This is more or less what I do using MS Paint, which anyone with a Windows computer has. If one doesn't, then there are some free "paint" apps available for download. You can cut and paste with it, meaning that if you can find a picture of a plant, you can copy and use it in the drawing. But the trouble with that is "cleaning" the plant from its background can be time consuming. If you have drawing/painting skills, it is much easier to just paint plants into a photograph. The process is much like oil painting. If you make mistakes, but not to many, you can use the "undo" button. If you drew something too long ago and don't want to undo because you drew more recent things that you want to keep, then you must draw over what you don't like. Or paste in a piece of the original photo to bring an area back to the starting point, and then draw on that. I use a touchscreen and a stylus. It's hard to draw detail with a finger on a touchscreen. If you have only a mouse/trackpad you can still draw using line and some other tools, so it will be more limited in what you can do.

The free graphics program "The GIMP" (www.gimp.org)

I take pictures, print them out and draw on them with colored pencils, or make copies of the files and scribble plants in with a paint program.

Yaardvark and I both consider the function, size and shape of a plant instead of the exact variety. Where he scribbles in a small flowering tree you can use a flowering magnolia, crepe myrtle or desert willow, depending on the climate.

I like the GIMP because it uses layers ... I can keep the original image on one layer and make more layers showing different ideas.

Layers sounds excellent. I will have to look into (which, of course, will take about a year.)

An easier to use free program is Paint.NET. It started out as a student project at Washington State University, and is now used internationally by amateurs and professionals, though it only works with Windows. It, too, uses layers.

Here is a plant with the background removed using the Magic Wand selection tool and the Delete key.

I use paint almost every day - appreciated very much the suggestion for paint.net. I've also been using photo shop elements for ~5 years now, like it very much.