You have to be careful with any kind of greens when taking Warfarin, Coumadin, etc., even salad greens. Patients are warned about that when they are started on the medications.
A bit off topic, but I wasn't the first one to bring it up----
It's not just greens. Cauliflower (a brassica also) is fairly high in Vit K.
But head/iceberg lettuce has less Vit K in it than Romaine. So, with some care salads can be eaten if you use head lettuce.
I'm the Vit K Kueen!