From what you described sounds like maybe root rot. I always thought of the cardinal flower as a bog or marsh plant though, so I dont see how that could be.

Take a look at your roots on the one you pulled up, are the roots brown and mushy or do they tend to look like something has eaten your roots??

These grow wild here. I've seen them growing in a ditch in the middle of the water. I don't think they rot that easily. I bet it's something else. Do you have voles? My friend lost tons of plants to those monsters.

if we are discussing Lobelia cardinalis, it is plant that needs poor drainage such as bog area to grow well. It might be some virus or fungi. Good news they are easily winter sown so you could start tons of it.

One other point - Lobelia has a very shallow root system - it's possible that a large plant could get weighted down by rain water and if in loose soil, would flop.

Insect damage perhaps. There is an ant like insect that I do not know the name of, very tiny and it seems to like the roots of plants to make its nest and lay eggs. They seem more prevalent in wetter springs and a stem will just fall over and you can pick it out without almost any roots left. I use soapy water once or twice to get them gone, but they may pop up under another plant sometimes. This year with all the rain they seem to be in more places. If you get them in time the plant will recover.

Voles and moles are undermining a large amount of my gardens this year. Yesterday a large clump of Oriental lilies all started leaning. Many of the Echinaceas and Rudbeckias are drooping. Penstemen and Monarda are also in peril. So far the cardinal flowers are ok, but it is just a matter of time I am afraid. Have tried many methods to get rid of these rhodents, but nothing is helping much.

Moles undermine your garden but do no harm to plants. Voles use the mole tunnels and eat plant roots. They are the problem, not the moles.

Voles do use mole tunnels, but also will make their own. Their diet is made up of a large variety of plant roots and they are the culpret most often when a plant falls out of the soil and dies.

However, enough mole tunnels in and around the root zone of plants can cause to plants loose their firm grip in the soil, especially seedlings or bedding plants and shallow rooted ones. This may cause the plant to wilt by affecting it's ability to take up moisture and in some cases, actually fall over.

This exact thing is happening to mine as well, it does look like root rot since they pull out of the ground easily and are mushy but that doesn’t make any sense since they are marsh plants. I live in georgia and when I first noticed it I thought I was over watering so I limited the water I gave them but its Been almost a month and they are still suffering. My best guess is that it‘a either under watering or some kind of fungus/disease. I’m going to attempt to give them more water and spray them down with a fungicide and see if I have any success, if you’ve happened to figure out what went wrong with yours I would greatly appreciate any advice.

Is there enough time to collect seed from Lobelia before a frost? My one plant just started to bloom and I'd like to try winter sowing more. It is a hybrid with dark leaves.