The opium poppies have now set seed, their leaves rattling like crisp packets, the vents beneath their pod lids open for the seeds to be scattered. Dotted throughout my flower borders, adding stature to the vegetable garden, Papaver somniferum is the breadseed poppy, its large, floppy petals unpredictable in colour: pale lilac, purple, pink or sometimes with fringed petals. The unexpected brings a freedom and a randomness to the garden.

Those blue-grey seeds are sprinkled on bagels and bread, baked into cakes or ground as a thickener for curries, adding protein, plant fats and trace elements such as manganese. But in its unripe state this is the plant that gives opium, a two-sided drug, both medicinal and destructive as a sedative – its specific name somniferum means sleep-bringing.

It is when the petals fall off, leaving a plump, unripe capsule the colour of celadon, that the opium is extracted. Damage the pod or break off a poppy stem, and a white, milky latex oozes out. It is this that contains a mixture of opiate alkaloids including morphine, heroin and codeine, something wildlife would normally avoid.

These poppies are also the source of a mystery in my garden. I’m sitting on the terrace in the late afternoon sun, feeling the absence of the swallows, when a wood mouse scampers past. It runs lightly up the stalk of a poppy, one that I’ve been enjoying for its late flowering. Speedily severing a stem, the mouse carries a pod several times the size of its own head, and manoeuvres it with difficulty into a hidden store in the drystone wall.

Curious to know if the alkaloids take their toll, I set a nightly wildlife camera. Several times the mouse goes back, eyes large in the momentary light, tail curling round a stalk. There’s a rustle of foliage and the sound of chewing before a heavy seedpod drops to the ground.

Bemused, I try some research. The general opinion is that unripe poppies would spoil when cached, that immature seeds have fewer nutrients, and that even a small amount of latex would be toxic to the mouse. I haven’t seen the field mouse since, but then maybe it has chosen something else to harvest.

• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024, is available now at guardianbookshop.com