Using a plant pot as an ashtray could start a fire. The potting plant soil could insulate the cigarette butt instead of extinguishing it. 

Many potting soil mixes on the market today contain as much or more organic material as inorganic material. For example, potting soil can include shredded wood, bark, and/or peat moss with minimal amount of "real" soil (dirt).

When you use a dried out plant pot as an ashtray, it will work to insulate the lit cigarette. The soil heats to ignition temperature and begins to smoulder. Smouldering can continue for several hours, and when sufficient oxygen is available, the material can break into flame. If other combustible materials are in close proximity to the planter, it may become involved resulting in a serious fire.

Use a metal can with sand or a commercially-made ashtray to extinguish smoking materials instead.

Safety tips:

• Keep planters well watered to reduce flammability and remove dead plants to lessen available potential fuel for a fire.
• Planters should not rest on or against flammable surfaces such as a wood deck or siding.
• Stored peat moss should be protected from contact with heat sources.
• Should a fire begin, thoroughly wet the contents of the planter. Empty the contents on a non-flammable surface and spread them to verify that all burning substances have been extinguished. In a garden bed, use a rake or shovel to separate the burning peat, and then apply water.