Even the lump of dirt with the sprig in it takes a little while to spawn.
I've seen plenty of mentions about waiting for plants to mature, but I think plenty of people would like to know that if you load the area for the first time and hit pause right away before scanning around to find plants and rocks and such, you'll likely see nothing anywhere.
Do something in the newly loaded area for a while; go fishing, bring a boom box and dance, wander aimlessly, whatever. After a bit you'll see things start to spawn gradually.
Of course, if your Sim has ever been to the area before (any lot in the area) things should have already spawned while you were throwing your dinner party at a friends house, finding new and exciting trash to rummage, or whatever you were doing there.
