Hallerbos is a beech wood south of Brussels whose floor turns solid purple-blue for about three weeks in April. The flowers have no choice about the timing. Wild bluebells need open sky, and the beech canopy closes in late spring, so millions of bulbs flower together in the gap, feed, and go back underground before the roof shuts. It is a slow plant on a fast window: five to seven years from seed to a first flower, for three weeks of it. For a few days in the middle the two things overlap — young beech leaves open but still thin, so acid-green light comes down over your head onto a floor at full purple. First light is the other hour to be there. Mist hangs between the grey trunks, the low sun arrives in separate shafts that land on the flowers, and the roe deer are still out grazing the edges of the blue.
The wood is gently hilly, and that decides what you see. Looking up a slope, the flowers stack into one unbroken sheet with the trunks rising straight out of it; on flat ground the same carpet shows its gaps and bare soil. Down at the path edge the colour turns back into plants, thousands of narrow stems each bowed under a row of nodding bells, with a faint sweetness you miss entirely from standing height. At the foot of any beech you find the bulbs pressed against roots much younger than themselves: the wood was felled in the First World War and replanted from the 1930s, while the bluebells underneath carried on. Among the new planting were giant sequoias, and a marked walk of about four kilometres leads out to them, red-barked and absurd among the beeches — the walk that still works in the eleven months when the floor is green. The ropes along the paths are not decoration. A bluebell that gets trodden on loses the leaves it feeds itself with, and the bald patch it leaves in the carpet is still there years after the boots have gone.