Most of the nesting aids you find at the garden centre are homes for just a handful of species: mason bees, a few leafcutters, a bit of bustle in spring. But the vast majority of our wild bees never move in. Around three quarters of all native species nest not in drilled holes or hollow stems, but dig their brood tunnels straight down into warm, open soil. For them you do not build an insect hotel. You build a sandarium.
At heart a sandarium is surprisingly unspectacular: a sunny patch of open sand, slightly sloped, deep enough to dig into. No kit, no drill, no roof. Still, a few things decide between success and failure, and we will work through them one by one. The best time to build one is early spring, roughly from mid-March to mid-May · CW 12 to 20, before the first mining bees go looking for nest sites in April.
Why ground-nesting wild bees need their own nesting site
When people think of protecting wild bees, they think of the insect hotel. That covers only a small slice. Mason bees, scissor bees and yellow-face bees gladly take drilled tunnels and pithy stems. But mining bees (Andrena), sweat bees (Halictus and Lasioglossum), plasterer bees (Colletes) and many nomad bees nest exclusively in open ground. To them a drilled hole is about as useful as a ladder is to a fish.
In spring these species look for warm, sparsely vegetated patches of soil: sunny banks, path edges, eroded slopes. In a mulched, densely planted garden they find almost none of that anymore. This is exactly the gap a sandarium fills, perhaps the single most effective measure for wild bees in your garden, and it costs almost nothing beyond an afternoon.
Choosing the right spot
The location matters more than any building plan. Ground nesters need warmth, because their brood develops in the sand and the sand has to heat up. Look for the sunniest corner you have.
- Full sun, facing south
The spot should get sun from morning to afternoon, at least six hours. A patch sloping to the south or south-west heats up fastest. North-facing and shaded spots stay empty.
- Rain-sheltered, but not roofed
Sand that is constantly soaked silts up and lets the brood rot. A slightly raised spot under an eave, against a house wall or beneath light-canopied shrubs keeps off the worst downpours without taking away the sun.
- Quiet and undisturbed
Not a place where the mower runs over constantly or children race across. A border by the fence or next to the dry stone wall is ideal. The bees are peaceful, but they dislike commotion right over their entrances.
- Close to food
Ground nesters dislike flying far. The closer flowering forage plants like viper's bugloss, thyme and wild carrot stand, the sooner the sandarium is taken up.
The right sand: the most common mistake
This is where most sandariums fail before the first bee has even stopped by. Pure, washed play sand is too fine and loose, the tunnels collapse. Coarse quartz gravel is too coarse, nothing can be dug into it.
What you need is unwashed, slightly loamy sand with a mixed grain size up to about 2 mm. The small loam fraction is the trick: it acts like mortar and keeps the self-dug tunnels stable without sealing them shut. At the builders' merchant it is often simply called masonry sand or path-building sand with grade 0/2. A quick test: moist sand should briefly hold a clump and crumble again when tapped. If it runs through your fingers like dry sugar, it lacks loam; if it stays hard like a brick, there is too much of it.
Depth and slope: what counts
Ground nesters dig surprisingly deep: depending on the species the tunnels run 20 to 40 cm straight down, sometimes more. A sandarium that is only shallowly heaped up is therefore rarely taken, so plan generously for depth. The slope does two things at once: it lets rain drain off instead of pooling, and it offers a warm, light-facing cut edge that many species particularly love. A patch sloping gently to the south with a small, almost vertical edge on the sunny side gets it exactly right.
Mark out and excavate the area
Mark out an area of at least 1 by 1 metre, larger if you can, and dig it out 40 to 50 cm deep. The depth gives the bees room for their tunnels and prevents waterlogging rising from below.
Build in drainage
Fill the bottom 10 to 15 cm with coarse gravel or rubble with no fine content. That lets excess water drain away and keeps the sand airy. On heavy, loamy subsoil this step is decisive.
Fill in and compact the sand
Fill in the unwashed sand until it rises slightly above ground level. Compact it in layers with your foot or a board, but not rock hard: the bees should be able to dig, not chisel. Lightly moisten each layer, that helps it settle.
Shape the slope and edge
Model the surface with a gentle fall to the south and cut a small, 15 to 20 cm high vertical edge on the sunny side. This warm wall is the most attractive part for many mining bees.
Frame the edge
Edge the area with a few larger stones or a timber frame so the sand does not spill out and weeds do not move straight in. A lying branch or stone is gladly used as a sunny perch.
Planting the sandarium without swamping it
A sandarium lives on open ground. Even so, it may carry a few plants, as long as they do not shade the surface. Set low, drought-loving species loosely along the edges and leave the middle clear. Leanness is an advantage here: the very pioneers that thrive on sand are also the best pollen sources for ground nesters.
Viper's bugloss is perhaps the most valuable forage plant for a sandarium: a biennial pioneer that loves lean, sandy sites and delivers blue, nectar-rich flowers all through high summer. It pairs well with creeping thyme, wild carrot, Carthusian pink and restharrow. All tolerate drought, none wants fertiliser.
Care through the year: do little, do it right
The lovely thing about a sandarium is that it is almost no work. The most common care mistake is doing too much.
- Keep it open, do not water, do not fertilise
The surface must stay free of vegetation. Pluck out emerging grass and weeds two or three times a year, ideally after rain. Never cover it with foil and never mulch, that suffocates the nests. Watering and fertilising only feed weeds and harm the brood; the sand may stay lean and dry.
- Leave the nests alone
From April on you will see small holes with cones of sand around them. Those are occupied nests. Do not rake them smooth, do not step on them, do not dig them up. The brood sits in the soil all summer and winter and only emerges the following year. Topping up settled sand once in March, CW 12 to 13, and recutting the sunny edge is enough.
When do the first bees arrive?
Patience belongs to the garden, and to the sandarium especially. Sometimes the first mining bees are there in the first warm days of April, above all if there are already populations in the neighbourhood. More often it takes until the second year. A sandarium is colonised better from year to year, because the emerged bees stay true to their birthplace and nest nearby again.
The best time to build a sandarium was last year. The second best is this spring.
Old gardener's wisdom, loosely
If you want to go further, combine the sandarium with more open, lean structures. A dry stone wall as a habitat alongside creates extra crevices and warm zones, and a well-considered offer of nesting aids and forage plants at the same time serves the cavity nesters. Together it becomes a small wild-bee habitat that sustains itself over the years.
Häufige Fragen
How deep does a sandarium need to be?
Plan for at least 40 to 50 cm of depth, plus a drainage layer of coarse gravel at the bottom. Ground-nesting wild bees dig their brood tunnels 20 to 40 cm straight into the sand depending on the species, some deeper still. A sandarium heaped up only shallowly is rarely taken, because the bees lack the room downwards and waterlogging endangers the brood.
Which sand is suitable for a sandarium?
Use unwashed, slightly loamy sand with a mixed grain size up to about 2 mm, often sold as masonry sand or path-building sand 0/2. The small loam fraction stabilises the dug tunnels. Pure, washed play sand is too loose and collapses; coarse gravel cannot be worked. As a quick test, moist sand should briefly hold a clump and crumble when tapped.
When is the best time to build a sandarium?
In early spring, roughly from mid-March to mid-May · CW 12 to 20, before the first mining bees go looking for nest sites in April. That way the surface is ready in time. In principle you can build a sandarium all year round, but built in spring you have the best chance of getting residents in the very first summer.
How do I care for a sandarium correctly?
As little as possible. Keep the surface free of vegetation by plucking out emerging grass and weeds two or three times a year. Do not water, do not fertilise, do not mulch and never cover it with foil. Occupied nests, recognisable by small holes with cones of sand, stay untouched. Topping up settled sand once in spring and recutting the sunny edge is enough.
Are ground-nesting wild bees dangerous, or do they sting?
No. Mining bees and the other ground nesters are decidedly peaceful and hardly ever sting. Many species have a sting so fine that it barely penetrates human skin, and they do not defend their individually built nests the way a honeybee colony does. You can put a sandarium in a family garden without a second thought and watch the bees up close.
Spotted a mistake?
