The speed at which systemic insecticides begin to work upon houseplant pests is, as you might expect, influenced by multiple factors including the plant species, the mechanism of delivery, how water soluble the insecticide is, and the mode of action of the active ingredient.
To provide some example timelines, in the case of a large Monstera plant treated with systemic insecticide via soil drench, it can take up to 7 days for the chemicals to be picked up by the roots and transported via the vascular system to all plant tissue. However, applying the same insecticide to the same plant by foliar spray, which acts doubly as a contact killer, can begin providing protection almost immediately for a duration as long as 90 days.
As the pests targeted by systemic insecticides are those that consume plant tissue (thrips, aphids etc) the application of a soil wash, or granules added into the growing medium will take longer to get into the plants system and therefore be less rapid to act than a spray application.
Irrespective of method of delivery however, once absorbed into the plant’s vascular system, every square inch of plant tissue will be reached.
Source link