Hi & Welcome.
It looks like a tree that has popped up in two places in my garden, so I will be looking forward in how to get rid of mine that are getting quite big Grrr! My two are growing amongst my shrubs which is annoying.
Just seen this on the web.... Roundup Tree Stump and Root Killer 250 ml Liquid Concentrate Weedkiller, there are others too.
Jackie

Does anyone nearby have a wisteria as they look very much like the leaves to me?

They aren't lawn weeds. Pretty sure they are tree suckers from somewhere near. Have a look at surrounding trees and large shrubs for one with matching leaves. If its on your land you can cut it down and treat the stump with a deep root killer following the instructions on the pack.

Agree with everyone else - there's a very large plant somewhere (wisteria or a tree) and its roots are under your lawn and they're now producing sucker growth. The only way to deal with it is to expose the roots which are causing the trouble, drill into them at intervals making holes about quarter inch deep, then filling the holes with SBK, a brushwood killer, cover and leave. Once they've died off, you'd probably need to replace your lawn.

Not only does it mean wrecking your lawn, it also means that whatever the roots are coming from will quite possibly be killed eventually. This doesn't always happen, sometimes these extra roots die and the main plant survives, but there is a risk.

If you don't deal with the roots in this way, you will have more and more sucker growth appearing in the lawn, and the only way to manage it otherwise is simply by constant mowing. It won't be a problem in late autumn and winter, just spring and summer.

Thank you all.

I will investigate nearby shrubs/trees further and try to report back. One neighbour's garden is very overgrown so it may have come in from his side :(

Looking again, I don't think they are Wisteria leaves, nor Laburnum- they could be Robinia pseudacacia though. They could just be off a root stock too - which would mean the topgrowth of whatever it is does not have the same leaves.

Could it be a Sanguisorba ? They've got leaves like that. Maybe it's come from spreading seeds.

Definitely not Sanguisorba, they have toothed leaflets. Robinia pseudoacacia seems very likely to me, it's well known for suckering everywhere.

Yes they do too ... lol I'd forgotten ...

I think false acacia too.

Looks like an ash tree to me. I've got one right next to my fence in the garden next door.

Pretty sure it's not ash - its got composite leaves, but many of these are rounded at the tip, ash tree leaves are pointed.

Have a look at this site....http://treedoctor.anr.msu.edu/ash/ashtree_id.html

It may help :o)

Our ash trees don't sucker though, just spread their seedlings around.

Is it a Rhus? I know they produce suckers by the thousand which will even come up through asphalt.

Never seen a Rhus with those rounded leaves though