There's lots of gardening advice out there, some old lore passed for generations, others newly created and circulated via the Internet. Lots of the tidbits work, but I've come across many that didn't work for me at all. Here's a list of those failures that come to mind........
...spread wood ashes (or diatomaceous earth, or coffee grounds, or sharp sand, or broken egg shells) to prevent slugs. Doesn't work at all.
...interplant marigolds with tomatoes to keep bugs off the tomatoes. Doesn't work.
...plant according to the moon charts. I never saw it make a difference in my gardens.
...never plant the same crop in the same spot twice. Actually good advice if you have insect or disease problem, but I often repeat crops twice unless I see a problem.
...only sow peas during the dark of the moon. Supposedly they wouldn't produce peas otherwise. But I've never seen any differences.
...lay down cardboard or newspaper in garden aisles to control weeds. I found that it gets slippery as heck after a few rains. Way too dangerous to walk on, especially when wet.
...use old carpet for weed control. The backing eventually rots away then you are left with miles of tangled yarn that is a nightmare to remove. Tried it once and never did it again.
...lay down landscape fabric. Just don't! It's a nightmare to remove and it doesn't control weeds.
...keep ducks in the garden for slug control. Mine were selective about slug eating, plus ate half the garden at the same time. Even just letting them in for a short period of time didn't stop them from wiping out the lettuce and spinach before looking for worms, bugs, and slugs.
...mix black pepper with your seeds before sowing them. It is suppose to control cutworms. Doesn't work.
...plant a morning glory vine among your pole beans. Supposed to draw bees, thus resulting in more pollination and a bigger bean crop. In reality, beans self pollinate and don't need bees. So it's worthless advice.
...add "water crystals" to the soil to maintain soil moisture. They didn't work.
...use shredded paper in the chicken pen. I find that they will eat it and thus poop paper mache. Nasty.
...use boiling water to kill weeds. While this might work, it takes a lot, lot, lot of boiling water. A very expensive and time consuming method to control weeds. Not worth it.
...use egg cartons for seed starting. Yes, the seeds germinate but the seedlings are stunted due to lack of room for root growth and never quite recover. Along this same line, one tidbit of advice says use eggshells to sow seeds in. First of all, I'd have to eat dozens and dozens of eggs to get enough shells, and besides, the shells are delicate and don't drain water. They would be very tedious to work with, a nightmare to water the seedlings without any drainage, and the seedlings would be root bound if the gardener failed to crack the shell when the seedling was planted into the garden. Seedlings will all be growth stunted because of the small container for their root growth. .... Oh yes, I just saw that idea of using scooped out orange peel halves for seed starting. Basically they would have all the same issues (no drainage, shallow volume for root growth) with the added benefit of getting moldy on the outside, rotting on the inside against the soil, plus inhibiting seedling root growth due the orange oil in the rind. My, my, a great suggestion over using a re-usable pot, eh?
...don't till the soil. No till hasn't worked for me. I suspect my soil composition is not suitable for no till approach. And besides, just about all the gardeners claiming to use no till actually do till in some fashion, such as forking in their old mulch layer, using a broadfork, raking the top few inches to loosen it for planting, etc. They claim to be able to easily plunge their hands many inches deep into their garden soil. I find that my soil settles and becomes too dense to do that after less than a month. Even after 10 years of adding lots and lots of compost and other soil amendments, my soil is not light and friable enough to productively grow food doing the no till approach. While plants will indeed grow, they aren't anywhere near as productive as those grown in tilled soil.
...interplant garlic and chives to repel insects. Doesn't work for me.
...feed cornmeal to ants to kill them. Doesn't work.
...use tobacco as an insecticide. Don't! Tobacco water and dust is toxic to humans too, and can be absorbed through your skin. Plus it will kill the pollinating insects too. It's a real bad suggestion, along the lines of suggesting you make your own firebricks out of asbestos.
...don't overhead water when the sun shines or it will burn the plant leaves. Supposedly the rains drops act as convex lenses. False. Dead spots on leaves aren't caused this way. Besides, Mother Nature creates "sun showers" all the time without ill effects.