These moths mainly inhabit deciduous forests, hedgerows, parks and gardens, at an elevation up to 1,600 metres (5,200 ft) above sea level. 
Acronicta psi has a wingspan of 34–45 mm.  These moths have grey forewings with bold black dagger-shaped markings. (The Latin specific name also refers to these markings, as resembling the Greek letter psi ψ.)  The hindwings are dirty grey, generally paler in the male.
This moth is very similar to the dark dagger (Acronicta tridens) and identification is generally only possible by minute examination of the genitalia. However, in general this moth is generally darker in colour than the dark dagger and always lacks the white hindwings often present in the male of that species. Moreover, the larvae of the two species are very different.
The larva of Acronicta psi is quite hairy, greyish or brownish below and black above, with red spots along the sides sand a bold yellow stripe along the back. It has a distinctive horn just behind the head (absent from the larva of dark dagger).
It feeds on a wide range of plants, mainly trees and shrubs (see list below). The species overwinters as a pupa.
Recorded food plants include: 
- Acer platanoides – Norway maple
- Aegopodium podagraria – ground-elder
- Alnus glutinosa, Alnus incana – alder
- Amelanchier spicata
- Betula verrucosa, Betula pubescens – birch
- Corylus avellana – hazel
- Crataegus oxyacantha, Crataegus coccinea – hawthorn
- Malus domestica – apple
- Photinia – red robin
- Populus tremula, Populus suaveolens – poplar
- Prunus domestica, Prunus cerasus, Prunus avium, Prunus padus
- Pyrus communis – pear
- Quercus robur – oak
- Rosa – rose
- Rubus idaeus – bramble
- Salix caprea, Salix phylicifolia – willow
- Sorbus intermedia, Sorbus hybrida, Sorbus aucuparia
- Spiraea salicifolia
- Tilia – lime
- Ulmus glabra – elm
Mature larva feeding on a Japanese cherry tree in early autumn
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- Chinery, Michael Collins Guide to the Insects of Britain and Western Europe 1986 (Reprinted 1991)
- Skinner, Bernard Colour Identification Guide to Moths of the British Isles 1984