I don't know if I would rely on spray of any kind... your best bet is in the selection of mulch itself... big hardwood chips will never mat down whereas if you water and walk on smaller chips of mulch they will soon form a hydrophobic layer over the surface of your slope...

Is anything growing on that slope???

 

Ive done some steep slopes what I have done is buy that p.o.s. black plastic edging that sticks up about 2 inches and put that in across the slope every 4-6 feet down and cover it with mulch it is hidden and provides a shelf to hold the mulch up

 

i've seen pics on here of mulched slopes. i just don't get it really. if you did that here one downpour and all the mulch would be washed away. even if you put edging to hold it, it would wash right over the edging.

 

On slopes, we install at a couple inch depth and then beat the snot out of it with a pitch fork as we install. Have almost never had a problem with mulch washing away... as mentioned before it will form a bit of a crust on top that water will run down for the most part.

 

i've seen pics on here of mulched slopes. i just don't get it really. if you did that here one downpour and all the mulch would be washed away. even if you put edging to hold it, it would wash right over the edging.

We are getting more and more heavy downpours and I am finding more washouts where there never were any before.

Plant ground cover!!!