I can't speak to the issue of the garden plants that Walmart sells, because - despite living on five acres of property - I do not garden. (I mow. That's plenty of work, on this much acreage.) But as for Walmart's house plants, I have found them to be sucky in the extreme.
Why are Walmart houseplants so terrible?
My Walmart plants look good, but haven't grown a single new leaf.
A few years ago I decided to brighten the inside of my home with some house plants. I studied up on my location's specifics, and cross-referenced a list of "plants that will do well in my home" with the ASPCA's list of plants that are not toxic to cats. (My cat has an abiding fondness for house plants.) I ended up with a short list of plants: Boston fern, Spider plant, Lipstick plant, and Nerve plant.
Luckily, these are all plants which are found in many stores. I bought several at Walmart, several from Ace Hardware, and a few from a nearby independently owned nursery.
After a few months, the difference was obvious. All of the plants I had bought at Walmart had not budged an inch. Sure, they looked nice: their leaves were glossy and plump, and they had an overall glow of health. But none of them had grown so much as a single leaf.
By contrast, 9 out of 10 of the plants I bought elsewhere had not just thrived but exploded with growth. (The 10th, a small Boston fern, just plain old died. No idea why. Just rotten luck, I guess. It happens.)
Why this discrepancy? I was blown away by the incredible difference in vitality. What could be going on here?
I have done some research, and frankly I am stumped. The only theory I can come up with is that the plants that are sold to Walmart are forced early in life to grow fast and hard, in order to get up to a size where they can be shipped off to Walmart to be sold. Much like cattle in a feed lot, perhaps these plants are pumped full of chemical fertilizers and pushed to grow too big, too fast.
My theory is that all this early growth burns them out, leaving the plants without the resources they need for a steady, measured growth. This is a completely unscientific theory, by the way. I don't know how else to account for it. If you have any other ideas on why Walmart indoor plants don't grow, please let me know! Frankly, I am stumped.