Our lupines (also called lupin in the UK) were done flowering for the year and were harboring an aphid infestation, so I mowed them. Are the "stumps" likely to regrow next year or do the plants rely on gathering a lot of energy through the foliage to survive the winter?
(They were originally weeds scavenged from a roadside that we multiplied here over a couple of years, so it's not a major loss if they won't regrow.)
It's hard to tell off-hand if they'll regrow or not, as it depends on a lot of factors such as how low did you mow it, how much damage have the aphids caused, how well developed is the root system, how bad is the winter, etc.
Usually, plants need to "prepare" for the winter by shutting down their flower/fruit production and stockpiling energy, and taking steps to reduce their energy consumption. Given that the plant has not had much chance to do any of this, I wouldn't necessarily bet my money on it. However, since there is some more time till winter, perhaps it might work out too.
I get the same problem with my lupines every year and about this time I harvest the seeds and cut them back. They always grow back and often I get some flowers again in September. I cut mine last week. That was the end of the first week of July. Good luck.
I hope they do well cause I just cut back a huge plant early season because it was blanketed in aphids 😔 Most years I cut mine back in early fall and they do just fine. They have a long tap root so that part should do ok. With that said, I'd overwinter some oil on the plant at dormancy and kill any overwintering eggs for an aphid problem
