In my garden I have several of the plants shown below. At first I thought it is a weed, but then it was quite robust against cold weather and continued to grow slowly:
The leaves are long and sharp, the example shown below is about 30cm high and the leaves are 8-12cm long. I haven't seen a flower or anything for 8 months and the temperature has varied from -3 to 35 C over this period (including rainy and dry times) and the plant hasn't died yet.
Click to enlarge
I believe it's Euphorbia Lathyris, commonly known as Mole Purge or Gopher Purge. Here's another article with a better picture.
If I've identified it correctly, the spike will grow to about 120cm (4 feet) and the flowers will be uninteresting (small and green). It spreads, but not aggressively; I'm familiar with it from a friend that grows it as a repellent for moles and it hasn't taken over his yard, though I know he's had it for several years. It's easily controlled too; seedlings in the lawn just get mowed down, and in flower or vegetable beds they pull up easily by hand when they're small. (At 30cm, yours are probably starting to develop a proper root system, so may not be quite so easy to pull.)
The sap is an irritant, so if you've got children or pets, you might want to remove the plants for safety; be sure to wear gloves if you do.
I'll confirm that ID. I have it growing in my yard in Chapel Hill, NC. If their numbers become excessive, they are easy to pull from the ground. The sap is indeed nasty, so be careful when handling it.
