Ok, so this plant is possibly in this forum a thousand times but I had not found specific information I need and I would appreciate help. Specifically, I need cultivars which are non-invasive and I need to know the plants life expectancy. I have been looking at "Blocking 14" strand but I could not locate a life expectancy. Also, what would be helpful is a reference to companion planting of comfrey - mostly spacing requirements, thanks.
How do you define 'invasive?' Comfrey no matter what cultivar will spread out depending on the soil and conditions but it's not too difficult to keep it in check depending on where or how you plant it. It's not for the most part a crazy fast spreader that takes over most everything at high speed but if it love the soil and conditions it will grow in every direction. If you're wanting to keep it in a small space I know people who plant in like they would mint with in soil barriers, like a buried tire. The most important thing to know though that it is 'invasive' in the sense that it is very hard to get rid of once it's been planted. You have to get rid of every single root bit as even the smallest bit will grow a new plant. I know of people that wanted to get rid of it and the only thing that worked was to set a few pigs into the patch.
As for life expectancy? It will last pretty much last forever. Like a lot of perennials some of the older plants die down in the middle but there are always new plants and offshoots replacing the old ones. You don't plant comfrey in order to have single plants. You plant for a patch. Single plants will eventually turn into an area full of plants of varying ages. I have a neighbor who has a comfrey patch on his property that was there when he bought and is still going strong 20 years later. He hates it and has constantly whacked and mowed it down. Though he's coming around now that he's learning that it's great for mulch and compost.
To give you an example I planted one plant in a spot 3 years ago. That 'plant' is now a patch about 4 by 3 feet wide.
Thanks for the detailed information. I am curious when you said that your comfrey spread 3 ft diameter in 3 years. At what rate will the spread slow down. After planting, hypothetically, will I have in 20 years a 20 ft diameter patch that I cannot remove?
I really think that it depends on where you plant it and what you plant it in. This plant went it a bed that was basically virgin soil with nothing else planted in it. It is also extremely fertile soil. I noticed though that it only spread out to the edge of the bed and stopat the edge of the sod so it spread more one way then the other. It did seem to stop in the other direction when it budded up against the valerian that was next to it.
It is a large perennial so getting to 3ft in that number of years isn't outside of how other large perennials grow.
I only just planted some comfrey in the fall and I abused it prior to getting it in the ground so I don't have first hand experience yet, but Hills recommends planting root cuttings on 4 foot centers if one aims to establish a large patch or field of it. He makes it sound like it isn't a crazy spreader. It needs to be scrupulously weeded and well fertilized as it establishes if one wants spectacular yields. He makes it sound like it will only spread out to about 4 or 5 foot diameter circles from a single cutting and after a few decades the center of that spot will decline in productivity.
If you're interested the whole book can be found at Steve Solomon's digital library soilandhealth.org
There is a whole book written about comfrey that you can download for free
just use the find function to get the link