Species of the Month - May 2019

 Jack-by-the-Hedge

Alliaria petiolata


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,


Alliaria petiolata

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.
 

Alliaria petiolata

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.
 

Alliaria petiolata

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!
 

Alliaria petiolata

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.
 

Alliaria petiolata   Alliaria petiolata

Jack-by-the-Hedge photographed in Inverness in 2002


Please send in your Jack-by-the-Hedge sightings using the form below, or email sightings@lnhg.org.uk with the details if you prefer.  If you are not sure of the identity of your plant, please send a photo to sightings@lnhg.org.uk, or put one on the LNHG Facebook page.
 

Date of sighting 
Location 
Grid reference 
Name of finder 
Your name (if different) 
Email (not needed if I already know it!) 
Any other details, e.g quantity, habitat    


 

By filling in this form you agree that the information contained in this form may be collated and disseminated manually or electronically for environmental decision-making, education, research and other public benefit uses in accordance with the LNHG data access policy.  Your email address will not form part of the record and will not be passed on to anyone.

Carl Farmer
LNHG Biological Records Manager


Note you can still send in records for past species of the month.  Here are the previous May species:

May 2018 - Speckled Wood
May 2017 - Bee-fly
May 2016 - Common Heath
May 2015 - Orange Tip
May 2014 - Large Red Damselfly
May 2013 - Early Purple Orchids
May 2012 - Small Copper
May 2011 - Green Hairstreak

Last month's page

Complete list of Species of the Month



All photos and other content copyright Carl Farmer except where stated.  Mouse over photos to see credits.