Interesting Cowplant fact I saw

It's level 5 gardening and it's called grafting. I do it all the time. It's not bugged in my game. A player has to graft in order to complete the gardening aspiration. Some plants can only be obtained by grafting, patchy, buying lots of seed packets, the gallery or cheating. I prefer getting the rare plants myself through grafting.

Rare Plant Grafting
Bird of Paradise (Rare) - Graft Tulip onto Chrysanthemum.
Bonsai Buds (Rare) - Graft Daisy onto Strawberry.
Bonsai Buds (Rare) - Graft Roses onto Grape.
Pomegranate (Rare) - Graft Cherry onto Apple.
Dragonfruit (Rare) - Graft Snapdragon onto Strawberry.
Cow Berry (Rare) - Graft Dragonfruit onto Snapdragon.
Orchid (Rare) - Graft Lily onto Snapdragon.
Death Flower (Rare) - Graft Pomegranate onto Orchid.

Here's a quick guide...
https://www.carls-sims-4-guide.com/help/grafting.php

Here's a complete gardening guide...
https://www.carls-sims-4-guide.com/help/grafting.php

Edit: A Sim needs level 10 gardening to buy rare seed packs or you can cheat and use debug buy. Some seeds can be obtained at the City Living produce market and through purchasing gifts on the phone or computer. Some seeds can be caught while fishing and harvested at the Spice and Romance Festivals.

Edit 2: Cow plants do not produce cow berries. There are obtained though grafting, rare seed packets, fishing and sometimes Patchy.