Why do we say "plant a kiss"?

The verb comes to us circuitously from the Latin plantare, which had the same two related senses that we use, the one agricultural and the other meaning to fix in place. In the latter case, we plant ourselves into a position to fight, we plant spies in the enemy's camp, and we plant kisses on the cheeks of our beloveds.

To plant suggests the idea of placing something in the ground (possibly with affection and or dedication) for future growth. The metaphor used for a kiss coneys the idea of a vigorous kiss, one that you are likely to remember.

From the OED

Plant a kiss:

• spec. to place (a kiss) on the lips, cheek, etc., usu. with gusto or deliberation.

  • 1906 Galsworthy Man of Property i. ix. 122 Moved by some inexplicable desire to assert his proprietorship, he rose from his chair and planted a kiss on his wife's shoulder.

  • 1937 G. Frankau More of Us xii. 125 Was this The ruleress of waves, R.N., all-British, Who stooped to plant the Cytherean Kiss?