How can I get my sim not to be afraid of all plants now?

You need to check in the fear bubble, it says there how to overcome your sims fear. I found this on the net regarding fear of cowplants. 

Fear of Cowplant

Cowplants are living plant/animal hybrids capable of eating a whole Sim. If a Sim gets swallowed and then spat out, they develop an intense fear of Cowplants. To get over this fear, let the Sim successfully Milk a Cowplant, while being Confident.

It's part of the wants and fears system. Sims develop fears from time to time which they can overcome. Sometimes next to wants, you see a special want with a spiky bubble. That's the way to overcome the fear. Read what you have to do and follow the instructions.

Alternatively, you can:

• Check the Rewards store from the aspiration panel. There's a potion that can remove fears.
• Use this cheat: traits.remove_trait trait_Fear_Cowplant

The Brave trait was introduced before the fears were (with the Scared emotion rather). Funnily, it only prevents two fears: Fear of the Dark and Fear of Ghosts (list might be incomplete). Traits that prevent the Fear of Cowplants are Green Fiend, Super Green Thumb, and being a PlantSim. I agree, it's not a good decision to have the Brave trait only prevent a few fears.

You can also remove Fears by turning off Wants and Fears in the Game Options. Unfortunately, this means you'll also lose your current Wants. You might want to wait a bit if you have a high-scoring Want pinned, such as Increase Handiness Skill for 100 points. 

Open the Options panel by pressing Esc (on PC).  Select Game Options and then Gameplay. Remove the checkmark next to Show Wants and Fears. Apply Changes. You can now turn Wants and Fears back on, if you want, by restoring the checkmark and applying changes a second time. Go back to Live mode and check your Traits on the Simology panel to verify that the Fear has been removed.

This will not get rid of any moodlets that came from the Fear. You'll have to wait for those to time out on their own or remove them with other methods, such as the Moodlet Solver potion from the Reward Store or the Potion of Emotional Stability made by Spellcasters.