21-60 days. Often called Mustard Spinach or Spinach Mustard, it’s neither mustard nor a spinach. Japanese cooks call it Komatsuna. Whatever its name, this delicious, leafy green is very easy to grow and produces flat, smooth, dark, glossy green leaves that can be picked as baby greens or grown to full size. Hardy and productive plants are slow to bolt and tolerate cold, heat and dry conditions.