Seeds, Waters and Fruits of Labor

"Remember, water with 115 is not for drinking. Water with 115 is not for drinking. Water with 115 is not for drinking." - Nikolai Belinski

Credit to djnapoleon, Reyhz - リース and Pure on Steam Guides for the screenshots and some info I didn't even know about the plant mechanics!

Seeds

Seeds drop as blue power-ups from zombies in Zetsubou no Shima. They can be planted at special circular marked stone planters found around the map. Depending on how the seed is taken care of, it can blossom into a number of plants. 

Waters

Water requires a Bucket to obtain. Four in total are scattered across the map, one of which is in the spawn room. (The Golden Bucket is also acceptable, however the game can easily glitch out causing you to not have a bucket for the remainder of the game so proceed with caution when doing that optional upgrade.)

Blue 115 Water

Blue 115 Water is found behind Lab A, near the web that leads to the Giant Spider mini-boss fight. 

Green 115 Water

Green 115 Water is found behind Lab B, right next to the exit of the sewer drain area.

Purple 115 Water

Purple 115 Water is found in the Spider Cocoon room inside the Bunker, near the floor plan that needs to be Mesmerized to start the Seeds of Doubt Easter Egg Quest.

"Rainbow" / Oil / Iridescent / Raw 115 Water

"Rainbow" Water can only be obtained by timing an interaction with the above 115 Rock when travelling from the KT-4 Chemical Benches to Lab B using the Sewer Drains fast travel across from Mule Kick. The opportunity to obtain it is frustratingly small, so don't be surprised if it takes multiple attempts to obtain it. 

Fruits of Labor

Standard Loot Pods

Standard Loot Pods grow from seeds 100% of the time when no water is given to the seed, or has a chance of spawning when water is applied but not for 3 consecutive rounds. They provide "loot" which tends to be lower-tier weapons, a small amount of points for the player that picks it up, or a live grenade. 

Purple Loot Pods

Purple Loot Pods grow from seeds when Blue 115 Water is given to the seed for 3 consecutive rounds, or has a chance of spawning when Blue 115 Water is applied but not for 3 consecutive rounds. They provide better loot than the Standard Pods, including higher-tier weapons and larger sums of points.

Mantis Arm Plant

Mantis Arm Plants grow from seeds when Green 115 Water is given to the seed for 3 consecutive rounds, or has a chance of spawning when Green 115 Water is applied but not for 3 consecutive rounds. Mantis Arm Plants will impale a nearby zombie / make it a crawler and "holds" the zombie in place for 10 minutes, giving teammates an opportunity to do objectives or take a break without disruption from zombies. Meleeing or shooting the plant will make it immediately kill the zombie it is holding. The plant dies when the zombie dies.

Carnivorous Plant

Carnivorous Plants grow from seeds when Purple 115 Water is given to the seed for 3 consecutive rounds, or has a chance of spawning when Purple 115 Water is applied but not for 3 consecutive rounds. Carnivorous Plants lure nearby zombies and Thrashers with pheromones before using tendrils to behead zombies and consume their heads one by one, until the plant dies. Players may also take damage from the plant if they stand too close to it.

Perk Fruit Plant

Perk Fruit Plants have a chance of growing from seeds when Blue, Green, and Purple 115 Water is applied to a seed for one round each during the 3 round cycle. Eating the Perk Fruit will either gift the player a free perk, or cause them to uncontrollably vomit earning the player the achievement/trophy "One Too Many" if they don't already have it.

Masamune Plant

The Masamune Plant grows from a seed when Rainbow/Oil/Raw 115 Water is given to the seed for 3 consecutive rounds in the special planter revealed by the Skull of Nan Sapwe's 'Mesmerize' function on the wall to the right of where the KT-4's plant is obtained from. The plant itself can be difficult to pick up due to a strange hitbox but the remainder of the plant will shrivel up immediately after obtaining it. The Masamune Plant, as the name implies, is used to upgrade the KT-4 into the Masamune.

Enhanced Purple Loot Pods

Enhanced Purple Loot Pods grow from seeds when the KT-4 / Masamune and Blue 115 Water is used on the seed for 3 consecutive rounds, or has a chance of spawning when the KT-4 / Masamune and Blue 115 Water is used but not for 3 consecutive rounds. Enhanced Purple Loot Pods are visually similar, but are covered in yellow pores/spores indicating they are the better variant. This version can drop even greater loot, such as Empty Perk Bottles, Monkey Bombs, better weapons, and the Anti-Aircraft Bullet needed for the Seeds of Doubt Easter Egg Quest.

Enhanced Mantis Arm Plant

Enhanced Mantis Arm Plants grow from seeds when the KT-4 / Masamune and Green 115 Water is used on the seed for 3 consecutive rounds, or has a chance of spawning when the KT-4 / Masamune and Green 115 Water is used but not for 3 consecutive rounds. Enhanced Mantis Arm Plants are practically identical to normal Mantis Arm Plants but will emit red mist from the planter and can hold the zombie for 60 minutes as opposed to 10.

Enhanced Carnivorous Plant

Enhanced Carnivorous Plants grow from seeds when the KT-4 / Masamune and Purple 115 Water is used on the seed for 3 consecutive rounds, or has a chance of spawning when the KT-4 / Masamune and Purple 115 Water is used but not for 3 consecutive rounds. Enhanced Carnivorous Plants kill faster and can kill more zombies than their predecessor.

Enhanced Perk Fruit Plant

Enhanced Perk Fruit Plants have a chance of growing from seeds when the KT-4 / Masamune and Blue, Green, and Purple 115 Water is used on a seed for one round each during the 3 round cycle. Eating the Perk Fruit will either gift the player a free perk, or cause them to uncontrollably vomit earning the player the achievement/trophy "One Too Many" if they don't already have it just like the normal Perk Fruit. However Enhanced Perk Fruits have a much higher chance of rewarding the perk and much lower chance of vomiting. Also it appears the water combination used for this plant when combined with the KT-4 / Masamune gives the highest possibility of obtaining Imprint Pods (see below).

Imprint Pod

Imprint Pods are a rare plant that can appear from any stage of seed both watered and unwatered when the KT-4 / Masamune is used on the plant as well. Imprint Pods will inflate when interacted with, revealing a human body inside. When imprinted, the pod will "remember" everything your player has equipped at that moment, and if that player bleeds out they will immediately respawn from the Imprint Pod with whatever they had at that moment (including Perks) and earn the "Herbal Remedy" achievement/trophy if they don't already have it. If a second Imprint Pod is used while one is already active, the first one will immediately wither as the new Imprint Pod overrides it.