Gardening myths, also known as urban legends, have always been with us. Unfortunately, with the rise of the internet and social media, they spread even faster than before. None of these have scientific merit.
1. Epsom Salt
The use of Epsom salt as a garden cure-all is one of the most persistent and bizarre myths in gardening. Usually people are told to try it, so they use Epsom salt along with doing some other things, and the plant responds favorably. For example, when adding a teaspoon of Epsom salt, and then watering over it, it is the water that helps the plant, not the Epsom salt. But because the plant improved, the person believes it was the Epsom salt.
Epsom salt, however, is only magnesium sulfate, not magic. Most garden soils have sufficient magnesium for normal plant growth, although fruit trees may occasionally need magnesium supplements. That need is best met with a commercial citrus micronutrient solution. Epsom salt can be used to treat tomato plants with yellowing between the leaf veins. That is a typical sign of magnesium deficiency, and a teaspoon of Epsom salt watered into the soil will fix it. But Epsom salt does not solve any other problem.
In large amounts, magnesium can inhibit the uptake of calcium, which, in tomatoes, leads to Blossom End Rot. Sulfate can be used to make soil more acidic, but, the amount of Epsom salt you would need to add, to have a measurable effect, would add harmful amounts of magnesium to the soil.
Blossom End Rot on tomatoes and peppers, for example, is caused by inconsistent watering, not usually by lack of calcium in the soil. If a tomato is not watered consistently, calcium will not be properly transported throughout the plant and fruit, leading to tissue malformation called Blossom End Rot. This condition is not related to magnesium or sulfate, but some misguided home gardeners recommend Epsom salt as the solution.
Magnesium sulfate is used in agriculture to add magnesium to the soil where it is deficient. But in home gardening, magnesium deficiency is rarely a problem. A group called the "Epsom Salt Council", however, advertising on the internet, states that an expert recommends the use of Epsom salt. Obviously, they are trying to increase the sales of their simple product without regard to its limitations.
2. Planting According to the Moon
Botanists who study the behavior of plants have identified many factors that affect seed sprouting and plant growth, but phases of the moon is not one of them. Since the purpose of university scientists is to improve agricultural productivity and raise farmers' incomes, it is hard to see how they could miss the moon as a factor if it had any merit.
Planting by the moon is a religion that goes back thousands of years when, in ancient times, the moon was worshipped as a goddess. Possibly that is why a majority of the devotees to this practice are women. Something about the moon seems to resonate emotionally and some become very angry if their beliefs are dismissed.
These planting-by-the-moon followers are not entirely anti-science, because some use science in an attempt to justify their beliefs. Usually the argument goes  the moon affects the tides,  animals alter their behavior according to moon phases,  therefore the moon must affect plants as well. While the first two facts are correct, the conclusion is false. Those devotees who claim to see the effect in their own gardens are victims of confirmation bias (see the end of this page).
3. Adding Coffee Grounds or Banana Peels to the Soil
Coffee grounds are acidic, but they add acidity to the soil slowly, and temporarily. Banana peels contain potassium, but decomposition releases it to the soil very slowly. Both however, tie up nitrogen in the soil as microbes decompose them, slowing plant growth. They are better placed on top of the soil or in a compost pile where they can decompose before being added to the soil.
Coffee grounds can be placed on the top of the soil as mulch and used to acidify the soil by watering through them. This presents two problems, however. First, coffee grounds contain many complex chemicals, evolved by the plant to deter or kill insects. A few of these chemicals can harm or inhibit seedlings. Some varieties of tomatoes, for example, are harmed by coffee grounds.
Second, when the season is over, the coffee grounds have lost their acidity but have not decomposed. At this point they must be scooped up and placed into a compost pile. If they are mixed into the soil like other mulch, they will tie up nitrogen as they decompose.
4. Adding Vitamins or Vitamin B1 to the Soil
Plants, and various soil microbes, make their own vitamin B1. Other vitamins needed by humans and animals are not needed by plants. There is never a need to add vitamin-mineral tablets to the soil.
When transplanting a plant, vitamin B1 does not help reduce transplant shock. Neither does phosphate or any other kind of fertilizer. The presence of a rich source of nutrients in the planting hole can delay root growth into the surrounding soil which is necessary to stabilize the plant.
5. Helpful Bacteria Will Die If You Till the Soil
This is an excuse made up to avoid the hard work of turning over winter garden beds in preparation for new planting. Tilling, or deep digging, home vegetable garden soil is necessary to aerate the soil, remove any perennial weed roots, and to find and remove harmful grubs.
Microbes, including bacteria and fungi, number in the billions. So, at the end of tilling, if some bacteria are exposed to the sun that were not exposed before, and die, what of it? Microbes multiply quickly, and the 95% of them that are not exposed to the sun will quickly replenish their numbers.
There is a no-till practice for large-scale agriculture to reduce wind erosion of the soil. It has nothing to do with avoiding microbe death, however, and is not relevant to home gardening.
SUPERSTITIOUS BEHAVIOUR is the act of doing some random behavior in a given circumstance, in the belief that it will change the outcome. An example is always wearing some item of clothing, perhaps red socks, whenever the home sports team is playing, in the expectation that the team will win the game. In gardening, this usually involves adding some unnecessary ingredient, or engaging in an unnecessary practice, like planting during a full moon.
CONFIRMATION BIAS is the tendency to search for, interpret, favor, and recall information in a way that confirms one's preexisting beliefs. Most people are not trained in the scientific method, so their experiments are accidentally construed to verify what they already believe.