Do you know this plant?

[Thomas G]

Well, I hope this is the right place to ask:

I'm wondering about what this plant might be, as it is immigrating into the garden.

[Frank Fremerey]

It is a baby tree and I have seen these sprouts before.

[Bjørn Rørslett]

It is the invasive weed Fallopia probably F. japonica that can grow to a height of 4-5 m and create extensive stands so dense all other vegetation is killed off.

Get rid of it quickly.

[Fons Baerken]

roots deeply but running roots break easily,
any rootbud left unattended well you know
i can think of a few weeds hard to get rid off
on the positive they flower abundantly with white flowers

[Bjørn Rørslett]

The dispersal, at least in the earlier stages, is purely vegetative by rhizomes and runners. In a few years, the damage is done and eradicating this tenacious plant is very difficult. There are several closely related species involved and they hybridise so much of the stock may have poor seed set, but the efficacy of the vegetative dispersal more than makes up for that "defect".

Two years from now, your garden might look like this;

[Thomas G]

Thanks so much!!

Uuuh, that's unhappy news.
Sounds like I'm supposed to bring the gardener weapons for these undesirable aliens.
ASAP.

[John Geerts]

I have a family member as well in the garden. Digging out the ground, and replacing it, did work well, for a year or five. But as Fons said, the roots break easy, and if one single tiny  piece of root remains, it will eventually pop-up again.

[Thomas G]

All right, I will be very careful.

[Bill De Jager]

Thanks for the the heads-up, Bjørn.  It's apparently now a widespread and serious invasive in the U.S., not yet in my county but quite close by: http://www.eddmaps.org/distribution/uscounty.cfm?sub=19655.  Reynoutria japonica is a synonym.  I'll keep an eye out for it.