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On poison in the shade of walnut trees.

Pliny on the dangers of being in the shade (Naturalis Historia 17.18):

Iam quaedam umbrarum proprietas: iuglandum gravis et noxia, etiam capiti humano omnibusque iuxta satis.

Now there is a certain distinctive property to some shades: that of walnut trees is unwholesome and harmful, both to human life, and to anything else close enough.

He lists several trees whose shadows are supposed to have positive or negative qualities, but the walnut (along with the silver fir and some kinds of pine) are non dubie venenum ‘doubtless poison’.

Of course the shade of trees itself, outside of blocking the light, does not in itself have much effect on other living things. The walnut actually does secrete a chemical called juglone, which discourages the growth of other plants around it, though this effect is much more noticeable in New World species of walnut such as the Black Walnut (Juglans nigra L.) than those native to Eurasia.

[For proprietas.]

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