Arctic animals:  How do Arctic plants and animals survive?
Biodiversity:  Which has more biodiversity, rainforests or the ocean?
Breathe:  How many plants are needed to produce enough oxygen for one person?
Chiles: Why do chiles fall to the ground before they turn red? What is the correct way to take care of chiles?
Clones: do plant clones look identical, branching at the same place?
Death: In the absence of clear life signs (respiration, circulation, etc.), how do botanists decide when a plant is no longer living?
Dinosaur plants:  What plants, from the days of the dinosaurs, live today in their original form?
Ever green: why do some trees and bushes stay green all year long?
Fairy Rings: what causes the big circles in lawns where the grass has died?
Fall colors of leaves: how do leaves change color in the fall?
Fall colors of leaves: 
Fall color, drought: Does a drought affect the color of leaves in the fall?
Flowers: why are flowers colored?
Flowers:  What is the colour of a red flower when it is exposed to blue light?
Flowers:  What part of the flower has the scent?
Flowers:  Do flowers attract male wasps by smelling like a wasp and do they have nectar guides?
Flowers:  What makes red petals red?  Why does a red flower appear red?  Why did plants evolve red petals? 
Forest: How do forests re-grow after they burn?
Glowing tree:  I saw a glowing tree at night from 40 feet away.  What caused this?
Grapefruit:  Where did the word 'grapefruit' come from?
Gravity: How does a seedling know which way is "up"?
Indian corn: why the colors?
Indian corn:  Is Indian corn poisonous?
Leaves:  Why are trees and grass green instead of red or blue or black?
Leaves:  How do leaves change color in the fall?
Leaves:  Does a drought affect the color of leaves in the fall?
Lichen: What is lichen? Is it a moss? Why does it look so funny?
Light: What's the difference for plants between artificial light and sunlight?
Oldest plant:  What is the oldest plant living right now?
Peanuts, grow: Where and how do peanuts grow?
Peanuts, salty:  How do peanut processors get salt into roasted, salted peanuts in the shell?
Peanut shells: What happens to the shells after processing peanuts?
Pine Needles: why are pine needles slender?
Poison ivy:  why does poison ivy itch and how did it evolve?
Poisonous plant:  What is the world’s most poisonous plant, and how deadly is it?
Red leaves: Is autumn color Nature's sunscreen?
Redwoods: how do giant redwoods hoist water up 30 stories to the top?
Sap: what happens to tree sap in the winter?
Sap trees:  Do sap trees blow up in the winter?  And how?
Talk:  Can plants somehow communicate with other plants?  I read recently of plants next to one with a torn off limb, responding to the nearby injury.
Watermelons: Why does the center of a watermelon taste sweeter than the rest of it?