The method I would use to clump the vegetation together is by tweaking the 'radius' value in the particle's children tab. Note that children type must be set to 'Simple'.

[![3d viewport radius][1]][1]
[![Children menu][2]][2]

This is the most simple and easy way to achieve the 'bush' look, but if you want more freedom and customization possibilities, a density texture is the way to go!

For example: I'm using a cloud texture to control where the hair can grow and where not.

[![texture 3d view][3]][3] 
[![clouds and ramp][4]][4]

For this example I'm using a cloud texture, but you can obviously paint a texture by hand and gain 100% control.

Make sure to check the influence of your texture:
[![density check][5]][5]

And assign it in the particle system as well:
[![enter image description here][6]][6]

A fundamental difference between these methods is that by using a texture, you're actually 'cutting' out portion of your scattered hair. By using the radius option under the children's panel, you control where the hair-particles are going to be scattered. So your amount of particles in the settings will be higher when using a texture.


  [1]: https://i.sstatic.net/dsytz.png
  [2]: https://i.sstatic.net/9tzNF.png
  [3]: https://i.sstatic.net/wfNXw.png
  [4]: https://i.sstatic.net/aXQmR.png
  [5]: https://i.sstatic.net/OG7S5.png
  [6]: https://i.sstatic.net/AYlRQ.png