The Grow Effect will change the scale of the plants mesh as the plant grows. You can find these settings in the plant blueprint like for example the BP_TomatoPlant then select the BP_PlantComponent and in the details panel in the Plant Settings category find the Growth category in that category is the Grow Effect setting like shown in the image below.
This can be enabled or disable using the Use Grow Effect setting.
The Grow Effect Start Scale is the scale the meshes start at when the plant is at 0 growth value, this can be used to change the starting size of the plant mesh.
The Growth Effect Rate controls how many times per seconds the plants mesh scale updates. Plants with long grow times like 10 mins or higher should have a low value for this setting like for example between 1 – 3 for the best performance. The default value of 24 is for plants with very short grow times (3 Mins or less) that need to update their scale very frequently.