Kale seeds I've seen are little black seeds. Arugula are generally brown.

However, there are probably different cultivars etc....

Good news is, Arugula Sprouts REALLY fast (it's nickname is "Rocket") and gets it's true leaves REALLY fast. So it's not that long before you'd be able to tell which one is Arugula. You could plant them both, and just pull up the Arugula when it comes up.

All brassicas are dicots. The emergence features two basic leaves. {{gwi:23382}}
I don't grow arugula. but would expect it to emerge in a different form.

Arugula is a mustard, and its basic leaves are indistinguishable from kale. Along what JW said, if the seeds are cast, and can no longer be distinguished between brown and black, one could try to keep the tray at 45F. Only arugula seeds will come up.

Do you also dislike cooked arugula? It has a neutral flavour (to me) and can be subbed for spinach in cooked dishes.

I'll try separating the brown from the black seeds and throw out all the brown ones. Once they emerge I can't tell them apart until the true leaves start to take shape. I have some about 6weeks along in the cold frame but I wanted to start some more to fill in the gaps. I was trying to eat it as I thinned it out today but just couldn't do it.

And Dr., I'll try it cooked, my wife usually puts spinach in quiche so we'll try arugula.

Thanks for the help.

Too late now but I have to ask the obvious - why didn't you just buy kale seeds rather than a mix? :)

But since that's what you have - do you intend to eat the kale as a cut and come again baby salad veggie? Or do you want to grow it on to full size and eat as greens? If it's the latter you could just thin out the rocket at a later stage when it is easy to distinguish. Separating seeds sounds like an awful lot of fiddly work compared with pulling out plants when they're a few inches high and easy to id.

Well, I didn't provide all the details because its kind of a long explanation but since you asked....I got a mix because it was all I could find around Christmas which is when I started the first round indoors. Now I need to start some more to fill in the gaps in the grid when I pull the arugula and I don't want to grow more arugula. Also, the mix contains 6 other lettuce varieties and makes a really good salad but I have to pull the arugula and endive out, but endive seeds are easy to distinguish. I enjoy fiddling with seeds. I start them indoors so it's not like i'm throwing handfuls of seed into a bed, just 40-50 seeds in a 2-liter bottle. Space is limited on my sunny window stools and beneath grow lights so I end up planting them out as soon as possible, maybe 2-3 weeks after planting which means true leaves are not always present or developed enough to differentiate. And lastly because space is limited in the cold-frame. I want to make sure every spot in my grid spacing is filled.