there's a strain of Russian Comfrey that has sterile seed, so won't spread that way, but still has invasive roots - all Comfrey spreads invasively from the roots, and most spreads seed along ahead, so it needs to be in an area where you can contain it's vigorous growth, either with a solid border it can't penetrate or a bare patch that can be kept cultivated .... even if you're 'harvesting' it for use, it won't be long before you can't keep up with it

Bill

It depends. In some places a plant could be invasive while in others it would be so slow growing that it will not spread. In British Columbia comfrey may be very well behaved while in Australia it may not be. Here, I planted some on the highest part of the property and it was not happy there so it did relocate, on its own, to a lower spot where the water table is closer to the soils surface. Apparently it is happy there and has grown from one clump to now 4 in 25 years but is not even trying to grow anywhere else, that I've been able to find. I let my comfrey blossom, because the bees really like it, and the leaves and seeds are harvested for the compost and I have not found comfrey growing from where that compost is plunked down, either.
Plant yours and watch what happens.

what is comfrey and how does it help compost?

This is one of many sites which praise the comfrey plant's properties for compost and mulch. Also good for fertilizer tea to be used on potted plants and veggies.

Here is a link that might be useful: the organic gardener

Hello, growing Comfrey plants in pots or containers seems to make a far less valuable resource than growing in the open. The reason is that you will have to feed the pot soil.

Comfrey plants growing in the open soil are apparently able to access potassium from deeper soil levels. This nutrient reserve would not be available to pot grown plants.

Last year I planted 3 Comfrey plants along the sides and corners of a new timber compost bin. The idea is that the Comfrey will benefit from and recycle nutrients draining out of the heap as well as being very conveniently placed to cut the leaves to use as compost activator.

Happy gardening.

I love growing comfrey! Mine doesn't spread because we don't have rain in the summer, and it wilts if left unwatered. I planted the first one under the hose bib to catch stray drops of water, and it's very happy. It gets 2.5 ft. high and 3 ft. wide in the summer, with many flowering stalks. When it starts encroaching on the path and getting in the way (it's at a community garden), I cut off the ratty leaves and whatever's in the paths to make comfrey tea for my plants, or to put directly in the compost bins.

To make comfrey tea for the garden, I fill a bucket with comfrey, cover it with water, and let it ferment for a few days. I think the key to controlling it is to use it. In northern Calif., at least, it can be cut back 3 or 4 times a year. I have been growing it for 2-3 years at the edges of my garden and last year I added 3 more plants next to the compost bins, and it has not spread, but I do cut it back often and use it either for garden tea or directly on the compost pile. It has been so well behaved that I'm always looking for more places to add it. Maybe under the raspberries? Maybe a whole row of it along the compost bins, instead of a few separate plants?

Comfrey mines the soil and brings up minerals, so it's not so effective in containers.
I did keep one plant alive in a container for a year or so, but it wilted sooner than any of the others and was not nearly as robust -- it hardly ever had enough (or big enough) leaves to harvest. I planted all of mine from small pieces of root -- a three-inch section is all you need. But if you want to dig it up, you will never get all the root pieces. I dug up one plant last year so I could send off some root pieces, and then I potted up the tiny rootlets that were left over. They all became plants! (And the original plant recovered quickly, too.) I've heard it can be eradicated by deep, persistent mulching in this climate.

In late fall or winter, the foliage dies back. A couple days ago I saw tiny green nubs coming up, which I will have to check on tomorrow morning because we are supposed to have another freeze tomorrow night.

An easier to manage compost crop is borage. It's a self-seeding annual with edible blue flowers and makes a wonderful border, though it does get mangy after a while. Even in my mulched garden, I get borage seedlings, and if I want more I can always find some growing as "weeds" in other parts of the garden. It gets up to 3 feet high. The bees love it -- it's a great plant to draw pollinators to the garden. I love eating the flowers (check for ants first, they also like the drop of sweet liquid inside the flower).

Finally, I have a big cardoon growing as a compost crop. I'm not sure I like it, mostly because it gets farmed by ants and it's quite vigorous, almost weedy. I let the foliage die off in the fall, and now it's already 3-4 ft. high and wide! Certainly it's a pest in the wild lands, where it thrives and spreads with no irrigation. In my garden, it's useful mainly because it thrives on neglect and produces greens for the bin when most other plants are dormant. Also, it's a big green presence, and I like the flowers (purple thistles).

I also use yarrows to encourage pollinators and beneficial insects. The yellow yarrow (Moonlight) spreads even with little water, but it is easy to dig up. I divided one clump into 24 plants a year or so ago, then I gave away a bunch of those after they had grown big, and I still have a wealth of yarrow. This year I'm removing some of the Moonlight and adding more of the species (Achillea millefolium), which is supposed to be more invasive but less drought tolerant, so I will see how it does.

I'm interested to hear that comfrey dies down to buds in Californian winters and yet remains as an evergreen clump in my UK garden. (Both Symphytum grandiflorum and S officinale, our native one) That seems very odd. Certainly frost never harms it here. I grow mine in a scruffy area down by the compost bins and rhubarb patch where the soil remains moist all year round. Our native comfrey grows along river banks so that shows its preference. Both will spread but can be kept in check by being sheared down to the base several times a season for their leaves and by having any runners pulled off as soon as you see them. Also give it a patch on its own as you'll never disentangle it if it gets in amongst your other plants.

At my former house, I had it planted next to the bins in its own little patch. I shudder to think of how far it's taken over without me to keep it in check (we moved 3 months ago).

I think I'll probably plant some in the soil at my new place, but will sink boards 2 feet deep around it to keep it in check. It's worked for mint, so it will probably be perfect for comfrey.

I have comfrey in three places here in Dallas but it must be too hot and dry for any invasive qualities, in fact if it doesn't have some shade it will not do well at all. In winter it goes back to limp small leaves until spring. I use it to control psoriasis. I run the leaves through a juicer at harvest time (late spring late fall) add a little rubbing alcohol as a preservative apply to affected skin

I must be the only one who can kill comfrey. Mine just kind of died ... :-(