Large seedlings - about 3-4cm across both seed leaves.
No interactions present.
Seeds can lay dormant for several years - I pulled the last seedling here in the garden at least 4 years after the last plant flowered and set seed.
A dreadful plant which crowds out native species. It has tremendous survival abilities besides the seed bank: if cut down it will sprout again, and if pulled up it can re-root from the nodes on the stem. As a final trick, it can produce cleistogamous flowers (they self-fertilise without opening fully).
I've pulled a good few tonnes in my time, sadly to little effect. Goat browsing is effective, but doesn't deal with the seed bank.
Dreadful is a little harsh - it is actually a very impressive, decorative and easily grown garden plant. If there was a sterile clone, that should be in every garden.
The problem is that it is hardy, produces plentiful seed and is a typical busy-lizzie in that it can be easily propagated by just about every method known, including taking of very small cuttings.
Great plant - wrong place ie. in the UK.
Lat/Lng: 53.0, -1.3
OS grid ref: SK5245