I left my cool basement office the other day (three colleagues and I share the the former scullery in the service wing) to visit the Walled Garden on a triple h-day last week to speak with Kim Johnson--the Walled Garden gardener/curator. (Kim's been responsible for the world-famous Walled Garden for over a quarter-century now). I was curious to see what plants were thriving during our summer of high temperatures and little to no rain.
By its very design temperatures in the Walled Garden can exceed 7-8 degrees above the ambient temperature in the rest of the Gardens. As Kim pointed out to me--even though the Walled Garden is irrigated, no irrigation system is truly perfect and with the multiplicity of plants and borders, hand watering and additional sprinklers are a necessity. So Kim as well as the other gardeners have been at OWG from early morning to late night moving sprinklers into various areas.
Kim also pointed out several plants that are thriving in this weather pattern (hot, hot & dry, hot & humid). As you can see in the photos the Walled Gardens' Zinnia and Rudbeckia speciosa (coneflower) are doing well since their native climate is the scrub and desert grasslands (zinnia) and prairies (Rudbeckia).
And of course the tropical plants--Nymphae (waterlily) and Nelumbo (lotus) love the hot & humid spells.