When it comes to undergrowth we can quite literally find ourselves with quite a thorny problem, and painting it seems no easier: how do you cope with all those similar, repetitive and often mundane shapes? Firstly, don't dismiss those mundane bits of a scene: in a composition we need quiet, mundane passages in order to make the exciting bits stand out, so they are important parts of a painting. Secondly, when you are out in the countryside don't forget to gather material like this for use in a painting, in sketch and photographic form. Now and again concentrate on these less dramatic features and deliberately record them carefully.
This photograph taken on Strumble Head in gentle spring sunshine will give you an idea of what I mean by recording the less dramatic. Posts, boulders, a dry-stone wall can break up the mass of undergrowth, as can a gate, tree, bush, rusty farm machinery, and so on. The undergrowth serves the extremely useful purpose of creating a lost-and-found effect here for the wall, which can look too strident if standing up above the ground by itself.
Of course the wild tangle of vegetation needs simplification by reducing it to fewer detailed shapes. Make some of the grasses and briars stand out more than others. With vegetation the spatter technique of splashing blobs of paint from a brush can work very effectively. If you wish to beef it up, as you will do from time to time, one of the best methods is to introduce more variety of colour - red and orange can be particularly striking and I often carry this out by dropping these colours into an area that I've already wetted with clean water. Substitute detail with colour. There is more on tackling vegetation in my Complete Guide to Watercolour Painting.