the availability of suitable nesting habitats and a sufficiently diverse range of flowering plants in the immediate vicinity. This was precisely the starting point for the Environmental Sciences student body’s decision to create a sandarium.

The selection of plant species for the Sandarium was based not only on ecological criteria, but also, deliberately, on aesthetic ones. The Sandarium is intended to demonstrate that nature-oriented design can be visually appealing and, ideally, inspire others to follow suit. The main priorities were that all the selected species should be native, well-adapted to the site and suited to the local wild bee fauna. Unlike the honeybee, which is polylectic and collects pollen from many different plant families, many wild bee species are oligolectic, meaning they specialise in specific plant families or even individual genera. When compiling the species list, we therefore first carried out our own floristic surveys on campus to determine which plant families were already present and which could be specifically added. The Asteraceae and Apiaceae families are of particular importance, as a large number of specialised wild bee species depend on them.

Particular emphasis is placed on the Campanulaceae, or bellflower family. This plant family has so far been scarcely represented on the campus, although several wild bee species specialise in its pollen. Three members of this family – Campanula trachelium, Campanula rotundifolia and Jasione montana – have been planted in the Sandarium. In addition, species from the Brassicaceae family have been included: Alyssum saxatile, Alyssum montanum and Aubrieta are classic wall plants that help to stabilise the dry stone wall whilst also providing one of the first sources of pollen of the season in spring.

Another key criterion was site adaptability: all the selected species are adapted to nutrient-poor, dry to moderately dry conditions. Care was also taken to ensure that something is always in flower throughout the year, from the early wall plants in spring to the later perennials in summer and autumn. Species such as Echium vulgare and Hyssopus officinalis were included not least because of their exceptionally high nectar and pollen value and are attractive to a wide range of wild bee species.

The species list is divided into several groups with different functions. Large focal perennials such as Ononis spinosa, Genista tinctoria and Eryngium planum serve primarily to provide spatial structure to the area and make the Sandarium visually engaging. Medium-sized perennial herbs such as Salvia pratensis, Anthemis tinctoria and Origanum vulgare form the ecological core of the planting scheme. Annual and biennial species such as Echium vulgare, Knautia macedonica and Reseda luteola enrich the mix and ensure dynamism in the development of the site. Ground-covering species and wall-clinging plants complete the overall appearance of the area without compromising its open, unpaved character, whilst also helping to stabilise the dry stone wall.

Ononis spinosa, Genista tinctoria, Dipsacus fullonum, Eryngium planum, Hyssopus officinalis, Festuca ovina

Salvia pratensis, - staminea, Anthemis tinctoria, Campanula trachelium, Stachys germanica, Origanium vulgare

Salvia sclarea, Echium vulgare, Knautia macedonia, Onobrychis vicifolia, Reseda luteola, Jasione montana

Campanula rotundifolia, Alyssum saxatille, Alyssum montanum, Aubrieta, Sedum acre, Potentilla verna, Hyppocrepis comosa, Thymus pulegioides, - serpyllum

Cymbalaria and Diplotaxis tenuifolia

The idea for the Sandarium was conceived in the summer of 2025 within the Environmental Sciences student body. Initial discussions on the choice of location began as early as July, followed by an initial planning document in August; in September, the list of plant species was finalised and the seeds were ordered. The project was supported by School 5’s funding scheme with a budget of 5,000 euros, which covered the cost of materials such as sand, natural stones, seeds and an information board.

In November, the plants were started off in the Botanical Garden of the University of Oldenburg, which had supported the project from the outset. Winter proved to be an unexpected hurdle: heavy snowfall darkened the greenhouse roofs, the seedlings received too little light, and a fungal infection destroyed some of the young plants. Part of the sowing had to be repeated. In January and February, the remaining seedlings were repotted and each given its own space. From this point onwards, a dry, nutrient-poor substrate was specifically used, and the plants were regularly placed in sunny spots to harden them off to the arid conditions of their future location. By spring, around 600 plants had developed and were ready for planting in the sandarium.

Final planning for the construction work began in April. Before the actual construction day, the use of the car park at the Wechloy campus had to be agreed with the building management team, as it was to be used as a delivery area for the building materials. The quantities of materials required were calculated independently and the delivery was then coordinated in collaboration with Tholen. The actual construction day took place on 24 and 25 April 2026, with up to 20 volunteers taking part at times. Over the course of two days, 35 cubic metres of sand were brought in and around 8.5 tonnes of Weser sandstone were used to build a dry stone wall. The material calculations proved to be very accurate: of the 9 tonnes of sandstone and 35 cubic metres of sand delivered, only small quantities remained, which the Botanical Garden and the building management team were able to use elsewhere.

The planting finally took place in mid-May. Contrary to the original plan, a decision was deliberately made not to adhere to a strictly defined bed layout. After a brief briefing on which species would serve as focal perennials and which as companion perennials or ground cover, the participating students were allowed to choose the locations themselves. Planting took place in a fixed order: first the focal perennials, then the companion perennials, and finally the ground cover and smaller perennials. Care was taken to group together species that form a coherent ecological and aesthetic unit, such as Salvia pratensis, Campanula trachelium and Festuca ovina. To further structure the area, a tonne of boulders was also used, along with deadwood, which was kindly provided by the Botanical Garden on Philosophenweg. Of the approximately 600 plants grown, as many were planted as the area could reasonably accommodate.

To ensure that the sandarium can fulfil its ecological function in the long term, a maintenance plan was drawn up right from the start. The area is maintained twice a year, once in the spring between April and May and once in the autumn between September and October. Maintenance is permanently in the hands of the Environmental Sciences student body and is carried out by volunteers, likely continuing in co-operation with the University of Oldenburg’s Botanical Garden. There are two main tasks during the maintenance sessions: firstly, removing plants that have self-seeded from outside the site; and secondly, monitoring the introduced species to prevent them from spreading excessively. The primary aim is to preserve the open ground areas, as only a site that is not completely overgrown can fulfil its function as a nesting habitat for ground-nesting insects. It is particularly important to consistently remove woody seedlings and young trees, which self-seed regularly and, if neglected, would quickly shade and crowd out the area. However, not every species that establishes itself naturally is automatically undesirable. Species from the immediate vicinity, such as cat’s-ear (Hypochaeris), hawkweed (Hieracium), knapweed (Centaurea) or daisy (Leucanthemum), are highly valuable forage plants from which many species of wild bees benefit. However, as the Sandarium is specifically designed to promote species that have so far been under-represented on the campus, regulatory measures are also taken against these species if they become overgrown. Particular attention is paid to certain species from our own species list which, experience has shown, self-seed profusely and can displace other species over time: ox-eye (Buphthalmum salicifolium), Macedonian scabious (Knautia macedonica), wild teasel (Dipsacus fullonum), reseda (Reseda luteola), viper’s bugloss (Echium vulgare) and stonecrop (Sedum acre) must be monitored regularly and cut back where necessary. Species such as Echium vulgare, Dipsacus fullonum or Reseda luteola also form large leaf rosettes in their first year, which cover large areas of bare ground and thus obscure precisely the structures that ground-nesting insects need to dig in. In principle, however, the maintenance required for such a sandarium is manageable: the arid, nutrient-poor site simply makes the area unattractive to many problematic species. No deliberate intervention is made in the natural dynamics of the area. Deadwood and boulders are left untouched, and dead plant stems are not removed, as they serve as overwintering structures for insects. The aim is to create an area that is largely self-regulating and is only managed where it is truly necessary. Over the years, the development of the species composition is also monitored: if certain species become permanently overabundant, the vegetation is selectively thinned out; if large gaps appear or species fail to establish, replanting may take place.

A sandarium doesn’t have to cover a large area. Even an area as small as one square metre can provide a useful structure that serves as a habitat for ground-nesting insects. If you have the space and the inclination, feel free to think on a larger scale and let your creativity run wild: old tree stumps, stones and deadwood are excellent structural elements that enrich the sandarium both ecologically and visually. Incidentally, deadwood can easily be requested from the Botanical Garden on Philosophenweg in Oldenburg; they’re happy to provide some.

The most important factor when choosing a location is sunlight. Ideally, the area should receive several hours of direct sunlight each day so that the soil dries out sufficiently. Places where water regularly pools after rain are unsuitable; the soil should be as dry as possible.

There are a few things to bear in mind regarding the sand. Washed sand, as is often available from DIY stores, is less suitable for a sandarium as it lacks the natural fine particles. We used standard fill sand or masonry sand, such as that used for backfilling paved paths. This still contains a small amount of clay, which is beneficial: a certain amount of clay ensures that nesting tunnels remain stable, but too much of it makes the substrate too hard and compact. Alternatively, sand can be mixed with a little clay powder, though care should be taken with the proportions. A mixture of ordinary garden soil and sand also works well; the key is that the sand should make up the vast majority of the mixture.

When it comes to planting, the rule is: less is better than more. Planting too densely blocks the open areas of soil that are essential for ground-nesting insects. Attention should also be paid to the height of the plants: species that grow too tall and shade the area are unsuitable, as they reduce the heat and light exposure that are crucial for the sandarium. When selecting plants, you should opt for native, drought-tolerant species that are adapted to nutrient-poor sites. Which species are particularly suitable and how we went about the selection is described in the ‘Plants & Why’ section, where you can also download the complete list of species.

If you wish, you can add a dry stone wall to the sandarium. It is important to build the wall without mortar, as this is the only way to create the gaps between the stones, which provide additional nesting opportunities for insects that nest in crevices and cavities. When planting the wall, care should be taken to choose species that can genuinely thrive in this extreme habitat – in other words, those that are drought-tolerant and adapted to barren, stony conditions. Suitable wall plants can also be found in our species list.