You can use the slider on the calculator above to simulate different vegetation levels on your fence. The more vegetation, the higher the load on your fence. Your fence should be kept clean of vegetation. Heavy vegetation will sap power from your charger over the length of your fence. Too much can load your charger down, and cause it to stop working!
Vegetation can be counter intutitive when you consider it's effect on resistance. Generally, higher ohms means more resistance, but the vegetation is parralled across the fence. Only in the case of vegetaion does a lower ohm rating mean degraded performance. A rating of 100 ohms on the slider indicates a loaded fence. Play with the vegetation slider to get a feel for the effects.