Most seed packets are not 100% pure and you will find a hitchhiker or two amongst the intended seeds. This plant is what I term one of the Weeds Of The World. Irregardless of knowing its exact identity or not, I suggest that you remove it. Best Regards!

If that is all you've got growing and no actual cucumbers I'd take the packet back to where you bought it from and complain.

I thought at first that it was arugula (Eruca sativa), but what I can see of the flowers doesn't look quite right. Any chance of a close-up of the flowers to confirm, Shep?

Usually these seed packets will contain information of the purety of content. If not, or if it says 100% pure you might get your money back or the seller might say those plant seeds were dormant in the ground to begin with and they germinated when you brought them to the top in planting or were present in added earth from another source. Plenty of ways to wiggle out of a refund but then the seller would loose a customer.

Hi guys thank you all for your quick response.. .so basically what I have here is a weed and it has to come out right????...

Tugbrethi...i just added close up of flowers ...weed or no weed???...thanks

Weed - it looks like Field Pennycress, Thlaspi arvense. If it is it will have a strong smell when crushed.

Well, it's not Field Pennycress, which has plain, simple leaves, and it's also not Arugula, which has cylindrical seedpods. To be safe, I wouldn't try to eat it, so free the space up for a fall crop. It's definitely a member of the mustard family, so consider it a cole crop for your crop rotation schedule.

Well guys just ripped it all out and straight in compost bin..replaced with tomato plants ...so all good...thanks everyone again for your help...

For future reference, Phillip, lemon cucumber seeds, like all members of the squash family, are shaped like pumpkin seeds, but a different size. Quite different from any member of the mustard family. If what you planted looked like small pumpkin or cantaloupe seeds, they didn't come up, and this did in its stead. If the seeds in the packet were tiny round seeds, some one at the seed company made a mistake.