If she appears in Graveyard Ops, she will summon large hordes of Dandelion Weeds to attack her assailants, while in Backyard Battleground, the area the Marigold currently is in will experience a noticeable increase in Weed spawning (any Weed except for the Dandelion Weed).
Her first appearance in the Backyard Battleground is in the mission "Ancient Knowledge", where the player has to find and interact with The Scroll of Instruction for the Zen Sensei. After interacting with the scroll, the chest that it lays on is revealed to be a hiding Marigold, which the player must then vanquish to retrieve the scroll. She also makes a minor appearance in another mission for Ol' Deadbeard, where two Marigolds hide in breakable barrels. They will rarely appear in the backyard, depending on the circumstances. However, after a period of time, if they have not been vanquished and are still being chased, they will disappear, forcing the player to end their hunt.
She resembles a normal Marigold, except that she seems to be made entirely out of valuables. Her body and "arms" are made out of solid gold, while her petals are made of diamonds and her face is a large gem made of gold. She appears to be growing out of a treasure chest located at her base, inside which she hides when not being chased.
▼ Knockback was being unintentionally dispelled, making it easy for her to be cornered (Attacks now applies knockback)
Abilities
Like the Treasure Yeti, the Marigold is extremely fast and will try to escape from the player. The Marigold has a special attack, which consists of the Marigold jumping into the air and doing a backflip before slamming her treasure chest into the ground, sending a shockwave around them, stunning all zombies in the radius, similarly to the Treasure Yeti's freezing attack. In contrast to the Treasure Yeti, she has no other attacks besides ramming into zombies and knocking them back. To compensate for this lack of attacks, the Marigold is a lot harder to hit and therefore vanquish.
Strategies
This boss is fast and disruptive. It is recommended to stay away from Marigold because she can use Chest Slam that can stun you, much like the Treasure Yeti's freeze attack. Using a sniper like Captain Deadbeard, Foot Soldier, Paleontologist, or Astronaut is suggested. However, this means that you will have to pick up all of the coins later on. Alternatively, it is possible to stay just outside the range of the Chest Slam while attacking. Although getting the distance correct takes some practice, maintaining this range is much more profitable than sniping, as your shots have a higher chance of hitting the fast Marigold and you can follow after her more closely and pick up all the dropped loot as you follow after her.
Keep in mind that despite her similarities in function to the Treasure Yeti, she is much smaller than the Yeti and is therefore harder to hit.
Plants vs. Zombies: Battle for Neighborville
Try camping near the zombie base (in both Town Center and Weirding Woods) as the Strange Well is located near it and the Marigold will also spawn nearby, which will save you a lot of effort once you pick the Crystal Dog up. After vanquishing it, clear the area of plants quickly before picking up the Crystal Dog, as it will vanish if it's left for too long. Finally, head to the Strange Well carefully while trying to not get yourself vanquished by plants (if any of them are present), as you will become extremely vulnerable against attacks; even a hit from a Wildflower's projectile or a Weeds's slap can reduce your health by 10 or more points, and if you get vanquished, the Crystal Dog will also disappear, requiring you to start over.
Gallery
Plants vs. Zombies Wiki has a gallery for Marigold.
She is the smallest boss out of all the plant bosses and also the only one that doesn't actively attack the player (unless forced to do so).
In Backyard Battleground, if there are no plant bosses, the AI zombie heroes/zombie bosses will all attack the Marigold the very moment she spawns in Backyard Battleground.
Sometimes if she is chased into the plant base and the zombie bosses are attempting to reach her through the doors, the AI Weeds will attack the bosses at the plant base, which can cause the main doors to open (since the doors open to any plant), therefore allowing the bosses to enter and attack the Marigold in the plant base until she vanishes or is vanquished.
If spawned in the Backyard Battleground and attacked, the Marigold can occasionally end up fleeing into the plant base if things go wrong, making it almost impossible to reach it without entering the base and getting a massive plant retaliation as a result.
This most often occurs if the Marigold is attacked by a stray zombie boss that spawned earlier and wandered over to the plant side of the backyard.
She has a very similar voice to the Sunflower.
It's possible that her voice is a pitched Sunflower voice. Her voice is currently too hard to hear to confirm this.
As she is made out of valuables, it is possible that she did not grow from a seed.
The Marigold also appears to be growing out of the treasure chest she otherwise hides in, supporting this theory.
When a player vanquishes the Marigold, it says "Vanquished Plant", despite being a boss.
This is either a mistake in the game, or what is meant to happen in the end game.
Despite being a plant, she can be eaten by Chomper and be Gooped by him.