To change a plants Growth 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 Growth category like the image below.
Here you can change this plants growth settings, you can hover your mouse over each settings name to get a tooltip with more info about each setting.
You can set here the requirements for the plants growth such as if it requires water and how much water it requires, this is the same for warmth, light and fertilizer. You can also set the Growth Time which is the number of seconds the plant will take to grow.
Growth speed can also be effected by the growth multiplier, if the growth multiplier is 1 then the plant will grow at its standard speed, if the value is 2 then the plant will grow x2 as fast.
Water, Fertilizer, Warmth and Light can effect the growth multiplier those settings can be found in those stats settings categories. This setting is called Growth Speed Multiplier in each stats category. Setting this to 0 will mean that stat has no growth speed effect on the plants growth