Comfrey..

..a useful herb/wildflower. The leaves can be added to your compost heap as an accelerator or decomposed down into a liquid and used as a fertiliser.
This comfrey stands at 5' tall and is an absolute thug in my vegetable garden. The first seed wafted over from the fields or was dropped by a bird 10 years ago and I thought great! I'll let it grow! (I had only experienced a white version that grew 1' tall before.) that first year it grew to 3' tall and fairly compact but the following year there were off-shoots and seedlings and I've been battling with it ever since! There's only so much compost one needs!!
The roots go a long way down - more than a forks' length so it's very hard to dig up!
Beware this giant - get the dwarf version!!!