The Cactus Petal is a physical item from the 
Desert, in the 
First Sea, which only exists for the 
Desert Merchant's Prickly Harvest secret quest. You collect 10 by hitting blooming cacti with M1s, then bring them back to the merchant to validate the secret and earn 5 secret levels.
How to get it
- Spend some time in the Desert, or server hop: the quest is not always available, the cacti bloom at random after a few hours in a server.
- Talk to the Desert Merchant and accept his cactus flower quest, Prickly Harvest.
- Hit the cacti with M1s: they drop petals. You need 10 of them.
What it is for
Only for Prickly Harvest. Bring the 10 petals back to the Desert Merchant: the secret is validated, you earn 5 secret levels and he gives you a 
Refreshing Drink.
No other use is documented: no crafting, no selling, no trading.
Good to know
- The Desert page words the quest differently: "destroy 10 blooming cacti by hitting them". Same result, 10 petals.
- Do not feel like waiting for the cacti to bloom? Server hopping is the recommended shortcut for every waiting secret.
- It is one of the 40 island secrets of the First Sea, exclusive to that sea since Update 30.
- The source finally specifies the weapon: blooming cacti are hit with sword or fighting style M1s. Both work, contrary to what a hasty reading might have suggested.
Basic
Beast Hunter
Berries
Berserkers Elixir
Big Head Elixir
Blaze Ember
Blessed Scroll
Bloxy Cola
Blue Icicle Berry
Bones
Brazil Cube
Brigade
Cactus Petal
Candy
Candy Concoction
Candy Corn
Candy Egg
Celestial Token
Change Race
Common Scroll
Confetti
Conjured Cocoa
Core Brain
Cup
Cursed Scroll