Since you have a 50:50 chance of a plant becoming a sim eating plant, just grow something quickly, such as lettuce (45mins), and get a couple of sims gardening at once to give a better chance. Just make sure you're growing something with the monster symbol on the seed packet first.
So go to inventory
Go to outdoor section
Get a vegetable patch
Put it in your gardens
Click on it
Select lettuce
Do this with several sims and you should luck out and get the monster. Or skip the growing time with LPs
Or easier still, skip the entire task using 5LPs.