A medium pressure power washing does the trick. However the old tried and true method is to put some bleach and water into a bucket, get a stiff brush on a pole, and build some muscles. Use lots of water once done to dilute any bleach that may have got onto the grass or plants.
The choice is yours. Be cool with all sorts of noisy tools, or go low tech and use a brush.
Great answer! LOL
if you choose the bleach method, let me recommend Clorox Outdoor bleach. I bought mine at Home Depot in the home chemicals aisle. It is made for outside uses only. It's a little thicker and will "stick" to outside surfaces better than regular laundry bleach. Well worth the additional cost (a few dollars more than regular bleach).