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Species of the Month - May 2019 Jack-by-the-Hedge Alliaria petiolata
Also known as Hedge Garlic or Garlic Mustard, this very distinctive crucifer is easily recognised by the clusters of small 4-petalled flowers and the large-toothed heart-shaped leaves. If in doubt, crush a leaf and it will smell of garlic. It likes sheltered spots with partial shade, and is often up against a fence, wall or hedge, or a tree trunk as in the photo above,
The leaves are strongly
veined and edged with rounded or bluntly pointed teeth. They are hairless,
stalked and have a wide sinus at the base.
The flowers have 4
strap-shaped petals, 4 much smaller green sepals which soon fall off, 6
stamens (4 long and 2 short), and an ovary that soon begins to grow into
a long narrow pod. The flowers are very densely packed when they
first open, but the inflorescence stem lengthens dramatically as the
fruits develop, and the pods end up well spaced down the stem.
The
flowers are very popular with the Orange Tip butterfly. Females
lay eggs on the flower stalks, and by the time the larva hatches there
is a well-developed pod for it to crawl onto and devour. The
narrow green caterpillars are well camouflaged against the pods.
Male Orange Tips visit the plant to look for females. Both sexes also feed on
nectar from the flowers, and may thereby aid in their pollination, which
partly makes up for their caterpillars eating the pods!
This photo shows how quickly the pods lengthen as the petals fall off. The female Orange Tip carefully chooses the right spot to lay her egg to synchronise the larval growth with that of the pod. They only lay one egg per plant, which is ideal from the plant's viewpoint as it means the female must move to another plant to lay the next egg, increasing the chances of cross-pollination. Jack-by-the-Hedge is scarce in the west of Scotland, particularly north of the Central Belt. It is much commoner in the east where it is quite frequent up to Inverness and a bit beyond. See the BSBI distribution map at https://bsbi.org/maps?taxonid=2cd4p9h.ybd (zoom in for more detail). There are very few records for Argyll but it is probably under-recorded here, as I know several locations for it in the Oban - Benderloch - Taynuilt area. Any records from other parts of Argyll would be of great interest. Orange
Tips have only spread north into Argyll in the last 30 years, and
Jack-by-the-Hedge is not spreading at the same rate, if at all.
Many of its west coast records are old ones (shown by paler colours on
the BSBI map). Fortunately the Orange Tips are equally happy with
Cuckoo Flower (Cardamine pratensis) which is very common in our area and
all the way up the west coast, so the butterfly's expansion won't be
held up by foodplant shortage.
Jack-by-the-Hedge photographed in Inverness in 2002
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Carl Farmer
May 2018
- Speckled Wood
Complete list of Species of the
Month
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