Keep all those worn out brushes or those that have spread because they were not cleaned properly, or those that have some parts of the brush welded together with hardened paint. These brushes are great for creating the interesting random marks you might need for such things like the painting of grass with acrylic/oil paint or creating interesting textures or patterns. Don’t forget the other end of the brush also. The wooden bit is useful for creating those tiny branches at the end of trees when using watercolour. Use a small blob of water colour at the end of the branch and drag it out with the wooden end of the brush, twiddle it a bit to make it look good!
Painting grasses and random flowers in a landscape can be tricky, but here are a couple of ideas that might help. Cut a piece from an old pair of corduroy trousers and make a simple pad from it, making sure the lines are not striaght. Dab on some colour and dab the pad on the canvas/board/paper and instant grass. This can produce rather organised grass which would not be suitable if you are after realism. A more random method is get a length of string paint it and and slap in on the canvas, it can create some interesting effects. If you tie a knot in the end of the string and paint the knot liberally in red and the string in green flick in onto the canvas and you have instant poppies. More in the Monet style than Andrew Wyeth, who? He died recently at the age of 91, he was a great realist painter. Google him!