Create larger onion bulbs by taking off the top.
Prematurely taking off the tops will stop the bulbing process, giving you a smaller harvest.

Add sugar to the soil when planting tomatoes to make them sweeter.
Sugar content in a tomato is predetermined by its genetics. Tomatoes produce their food/sugar through photosynthesis and not by uptake through the soil. This will not help your fruit be sweeter.

When onions go to seed, pinch off the seedpod.
Older heirloom varieties of onions were known for taking off out of control. The new hybrids of
today are different. In fact, for today’s onions, pinching off the seedpod will create a smaller onion
by stopping the center core from growing.

To ripen the fruit of a tomato plant, cut off extra foliage to allow more sunlight to come through and produce more fruit.
Don’t do this! This is not a good idea as the tomato plants get their nutrients from the foliage.
The more foliage, the more surface area to capture the sun and create more food to grow
better tomatoes.

Stressed plants need more fertilizer.
Not necessarily! Before adding fertilizer, test the soil to find out if it's lacking any nutrients. It could be other environmental conditions that are stressing the plant. Check the light level your plant is receiving, make sure the soil isn't compacted, and ensure that it is getting enough water.

Don't water plants on a hot, sunny day or you will burn the leaves.
We like to stay hydrated when we're outside on a hot day and plants do too! The sun's rays are not strong enough to scorch leaves. It's best to water in the morning to reduce evaporation, but you won't be harming your plant if you wait until midday to water it.

Organic pesticides are safer than synthetic pesticides.
Organic simply means that it is made from all-natural ingredients. It doesn't necessarily make a pesticide any safer. In fact, some all-nautral ingredients can actually be more toxic than ingredients in synthetic pesticides. It's always best to read the label.

Gravel in the bottom of containers will improve drainage.
Water can't easily move from fine soil to coarse gravel, so adding gravel will actually hinder drainage.

Walk around the lawn in golf shoes to help with aeration.
The spikes on golf shoes aren't long enough to reach grass roots, so it's unlikely to make a difference.