With that kind of rain, Andi, the soil would be saturated, so again, it's not the heat that causes the problem, but that the plant cannot establish its root system.
The only analogy I can come up with when I talk about the root system of a plant is to think of it as kind of like a vital body part. The roots take the energy produced by photosynthesis to feed the plant and they also send moisture up to the upper parts of the plant to hydrate it so that the leaves can function properly. If the root system is not completely functioning for any reason, then the plant will be stressed.
So, if a plant is root bound, that can cause stress. If it is water logged so it can't grow what I am calling new feeder roots ... those are the tiny roots that absorb moisture and nutrients from the soil and feed the rest of the plant ... the plant is stressed. If the soil is too dry and the roots can't grow, the plant will be stressed. If there is major root competition ... more stress.
We tend to think most of the plant failures are caused by the things we can see and feel, but often it's what is happening under the soil when we first plant a rose that has the biggest impact on its health and survival.
It's always a judgement call that the gardener has to make depending on the condition of the plant in the pot and the climate he or she is gardening in and what is happening in that climate in a given year.
