Upgrade plants are upgraded versions of some plants from Plants vs. Zombies. To unlock them, they all must be bought from Crazy Dave's Twiddydinkies at costs that vary from $3000 to $20,000, depending on the plant. Once purchased, they do not need to be bought again to use them in levels afterwards, as the shop will classify them as "Sold Out." To activate an upgrade, it must be planted on its weaker variant. They all have a very slow recharge in game.
There are a total of eight upgrade plants.
- Main article: Gatling Pea
Gatling Pea is the upgrade of the Repeater. It shoots four peas per round of fire instead of the Repeater's two peas per round. It costs $5000 in the shop, and it costs 250 sun during levels except for Survival: Endless.
- Main article: Twin Sunflower
Twin Sunflower is the upgrade of the Sunflower. It provides two units of sun every time it glows instead of the Sunflower's usual one unit. It costs $5000 in the shop, and 150 sun during levels except for Survival: Endless.
- Main article: Gloom-shroom
Gloom-shroom is the upgrade of the Fume-shroom. Instead of shooting a fume straight in front of it, it shoots four bursts of Area of Effect fumes into all adjacent spaces per round of fire. It costs $7500 in the shop, and 150 sun during levels except for Survival: Endless. If it is used in a daytime level, a Coffee Bean is needed for activating it, thus upping the cost to 225 sun total except for Survival: Endless.
- Main article: Cattail
Cattail is the upgrade to the Lily Pad. It shoots two Spikes per round of fire that automatically home in on a target and can cover the entire lawn. They can also shoot down Balloon Zombies like the Cactus. It costs $10,000 in the shop, and 225 sun during levels except for Survival: Endless. If the Lily Pad is occupied, however, it can't be upgraded.
- Main article: Winter Melon
Winter Melon is the upgrade to the Melon-pult. It lobs frozen melons that deal heavy immediate damage to its intended target and moderate splash damage to nearby zombies. Unlike its downgrade, the melons it lobs also add a slowdown effect to all zombies they hit. It costs $10,000 in the shop, and 200 sun during levels except for Survival: Endless.
- Main article: Gold Magnet
Gold Magnet is the upgrade to the Magnet-shroom. Instead of collecting metal headwear from zombies like its downgrade, it periodically collects silver coins, gold coins, and diamonds for the homeowner. It costs $3000 in the shop, and 50 sun during levels except for Survival: Endless. Note that if it is planted on a sleeping Magnet-shroom, it will wake up, even without help from a Coffee Bean.
- Main article: Spikerock
Spikerock is the upgrade to the Spikeweed. It spikes zombies at a rate of two damage per round of fire instead of the Spikeweed's usual one damage. It can also sustain nine Gargantuar blows or tire pops. Its spikes are indicators of its remaining health, as it loses one big spike every three Gargantuar blows or tire pops. It costs $7500 in the shop, and 125 sun during levels except Survival: Endless.
- Main article: Cob Cannon
Cob Cannon is the upgrade to the Kernel-pult. It is a reusable Cherry Bomb with a recharge rate of 36.41 seconds. Clicking or tapping the Cob Cannon will allow the player to select where to fire the corn cob. The Cob Cannon's corn cob can remove ladders, has an explosion radius of 3x3, and is an instant kill, but it takes time for the corn cob to reappear after firing. It costs $20,000 in the shop, and 500 sun during levels except for Survival: Endless. It requires a 2x1 area of Kernel-pults to plant.
- Imitater cannot imitate an upgrade plant without hacking, possibly because the player may not have the upgrade plants when they purchase the Imitater. However, the player can upgrade an Imitater version of a plant, it just will not be grayscaled.
- When a Pumpkin is placed on a Cattail, if the player removes the Cattail with the Shovel, it will downgrade into a Lily Pad without destroying the Pumpkin.
- The same applies to when a Bungee Zombie steal the Cattail, as it will also downgrade into a Lily Pad with the Pumpkin.
- In Plants vs. Zombies 2, the Twin Sunflower, Spikerock, and Winter Melon all appear as independent plants.
- In the Chinese version, Cattail and Gatling Pea are also individual plants. However, Cob Cannon remains as an upgrade plant (needs two Kernel-pults side-by-side).