Sycamore is a highly vigorous tree and many man hours and pounds are spent by conservation bodies in clearing it. I'd never really thought about why this was so, until this spring when our back garden has been almost overtaken by Sycamore seedlings. Every week for the last month or so, we've pulled up anywhere between 20-60 new seedlings from the lawn, much less from the borders and one or two from paving cracks (our garden tends to be very damp through the winter so 'hoovering' up the seeds and seedlings with the lawn-mower is not really possible). The nearest stand of Sycamores to our garden is only c40M away, but the helicopter seeds obviously did very well last autumn. Just goes to show that any untended land with nearby Sycamores would quickly become an self-set Acer acre.
Nasty invasive little bastard.
Moments later, these vermin were unceremoniously executed.