We understand from the passage that one reason why some can be particularly troublesome is ----.

Weeds are plants out of place, either as the wrong
plant in cultivated ground, or as any plant where none
should be. They can cause considerable financial loss
through the cost of their control and the damage they do
to crops. Plants which become really troublesome as
weeds are those which persist despite man's efforts
to control them. Such persistency is due to several
factors of which perhaps the most important are
prolific seed production, coupled with die often
remarkably long periods of dormancy of the seed, and
the ability of vegetative parts of some plants to survive
mechanical damage and adverse conditions and to set
up new plants. Weeds may be controlled by hand, by
cultivation and other mechanical means, by biological
means and by chemical weedkillers. Chemical
weedkillers are widely used, either to give a total kill
and suppress all vegetation or to control weeds
selectively in crops.