Saturday, 6 May 2017

Early Purple Orchids at Westwell Gorse, and May Moths

On Thursday, w

e went to Westwell Gorse where there is a small patch of Early Purple Orchids (Orchis mascula).  I am keen to see what the fruit set efficiency of these orchids is.   We carried out an exercise last year and got a figure of 32% of flowers setting fruit, but the result was unreliable because of weaknesses in our survey technique - too large an area, too late in looking for fruits.

This year we restricted the survey area to roughly 4m x 6m, where we counted 83 plants, measuring height and the number of flowers.  Individual plants were not marked but the plan is to go back in 6 weeks or so, and count the number of pods on as many of this cohort as we can find.


Naturally, larger plants have more flowers.   The average height of the flowering spike was 16.5cm, significantly less statistically  than the average last year of 20.3cm, probably because of the very dry period from late-March through April.   The number of flowers per spike averaged 9.8, higher than last year but significantly so.  Hopefully in addition to fruiting efficiency overall we might get some data on whether plants with more flowers have a higher percentage fruit set.

Incidentally we did not see any pollination activity, probably because it was dull, cool, and breezy.

My wife made an interesting observation.   Earlier in the week we were examining a nice population of greater butterfly orchids in the Chilterns.   There were very roughly 3 non flowering to each flowering plant (or to be exact, in bud).  With the early purples, there were few non-flowering plants, probably no more than 1:1, though small plants are difficult to spot in the sward.

Moth trapping at home has been a bit hit and miss.  Since the start of the year only a modest 16 species; on Thursday the catch was  moths, 3 species - a breezy, cool night.   But there was this rather grand Nut-tree Tussock.


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