Thank you to Rocket Money  for supporting today’s video.

Rocket Money helps you reach your financial  goals by canceling unwanted subscriptions, negotiating bills on your behalf, and budgeting. To download Rocket Money for free,  head to rocketmoney.com/scishow or click the link in the  video description down below. [ INTRO ] Maybe you’ve noticed that those tomatoes you  planted in your backyard aren’t doing so great.

If you planted them anywhere  near a black walnut tree, then you can take comfort  in knowing that this time, your inadequate gardening skills aren’t to blame. You and your tomatoes could  be the victim of sabotage. But not the jealous neighbor  kind, the botanical kind.

It’s called allelopathy, and it’s kind  of like the plant version of Round-Up. You see, some plants produce chemicals that  can suppress or even kill certain other plants. These are called allelochemicals.

They give the plants that  produce them an advantage when competing with other  species for limited resources. In other words, they help the  plant bushwhack the competition. One of the most well-known examples is  the aforementioned black walnut tree.

The Roman naturalist Pliny the Elder was the  first to observe that black walnut trees tend not to have many leafy green neighbors, but  now we know why they’re such lonely trees. The tissues of the black walnut tree  contain a chemical called hydrojuglone. Hydrojuglone is found in the leaves and stems, but it’s more concentrated in the  walnut hulls, buds, and roots.

Now, hydrojuglone on its own is  harmless, but when it’s exposed to air, it becomes a toxic chemical called juglone. Scientists are still figuring  out exactly how juglone works, and why it’s so toxic to the  plants that are sensitive to it. One of the ways juglone is thought to  work is by getting into the mitochondria in the roots of certain plants, which  can prevent them from taking in oxygen.

Because the roots of the tree release  juglone, juglone-sensitive plants can’t flourish in the soil around the root system. But the tree also drops leaves  and nuts, and as these decompose, they add even more juglone to the  soils surface, which means the ground under a black walnut tree's canopy is a   particularly hostile place if  you are sensitive to juglone. Now Black walnut trees aren’t  the only plants to do this, and juglone is not the only toxin  that plants have evolved to make.

A cereal grain called sorghum has  taken its sabotage underground. Sorghum is farmed for livestock  feed in the southern United States, and is also used to make sweetener and ethanol. But, sorghum exudes an allelochemical  called sorgoleone from its root hairs.

Sorgoleone reduces the shoot growth of  competing plants in a couple of different ways. It’s thought to interfere with photosynthesis  by binding to an important protein complex called photo system II or PSII, which inhibits the transportation of electrons  that are necessary for photosynthesis. Hank makes a PS5 Joke here, should we leave it?

It might also block an enzyme the  plant use to make carotenoids,   compounds that collect energy from light. And it seems to impede water uptake in  the roots which is, you know, important. Another plant vying for the title of  “least friendly neighbor” is musk thistle.

This large, invasive plant is native  to parts of Europe, Asia, and Africa and now grows in pastures and along  roadsides in much of North and South America, as well as New Zealand and Australia. The plant grows a rosette of leaves  at the ground level in its first year, then the following year it produces a stalk  from which the plant’s purple flower grows. These plants die after flowering once, and  when the musk thistle rosette decomposes, chemicals within the rosette  impede nitrogen fixation and upset the long-term  nitrogen cycle in that area .

This inhibits the growth of certain pasture  plants, and it’s especially harmful to legumes, the group of plants that includes  things like beans and peas. This can pose a problem for farmers since we like   to grow and eat those things,  so we want them to do well. Another thistle-like plant called spotted knapweed has roots that exude a type of  allelochemical called catechin, which prevents seed germination  in competing species.

Catechin is so good at preventing germination that  it even inhibits germination of spotted knapweed, which scientists think might be a way for  the plant to control its own population a nd prevent competition over  limited resources in a given area. But, Allelopathy isn’t always foolproof. Some plants are immune to allelochemicals.

We know that while tomatoes, cabbage, and  eggplants are especially sensitive to juglone, other foods like onions, parsnips, and  snap beans are not affected by it at all. What’s more interesting is that exposure   to certain allelochemicals may  even help some kinds of plants. Juglone, for example, has been shown to increase the growth of young muskmelon plants  and the germination of white oak trees.

For muskmelon, scientists  think this might have something to do with the cells that transport water  and nutrients from the roots of the plant. The radius of those cells seems to increase when   juglone is applied to the  seeds before germination. Juglone also seems to be associated with an  increase in the protein content of the seedlings.

What’s really interesting  is that these experiments also tested the effects of juglone on cucumbers, which are close relatives of muskmelons. And the juglone had the opposite  effect in the cucumbers. Which is to say, it did what juglone usually does, and suppressed their growth.

The researchers don’t have a clear idea why  these two plants react so differently to juglone, which is yet another reason we’re so  interested in these allelochemicals. Besides being a fascinating look into  the cutthroat world of plant evolution, allelopathy also has huge  implications for horticulture. Recent studies have suggested that  commonly used herbicides like glyphosate could be harmful to the health  of humans and other animals.

But herbicides have helped us grow  our crops efficiently for decades, so to get rid of them completely  would harm our food supply chain. We need more herbicides and safer herbicides, and  as it turns out, they’re right there in nature. Scientists are still figuring out  exactly how allelochemicals work, so it might be a while before  you’ll be able to pick up an   allelochemical herbicide  at your local garden store.

In the meantime, make sure to plant  your tomatoes a long, long way from that black walnut tree. This SciShow video is supported by Rocket Money, an all-in-one platform that helps you manage  your money, monitor your credit score, and build your personal savings. Managing your money can look like a lot of things.

For some people, it’s being reminded when you’ve overstepped  your monthly, quarterly, or annual budget. For others, it’s getting rid of  recurring expenses you don’t use. And Rocket Money does all of that!

They’ll even negotiate your bills for you,  from internet service to credit cards, so you’re not overpaying for  the services that you do use. After all of that, you  could end up with more money in your bank account than usual. And Rocket Money can also help  you set up a smart savings account to hold onto that money for  when you really need it.

You choose how much you  want to save, and how often,   and they’ll automatically deposit that  amount into a secure account on the app. From there, you can withdraw anytime. To try it out for free, head  to rocketmoney.com/scishow or click the link in the  video description down below.

And thank you for watching to the end! [ outro ]