To change a plants Respawn settings go to the plants blueprint so for example the BP_TomatoPlant and select the BP_PlantComponent in the Components panel, then in the Details panel and inside the Plant Settings category find the Respawn category like the image below.

Here you can change this plants respawn settings, you can hover your mouse over each settings name to get a tooltip with more info about each setting.

Respawn


If Should Respawn is true when the plant is destroyed it will respawn.

Respawn Time is how long after being destroyed in seconds the plant will wait before respawning.

If Should Respawn After Harvesting is true then if the plant is set to Destroy On Harvest (This setting is in the Harvesting category) it will respawn.

If Use Starting Growth On Respawn is true when the plant respawns if the Start Fully Grown setting or the Start Vegetables Fully Grown setting are true on respawn the plant will respawn fully grown and/or with fully grown vegetables.