A spirea that grows into a tree?

Zone: 5b. Toronto, Ontario (East York). South facing

Greetings. There are some seedlings growing in one of my beds, right through a goldflame. There are quite a few actually, only 3 have gotten away from me. I thought they were spirea, except they don't behave like spirea. For one thing, you can't kill them - the root goes down forever and can't be pulled out. No matter how many times I cut them (or mow them as in the case of the ones in the lawn) they keep coming back. In itself, that's not very spirea-like as I have pulled PLENTY of spirea seedlings out from around my garden with no difficulty. I have 15 spirea shrubs of bridal wreath, goldflame, goldmound and Anthony Waterer varieties. LOTS of seedlings have been pulled out, but none others behave like these ones.

The kicker is that the 3 that I couldn't reach when they were little (they were under the goldflame at the time) they looked identical to the one in the lawn. But, now at 2 yrs old they are over 6 feet tall, their leaves are larger than spirea but the same shape. The leaves come out slightly red in the spring, just like the goldflame spirea. The "lawn" ones have gotten to over a foot tall before I've gotten around to lopping them off in the spring, so they are the same as the tall ones.

I thought they might be linden trees, as there's one in the neighbour's yard. But, the leaves aren't the same shape at all and the linden's leaves don't come out slightly red in the spring, either.

Any ideas? Thanks in advance...veronica

 

I would suggest that its a Plum.

 

Hi

Or perhaps root stock from somthing?

 

Sprouts from rootstock of purpleleaf plum in second photo.

 

Interesting! Ron, are you suggesting the unknown hard-to-kill plants are sprouting from the roots of the standard trunk that the the Purpleleaf Sandcherry was grafted onto? I googled to find reports of sprouting but nothing came up. Still, it makes sense to me, due to the location of the little plants and the standard.
Any idea of how to get rid of these suckers/sprouts? Is it just a case of picking them off when they appear? I could get a longer 'dandelion' spike to cut the sucker plant deeper down, but I'd have to be very careful not to harm th Sandcherry roots. Dabbing the leaves with herbicide is out of the question if the roots are connected to the Sandcherry. Any advice would be appreciated.