Learn Farsi: ما باید در این پویش به کاشت نهال ادامه دهیم. - We must continue planting saplings in this campaign.

When you want to say you continue doing something with ادامه دادن (edâme dâdan), the verb requires the preposition به (be) followed by a noun or an action noun. Here, به links the main verb to the noun کاشت (kâsht - planting). Literally, the sentence says "we give continuation to the planting".

Why is کاشت نهال (kâsht-e nahâl) connected with an Ezafe instead of just being a verb and its object?

Because کاشت is acting as a noun here ("the planting"), it cannot take a direct object like a verb would. Instead, Farsi uses the Ezafe (the invisible "-e" sound) to link the action noun to the thing being planted, which literally translates to "the planting of saplings".

Why doesn't the subjunctive verb دهیم (dahim) have the بـ (be-) prefix that usually follows باید (bâyad)?

In compound verbs formed with دادن (dâdan), it is very common to drop the subjunctive بـ (be-) prefix, especially in formal or written Farsi. You will often hear ادامه بدهیم (edâme bedahim) in everyday speech, but ادامه دهیم is perfectly correct and natural here.

Is پویش (puyesh) the only word for "campaign" in Farsi?

No, you will also frequently hear the English loanword کمپین (kampin). However, پویش is the native Farsi equivalent and is widely used for social, environmental, or public awareness campaigns, making it the perfect choice for a tree-planting drive.