When speaking of equipment needed for botanical illustration, one can't forget items that are beyond the art table.  One item that is essential is a pair of "Wellies".  Wellington-type garden / rain/ mud/ snow/swamp/ meadow/ bog / boots.  The photo below is of my pair from L.L. Bean - these have been essential to me for tramping around collecting plant specimens or digging plant material to work from a live plant.  Mine aren't lined, so I have other boots for deep winter, but these are my go-to shoes for 90% of what I do that is involved with growing or drawing plants.