Allelopathy | The Peterborough & Area Master Gardeners

by Emma Murphy, Master Gardener

Gardeners have been passing down tips, tricks, and knowledge for generations. Some of this advice is science-based and works, but much of it is a mix of folklore and superstition. While some of it is benign, some of it may actually harm your gardens or have you spending money when you don’t need to.

Here are five of my favourite garden myths – there are hundreds out there – just google ‘garden myths’ and you’ll see what I mean.

I think social media has intensified the problem – in the past information spread through word of mouth, often handed down through generations. Now anyone with a computer can claim to be an expert, and provide inaccurate information that others will share.

Part of my reason for becoming a Master Gardener was to expand my gardening knowledge and share my passion for growing with others using solid, science-based information. We offer our services to the public in many forms – through presentations, advice clinics, answering email queries, and publishing blogs like this one!

The Master Gardeners of Ontario (MGOI) website shows where all the Master Gardener groups are located – find yours and their website and take advantage of their knowledge and expertise. If they don’t have the answer they will go research it for you.

MGOI also has a great Facebook page where you can post questions to Master Gardeners. And I highly recommend The Garden Professors Facebook page for science-based gardening information.

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Myth: You Should Stake A Newly Planted Tree

Truth: Unless it’s top-heavy or in an especially windy site, your tree does not require staking. Some movement is actually good for young trees. I loved this description “Just as our muscles grow larger with exercise, tree trunks grow thicker and stronger when they’re allowed to move.”

The response of trees and plants to wind is called thigmomorphogenesis (yes that’s a word!). The buffeting from winds releases ethylene gas, a growth mediator that triggers the formation of wood-strengthening lignin.

While staked trees tend to grow taller, their trunks are skinny and weak, so if you decide you must stake, stake as loosely as possible and only for a short time (no longer than six months). Make sure to use something soft against the tree bark to keep from cutting into it. But best to practice tough love – your tree will appreciate it.

P1030592-e1555428966560 — Photo used with permission of The Garden Professors http://gardenprofessors.com/

Myth: Gravel in The Bottom of Containers Improves Drainage

Truth: This myth will not die. We’ve all been told to place stones or pieces of pot at the bottom of our containers “for drainage”. The reality is that added gravel or rocks to the bottom of your pot will actually accelerate the potential for root rot, rather than preventing it. Water is pulled down through the container by gravity and builds up near the drainage hole. A layer of gravel at the pot’s base serves as the drainage hole and collects water in the same way. So gravel actually moves the pool of water higher up the pot, where it damages your plant.

As long as there is a hole in the bottom of the container, water will find its way out without the need for stones.

Myth: Add Epsom Salts To The Soil Helps Tomatoes Grow

Truth: You would think it was a miracle cure for everything
“It helps seeds germinate”
“It makes plants grow bushier”
“It can prevent blossom end rot in tomatoes” (see bonus information at the bottom of this blog!)
“It can prevent transplant shock”
“It results in more flowers”
“It increases chlorophyll production”
“It deters pests, such as slugs and voles”
“It reduces the total amounts of fertilizers needed”

Magnesium sulfate, or Epsom salt, is a naturally occurring mineral consisting of magnesium and sulfur (MgSO4). Magnesium is a necessary element for plant growth but adding unnecessary salts to your soil will destroy your soil structure over time. You are better to simply add compost or worm castings to the soil. There are decades of research that has documented damage done to both plants and soil with overuse and misuse of magnesium sulfate. Want to read more about it? Check out this peer reviewed study by Washington State University’s Extension Center (click here for more of their excellent science-based studies). The best thing epsom salts can be used for is a nice hot bath after working hard in your garden all day. sphynx-1521190

Myth: You Can’t Grow Anything Near A Black Walnut Tree

Truth: This one is a personal favourite, since I have extensive flower and vegetable gardens in the vicinity of two almost 150-year-old black walnuts. While the roots of black walnut (Juglans nigra) do release an allelopathic chemical known as juglone that inhibits the growth of some plants, the idea that nothing grows under a walnut started gaining traction in the 1920s when a Virginia researcher saw his tomatoes were suffering and just assumed the nearby trees were at fault based on folklore he had heard. Washington State University put out an excellent peer reviewed paper in 2019 explaining the history of walnut allelopathy.

Enjoy your walnut trees! Not only are they robust landscape plants but they provide food and habitat for wildlife and birds. Here’s my lovely perennial garden under my walnut tree (and featured in Linda Chalker-Scott’s paper mentioned above) and my previous blog specifically on this topic.

(Bonus – information on Blossom End Rot)

Research has shown that calcium deficiency is not the cause of blossom end rot. Egg shells won’t correct it. Epsom salts (which are magnesium sulfate) won’t correct it. Nor will coffee grounds, Tums, calcium sprays, dairy products, or any of the other things that are usually recommended for it. It just corrects itself. It’s not caused by a fungus, bacteria, or virus. It’s not in the soil. It’s an internal condition of the plant. The cause appears to be simply environmental: low temperatures at night, fluctuating temperatures, watering too often, etc. Ammonia fertilizer is linked to it, so some forms of plant food can be a problem. But basically: just pick off the fruit that are affected, water more deeply and don’t allow the plant to get severely drought stressed (daily watering is probably not necessary unless you have unusually fast-draining soil or are growing in a container). The next fruit will probably be fine. It’s usually a problem with the first tomatoes, peppers. and squash that set in the season and the rest come along fine.

by Emma Murphy, Master Gardener

As a Master Gardener, one of the most common questions I get asked is about the toxicity of walnut trees (Juglans nigra).

“Well I heard that the juglone stuff in the roots kills everything and that I can’t plant anything under or anywhere near a walnut tree.”

Well yes Virginia, you can plant a garden under a walnut tree, and have it thrive. Let’s look at Exhibit 1 below – an established perennial bed under a walnut tree. It’s at our house, so I guarantee it’s real, and it’s been there since late 2007.

Lots of lovely perennials here – hostas (Hosta spp.), daylilies (Hemerocallis), bearded iris (Iris germanica), summer phlox (Phlox paniculata), wild ginger (Asarum canadense), purple coneflower (Echinacea purpurea), ditch lilies (Hemerocallis fulva), although those last ones are doing just a little too well LOL. Don’t believe it’s a walnut tree? Here’s a photo from a bit further back on the street.

Yes, she’s a big old lady – we think she was planted around the same time the house was built, making her about 140 years old. I think she’s getting a bit tired out – most years she doesn’t produce too many walnuts. Her slightly younger cousin is to the left of the barn in the background – LOTS of walnuts off her (and yes a garden under that one too).

So where does this fallacy come from that ‘nothing grows under a walnut’? Well certainly all parts of the walnut tree contain a chemical called juglone (heck it’s even in the Latin name!). Juglone is a chemical that affects other plants growing nearby (a phenomenon called allelopathy). Simply put, allelopathy involves “living or dead plant parts that release chemicals into the soil which have an effect on other plants—positive or negative.” For walnuts it seems like an attempt at self preservation, with juglone acting like a natural herbicide on other plants.

As Professor Linda Chalker-Scott explains in her recent (2019) peer-reviewed Washington State University Extension paper, damage to tomatoes and other crops near walnut trees in the 1920s caused people to believe that toxic chemicals were involved, and this perception persisted and became widespread despite there being no evidence (and this was before social media existed!). The US Department of Agriculture did field testing – no problems. When applied in a laboratory setting to seeds and seedlings it did cause stunting, wilting, and necrosis, but the specific way it did this was unclear. The most recent science suggests that juglone disrupts photosynthetic and respiratory pathways and interferes with water uptake in plants.

So! It does affect plants – the laboratory says so. Well yes, and no. Field tests do not support the laboratory work, which doesn’t accurately mimic real life conditions in your average residential garden (again, for more detail read the excellent paper referenced above). Two very old University Extension papers (1973 and 1993) continue to be used to state which plants are ‘sensitive’ and which are ‘tolerant’. However these were simply observational papers—meaning that they correlate the presence of walnut trees with damage to other species but do not confirm a causative relationship. Neither should be considered good scientific evidence.

Gardening With Walnut Trees – My Story

I am sure the scientists, arboretums, farmers, and garden writers will continue to debate this topic for a while. Meanwhile, here’s our story. In 2007 I wanted a garden bed under our black walnut in my front yard. At the time I had heard the walnut horror stories, so I thought – well, how about I just don’t disturb the roots of the tree? (not a good thing to do when establishing any garden beds under a tree). I put good topsoil and compost down, making sure to minimize tearing up of the soil and roots, and planted, and watered, and waited. Things grew. Winter happened. Next spring plants came up. For the most part I just moved perennials that were already on site, although some hostas were new. Here’s the garden in 2008 in the fall.

Fall asters (Astereae spp.), sedums (Sedum ‘Autumn Joy’ in this case), hostas (Hosta spp.), wild ginger (Asarum canadense), fall rudbeckias (Rudbeckia fulgida and triloba), purple coneflower (Echinacea purpurea), ninebark (Physocarpus opulifolius), iris (Iris spp.), astilbe (Astilbe spp.), sweet woodruff (Galium odoratum), Ozark sundrops (Oenothera macrocarpa), coral bells (Heuchera spp.), cranesbill (Geranium spp.). All doing just fine.

And 2009 below, in the spring. Irises (Iris spp.) lemon lilies (Hemerocallis lilioasphodelus), lungwort (Pulmonaria spp.), poppies (Papaver rhoeas), ditch lilies (Hemerocallis fulva), variegated solomon’s seal (Polygonatum odoratum ‘Variegatum’), bracken (Pteridium aquilinum), bleeding hearts (Dicentra spectabilis), daylilies (Hemerocallis spp.).

Certainly, planting in general under walnuts is challenging – they cast dense shade and have extensive, water hungry root systems. Watering after establishment and for the first season is critical. We also had our walnut tree pruned professionally (it needed it) so it actually gets decent sunlight in the latter part of the afternoon. Like any fruit or nut tree they are messy, from their spring pollen to their leaves and nuts…oh those nuts.. 2017 was a crazy year – buckets of walnuts (I even had to engage my neighbours’ lovely children from across the street to help collect them) to 2018, with almost no walnuts. My trees are old too – although well pruned, their leaves drop at that first hard frost.

In Defence of Walnut Trees

Black walnuts are not all bad, and I will continue to treasure them in my yard. They are an amazing shade tree, are highly valued for their fine grained dark wood (for furniture), a great food source for wildlife and birds, and my white breasted nuthatches’ favourite spot to hide their seeds.

We have definitely had our challenges with our walnut trees, and I’ve learned a great deal over the past 20 years. But one thing I know – I can garden with them around. You can too.