We gardeners love to give advice, and that seems to go double when talking about remedies for common problems. But the remedies and ideas you hear aren’t always based in sound horticultural science. You’ve probably heard some of these myths before. They get repeated, often with great assurance, and it’s easy for a new gardener to believe it. 

There are entirely too many gardening myths to cover in one article, but here are a few more commonly repeated pieces of bad advice to watch out for. 

Use eggshells for calcium

Eggshells have calcium, and plants need calcium, so it seems a perfect match, right? The myth goes that placing crushed eggshells at the base of your plants will provide them with calcium. It’s sometimes touted as a cure for blossom-end rot of tomatoes and other vegetables, which is caused by a lack of calcium in the fruit (usually due to uneven watering and soil moisture issues). 

Eggshells are indeed a good source of calcium, but not for your plants in the near term. Plant roots can’t take up bits of shell; they need the calcium to be in elemental form. The shells have to be broken down by the microbial life in the soil, and that takes time. It often takes years. If you’ve ever tossed eggshells in the compost pile, you know they still look like eggshells months later. If you’d like to speed up the process and help the eggshells to break down sooner, bake them and crush them to dust first. 

Epsom salts for tomatoes (or anything else)

Bounce around the gardening pages on social media, and you’ll see all manner of advice about using Epsom salts. Fix your tomatoes. Sweeten your peppers. Cure diseases. Most especially, Epsom salts are talked about as a remedy for blossom-end rot of tomatoes. The problem is twofold: Epsom salts do not contain calcium, and they are a salt that can build up in your soil.

Epsom salts contain magnesium sulfate, and magnesium is indeed a needed plant nutrient, but few garden soils are short on it. They do not contain calcium, and despite being cheap, are not a DIY remedy for the garden. What applying Epsom salts can do is build up excess magnesium, which affects plant roots' ability to take up other nutrients, and can build up to toxic levels in the soil. Excessively salty soils don’t grow any plants. Do your garden a favor and leave the Epsom salts on the shelf.

Coffee grounds make soil acidic

Coffee is acidic, it’s true. But the grounds give up most of their acid properties when we make the coffee. Adding coffee grounds in the amounts we generate in our homes won’t appreciably change soil pH. While dumping a load of coffee grounds at the base of an acid-loving plant won’t sour the soil, they are a good source of organic matter and nitrogen. 

A better use for used coffee grounds is to put them, and the unbleached coffee filter, in the compost. Although they are brown in color, they act as a green, supplying nitrogen to balance out the other carbon sources in your compost pile. 

Pine needles make soil acidic

People often avoid using pine straw mulch (raked up pine needles) for fear it will acidify the soil. I suspect the myth comes from the idea that pine trees often grow in acidic soil. People hear pines grow well in soil with an acidic pH and assume the pines made the soil that way. 

Green pine needles are indeed acidic, but by the time they’ve browned and fallen from the tree, they no longer possess enough acidity to alter the pH of the soil. Rake them up and use them. Pine straw mulch is not only effective but quite attractive as well, and free to gather if you have pine trees in your yard.

Put rocks in your pots for drainage

I fell for this one as a new gardener. Add some rocks to the bottom of a pot to help with drainage. After all, water drains much more quickly through sand or gravel than it does through soil. 

The issue here is how water moves through media with different textures. Water moving downward through a material of finer texture, like potting soil, will stop moving when it hits an area of larger texture and more pore space, like gravel or pebbles. It won’t continue draining until the upper soil area is saturated, which is bad for our plants. The situation is called a perched water table when it happens out in the ground, and it can happen in our pots as well.

Pots with adequate drainage holes and good potting mix will drain well even if the holes appear to be covered with soil. If the pot doesn’t have any drainage holes at all, that’s a different problem.

Poor soil prevents seeds from germinating

Recently, a gardening influencer on social media was explaining how this one mistake had led to her garden failing the previous year. The mistake, she explained, was not realizing how poor her soil was, and that’s why none of her seeds germinated. 

While poor soil, by which people usually mean sandy, rocky, and low in organic matter and fertility, does cause issues with plant growth, it won’t by itself keep a seed from germinating. Remember, seeds don’t need any soil at all to germinate. Check out our article on what seeds do need here.

Poor soil can make it more difficult to achieve the proper conditions needed for germination, namely moisture, temperature, and oxygen, but by itself it won’t prevent sprouting. If the seeds aren’t popping up, you may have issues with soil toxicity killing seedlings after they germinate, hard soil preventing them from breaking the surface, or most likely, predation (something ate them).

Don’t water at midday (you’ll burn the plants)

Despite years of attempting to dispel this myth, and many university studies, people still cling to this one. The idea is that water droplets on the leaves act as a lens, focusing the sun’s energy like a magnifying glass and therefore sunburning the leaves. 

While watering at midday on a sunny, hot day is inefficient from a water use standpoint, it won’t hurt your plants. A summer rainstorm followed by a break in the clouds and the return of the sun doesn’t spell doom for your garden, and neither does a shot from the sprinkler. In fact, while some water is lost to evaporation, it’s far better to water plants in need of moisture in the middle of the day than waiting hours for the sun to go down. Moisture stress on plants can cause wilting and eventually damage and even death. Water when you need to, or when you can, and don’t worry about causing sunburn to your plants from water droplets.

Marigolds control pests

A few anecdotes and a misunderstood study have given rise to the idea that planting marigolds around your garden will deter pests. Everything from deer and rabbits to nematodes is supposed to be repelled by the strong odor of marigolds. 

While a deer won’t often eat a marigold, I’ve watched them walk right by them to eat other nearby plants, and the same happens with rabbits. Perhaps if you completely ringed your bed with marigolds like a fence, it might have some effect, but a marigold at each corner isn’t keeping any plant munchers away. 

The nematode issue is more nuanced. There is evidence that specific species of marigolds repel specific species of harmful nematodes, but it isn’t on a one-plant-here-or-there basis. The research in question involved planting marigolds as a cover crop. If you plant your entire garden bed to marigolds and nothing else for several months, the right marigold can repel a particular nematode. Unfortunately, a few marigolds at the edge of the bed or as companion plants won’t have the same effect.

Grow citronella plants to keep mosquitoes away

Citronella plant, a type of scented geranium, does indeed smell like the citronella oil sold to keep mosquitoes at bay. I grow a large pot of this plant every year on my deck, because I think it has gorgeous foliage. However, I am sad to report my confirmation of what the literature says: the mosquitoes are not bothered by the plant. I often brush off, swat, or otherwise swipe at mosquitoes while sitting not two feet from this large planter with three or four huge citronella plants. 

Like many herbal or aromatic mosquito repellent plants, the oils that are effective at repelling biting insects are only released when the leaves are crushed. You could pluck a leaf and vigorously rub it on your arms (I don’t know if that works), but the plant just growing by itself unmolested is not a repellent. 

Peonies need ants to bloom

Peony buds are often covered with ants, crawling all over doing their busy ant things. In fact, many have observed that the peonies won’t open until the ants have visited, so they must need the ants, right? Not so fast. That’s like saying you got sunburned at the beach, so the beach must cause sunburns.

Ants and peonies do go well together, but that’s because of a mutually beneficial relationship. The peonies exude a sweet liquid the ants like, and the ants visit and remove flower-eating insects while they are at it. The peonies will still bloom and open even without any ants. Once the peonies are done blooming, the ants will wander off in search of other food.

Add sand to clay soil to fix drainage problems

Gardeners often wish for a loamy garden soil. Loamy soils are part clay, part silt, and part sand, so it seems reasonable that if your soil is heavy toward clay, adding sand could help. And, sand is readily available and cheap. 

As many gardeners have found out to their dismay, adding sand to heavy clay soils usually results in something akin to concrete. It takes roughly equal parts sand to clay by volume to change the soil's texture. That’s a lot of sand (tons), and you’d need to mix it effectively.  Anything less just fills the gaps between clay particles, making the problem worse. The two most effective ways to deal with overly clayey soil are to add organic matter (lots of it over time) or create raised beds.

Houseplants will purify the air in your home

A misrepresented NASA study about using houseplants to purify air on space vessels is responsible for this one.  Titled "Interior Landscape Plants For Indoor Air Pollution Abatement," and released in 1989, the study found that certain plants, many of which we enjoy as houseplants, did indeed remove volatile organic chemicals (VOCs) from the air. The trouble is the math. 

The study utilized plants in a very small airspace, about the size of a cupboard. It was academic, but not practical. While adding houseplants will indeed improve the feeling of your home, the effect on air quality is unfortunately negligible. More recent research on this topic shows that you would need hundreds of these plants in your home to have a measurable effect. Two pothos and a snake plant won’t do it.