Why solitary bees need a helping hand

The UK is home to around 270 species of bee, and the great majority of them are solitary. Rather than living in a colony with a queen, each female works alone: she finds a tunnel, provisions it with pollen and nectar, lays an egg, seals it up and moves on. The red mason bee and the leafcutter bee are the two you are most likely to see in a British garden, and both are excellent pollinators of fruit, vegetables and wildflowers.

What they struggle to find is somewhere to nest. A tidy garden offers very little in the way of dead wood, hollow plant stems and beetle borings, which are exactly the cavities these bees rely on. The good news is that a small block of drilled timber and a bundle of stems will fill that gap. It costs very little, takes an afternoon, and can support dozens of bees within a single season.

Start with the right timber

Untreated hardwood is the material to aim for. Oak, ash, beech or sycamore all hold a clean hole well, and offcuts from a timber merchant or a friendly joiner are usually free. Avoid anything that has been pressure-treated, tanalised, painted or varnished, as the chemicals and the sealed surfaces are harmful to developing larvae. Softwoods are also best avoided — they tend to splinter internally, leaving rough fibres that damage delicate wings.

• Depth: 100–150mm, which suits most species and fits neatly inside a small roofed box
• Finish: bare, untreated, rough-sawn on the outside is fine
• Size: roughly 150mm tall and wide, or a log section of similar dimensions

If you have an old fence post or a length of seasoned log, that will do the job admirably. The bees are not fussy about appearance.

Drilling tunnels that actually work

The type of drill bit matters. A sharp brad-point bit or an auger bit cuts a clean, smooth-walled tunnel, whereas a standard twist bit tends to tear the grain and leave splinters around the entrance. Drill straight in, and stop about 10–15mm short of the far side so each tunnel has a closed back. An open-ended tube lets light through and puts the bees off.

Vary the diameters across the block rather than drilling a single size, and space the holes roughly 15–20mm apart on centre so the timber does not split.

• 3–5mm: smaller species such as the yellow-faced bee
• 6–8mm: the sweet spot for red mason bees and most leafcutters
• 9–10mm: larger leafcutter bees, which need room for their leaf-lined cells

Once the drilling is done, tip the block upside down and tap out the sawdust, then sand the face lightly so no rough edges remain around the entrances. Angle the finished block very slightly forward and down when you mount it, so rain runs off rather than sitting in the tunnels.

One thing worth thinking about before you site it: mason bees need a reliable supply of damp mud to build their cell walls, so a pond margin, a boggy corner or a patch of heavy clay that you keep watered is a real asset. Leafcutters, meanwhile, need soft leaves — roses, birch and lilac are favourites, so resist the urge to clip everything back.

Bundling hollow stems

A drilled block and a stem bundle complement each other beautifully, because different species have different preferences. Bamboo canes are the easiest starting point, along with the dried stems of teasel, hogweed, cow parsley and elder. Cut each stem just beyond a node, so one end is naturally sealed, and trim to 150–200mm.

Gather a good handful, keeping the cut ends level, and bind the bundle tightly with garden twine. Push it into an open-fronted tube — a short length of drainpipe or an old tin with the base removed works well — so the stems cannot shift. Aim for a mix of internal diameters between 3mm and 10mm, and check that each stem is genuinely hollow and clear by pushing a thin cane through it.

Avoid glass tubes and plastic drinking straws. Both trap condensation, and mould is fatal to the larvae inside.

Siting and weatherproofing

Position is everything. Choose a spot that faces south or south-east, so the entrances catch the morning sun and warm up early. Mount the hotel between one and two metres above the ground, firmly fixed so it does not sway, and shelter it from the prevailing south-westerly wind and driving rain. A roof overhang of at least 100mm will keep the front dry through the winter.

Keep it away from bird feeders, because blue tits and great spotted woodpeckers will happily peck out overwintering larvae. If predation becomes a problem, a piece of 25mm wire mesh fixed across the front will deter birds while letting the bees straight through.

Looking after it through the seasons

Solitary bees lay their eggs through spring and summer, and the larvae stay inside the tunnels over winter before emerging the following spring. That means the single most important thing you can do is leave the hotel exactly where it is through the cold months. Moving it into a shed or garage disrupts the temperature cues the bees rely on and often kills them.

Come autumn, you can gently slide out the stem bundle and replace it with fresh stems, storing the old ones in a cool, dry, sheltered spot outdoors until the bees have emerged. Replace the stems every year or two, and re-drill or brush out the wooden block if the tunnels become clogged. You will also start to notice other visitors — tiny parasitic wasps, pollen mites and the occasional spider — which are all part of a functioning garden ecosystem rather than a sign of failure.

Set it up this spring and you can reasonably expect mason bees to find it within weeks. From then on, it becomes one of the busiest corners of the garden.