Erythronium grandiflorum (hopefully)

Yellow avalanche or glacier lily. From eastern US. Earlier than E. revolutum flowering in early September. to 30cm high, with elliptic, bright green leaves. In spring, upright stems bear 1-3 large, nodding bright yellow flowers with spreading, recurved petals.  Woodland conditions are great in warmer climes, lots of sun and moisture, even wet and soppy, while flowering. Once it is dormant it can tolerate drought and denser shade. It does not like excessive moisture in its dormant phase. It must have great drainage to prevent rotting. In my garden they are fine, 20-30cm. 

For all these reasons I believe my colonies to be this species. However I am not much good at this sort of thing. So it could be E. tuolumense.  Supposedly this is easier.  I will be carefully checking in spring, now I realise there may be doubt.

Native habitat photograph: Bill Bouton from San Luis Obispo, CA, USA, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons

Closeup: Same photographer - Avalanche/Glacier/Fawn Lily, Erythronium grandiflorumUploaded by Orchi, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=20376534