Harcourt Arboretum

In the last couple of weeks the leaves have finally started to take on their autumn colours. Alas, this was rather too late for us to appreciate them during our visit to Harcourt arboretum.

Eight of us braved the traffic on the Oxford southern bypass (it gets worse) and met at the arboretum on the morning of Sunday 15 October. It was a bright, dry but cool day and the overnight temperature had dipped below 0C for the first time this autumn. Until then nighttime temperatures (at RAF Benson) had been over 10C and, because leaf colour depends both on day length and temperature, it had been just too warm for the leaves to start changing.¹

Although the leaves were predominantly green, some of the maples in the Acer Grove had started to change colour and would surely be spectacular by now. Despite the disappointing leaf colours there were other things to see, especially fungi.

Just after the Acer Grove we surrounded a chap photographing a rather fine immature shaggy parasol, Macrolepiota rhacodes; everyone took pictures on their mobiles! He didn’t seem to mind the invasion and was happy to chat. The others moved on but I stayed and we had (for me) a very informative conversation about macrophotography and the finer points of ‘focus stacking’ – the way to get very sharp macro images, if you have all the (rather expensive) kit.

There were quite a lot of parasols, Macrolepiota procea, present, especially in the meadow area. The woodchips used as a mulch at the bases of some of the trees were a rich habitat for fungi, particularly redlead roundhead, Leratiomyces ceres, and a coral fungus, possibly upright coral, Ramaria stricta. Some spindle toughshanks, Collybia fusipes were growing at the base of a huge oak. (Note: these ID’s are tentative and should be confirmed by an expert!) There were probably other fungi to be found had we looked more closely.

So, although the leaves weren’t performing, we did find some colours and species typical of autumn and it was a lovely sunny day for a change.

John Cobb 1 November 2023

1. The Woodland Trust has an article about the dates that leaves change colour (see here). They say that this can happen any time between September and November, so mid-October seems like a fairly safe bet. The date doesn’t seem to be influenced a great deal by global warming, although the Woodland Trust’s phenology studies suggest that the date when some species of tree completely lose their leaves has got later by a few days between 1999 and 2022. Nighttime temperatures, however, are important for leaf colour. Between 2013 and 2022 the first date that the temperature fell below 5C at Benson was between 1 and 9 October; this year it didn’t happen until October 14. It seems that we were a bit unlucky. ↩︎