One way to hold this word: plant + -er, the one that does the planting. The -er does not care who or what it is. It can be a person bending down to put a seedling in the soil. It can be a machine dragged behind a tractor, dropping seeds row after row. It can be someone who owns a whole of tea and lives off what is planted there. And the big container on a doorstep sits on the same line: it is the place the planting goes into, so it takes the same name. This is a picture to hold the word together, not a claim about how the word grew.
Wiktionary's etymology tree for planter runs plant + -er, with -er going back through Old English -ere and Latin -arius.
Wiktionary 的字源樹顯示 planter 就是 plant 加上 -er，而 -er 這個字尾可以一路追到古英語 -ere 和拉丁語 -arius。這正好對上今天的用法：不管是人、機器還是容器，都掛在「種」這個動作旁邊。
Wiktionary entry for planter (kaikki extract, source_ref kaikki:3f6f69a190036ae4:1328198891:18736), etymology tree: English plant + English -er
Three branches
動手種的那一個：人與機器
這一組最貼近核心畫面：誰在種、什麼在種。差別只在動作的是一雙手還是一台機器。
noun · B1
Someone who plants things — a person who puts seeds, young trees or other plants into the ground. The word is also used about a person who puts an idea, a story or a feeling into someone else's mind.
種植的人
靠種植吃飯的人：莊園主人
把「種」放大到一整片土地，管這片土地的人也留在這個名字底下——雖然真正下田的並不是他。
noun · B2
Someone who owns a plantation or is in charge of one — a large farm, usually in a warm country, that grows one crop such as tea, coffee, sugar or cotton. In history writing the word also names a whole landowning class whose fields were worked by farm workers or by enslaved people.
莊園主／種植園主
被種進去的那個地方：花槽
前面兩組是「誰在種」，這一組換個角度：種到哪裡去。容器不會自己種東西，但它是這個動作發生的地方。
noun · B1
A container for growing plants in. A planter is usually large and chosen to look good, and it often stands on the floor, on a balcony rail, or along a street, with one or several plants growing inside it.
大花盆／花槽