There’s very few flowers as iconic and well known as the sunflower. No one can fail to be cheers up by them.
But if you’re planning on planting sunflowers in your garden you need to be aware they don’t get on with everything. In fact there are some plants and flowers that you absolutely shouldn’t plant with sunflowers. Well, unless you want them to fail and not grow.
So I’m going to take you through the worst companions for your sunflowers and why. And then I’ll give you a few good options for planting with them, as well as tips so you are all set for success.
Understanding Sunflowers
Last summer I watched my neighbor’s gorgeous sunflower patch slowly wither away. And it really broke my heart.
Sunflowers are such beautiful flowers it’s always a shame to see suffering. So what went wrong?
Well she’d planted them alongside what looked like a beautiful mixed garden. But by mid July they were drooping and struggling. The culprit? Poor companion choices that meant they never had a chance.
Sunflowers may be pretty but there also quite demanding in your garden. They are allelopathic plants, which means they release natural chemicals that can stop the growth of any nearby plants.
But it’s not just about those chemicals. Sunflowers are also notorious for hogging resources. Their huge root systems can spread 2 to 3 feet in all directions and so steal the water and nutrients.
Having giant flower heads look great but they need serious energy to develop. That means sunflowers need virtually every bit of nutrition they can get their roots on.
And on top of all that sunflowers create a significant amount of shade as they mature. And that blocks the sun from reaching any smaller plants below them.
I’ve seen entire vegetable patches fail because someone thought it would be a good idea to plant sunflowers nearby without considering the shadow effect.
So getting the companion planting wrong with sunflowers will mean more than disappointing flowers. It could be as bad as total crop failure, stunted growth across your entire garden and months of wasted effort.
I’ve been there and it’s not fun. So let’s look at the plants to avoid planting with sunflowers so you don’t have to go through it.
Plants You Should Never Plant with Sunflowers
Black Walnut Trees
If you know about allelopathy you’ll know this is a no no. Black walnut trees produce juglone, a compound that’s toxic to sunflowers and will literally poison them.
The roots, bark and even fallen leaves contain juglone. And it can stay in the soil for months even after you’ve got rid of a black walnut tree. So if you’ve ever had black walnuts on your property make sure you test your soil before planting sunflowers. It’s not worth the gamble.
Potatoes
Potatoes might seem like a good companion but they really bring the worst out of each other. There both heavy feeders and compete aggressively for nitrogen and phosphorus.
Worse yet sunflowers can actually stunt potato growth through their allelopathic properties. And the potatoes aren’t completely innocent. They can harbor diseases that affect sunflowers too.
I know about this first hand as I tried to maximize my small garden space by interplanting them. The potatoes never got bigger than golf ball sized and the sunflowers didn’t get close to their full height. It was a complete disaster.
Tomatoes
Here’s where things get a little bit tricky. Some sources will tell you tomatoes and sunflowers can work together.
Personally I don’t believe they can. At least not successfully. (But I will say this is entirely my experience and may not be the case for you)
Tomatoes need consistent moisture and nutrients. But because sunflowers are so aggressive in now they compete with other plants they’ll steal both from tomatoes. And on top to that the shade from mature sunflowers will almost certainly reduce how much tomatoes produce.
Then here’s the allelopathic compounds from sunflowers. They can affect how the roots develop or tomatoes. I’ve seen tomato plants planted near sunflowers develop weak and shallow roots that make them more liekly to suffer from drought stress.
Beans (especially pole beans)
If you know about beans then this one might be a little surprising to you. But they make terrible companions for sunflowers.
Pole beans need support and while sunflower stalks might seem perfect they’re far too smooth and unstable. The beans end up competing for the same vertical space and neither plant does well.
Bush beans do slightly better but still struggle with how much competition there is for shade and root space. Even though beans fix the nitrogen levels in the soil it’s ruined by how aggressive the sunflowers are.
Fennel
Fennel simply doesn’t work well with anything. Sunflowers are no exceptions.
Fennel releases allelopathic compounds as well, and they stop the growth of most plants, including sunflowers. Even worse than that fennel attracts beneficial insects away from sunflowers which messes with their pollination.
Lettuce and Leafy Greens
Lettuce, spinach, arugula and other leafy greens are crops that don’t do well with shade. So much so that the shadow of a fully grown sunflower will stop them from growing.
They also prefer consistent moisture which sunflowers, with their large roots, steal all of.
The allelopathic effects are particularly harsh on lettuce family plants too. I’ve watched lettuce planted within 6 feet of sunflowers grow bitter, tough leaves and bolt prematurely.
Onions and Garlic
While onions and garlic are usually great companion plants they don’t work with sunflowers. They like well drained soil and so can’t compete with how much water and nutrients the sunflowers need. And take.
The allelopathic compounds from sunflowers also seem to affect how the flavor develops in onions. They come out with less flavor and a much less strong smell.
And while this is less important – the timing is all wrong for these two. Onions and garlic need to be planted early in the season when sunflowers are just getting established. So you’re going to be juggling different care needs.
Peppers
Peppers, like tomatoes, struggle with both the shade and how much competition sunflowers put up. But peppers are even more sensitive to the allelopathic compounds. I’ve seen pepper plants with yellowing leaves and less fruit simply from being planted near sunflowers.
Hot peppers seem particularly affected. They will produce much smaller and less spicy fruits if you grown them near sunflowers. Sweet peppers do a little better but you still still get a smaller yield.
Carrots and Root Vegetables
Root vegetables like carrots, parsnips and radishes need loose, well draining soil to grow properly. The roots of sh flowers are so dense that they make the soil compacted. This means root vegetables roots can’t push down deeply into that soil.
Then you’ve got the competition for nutrients which means root vegetables often develop weaker roots instead of the plump vegetables you’re hoping for.
Carrots are particularly sensitive to the allelopathic compounds. You’ll find your carrots have forked or stunted roots when grown near sunflowers.
Brassicas (Cabbage, Broccoli, Cauliflower)
Brassicas and sunflowers are totally incompatible. They’re just at polar ends of the spectrum when it comes to what they both need.
Brassicas like steady moisture and cooler temperatures and sunflowers create hot, dry conditions because of the shade from their height and how much water they consume. The allelopathic compounds from sunflowers can also interfere with the sulfur compounds that give brassicas their flavor.
I’ve seen cabbage heads fail to form properly and broccoli produce tiny, bitter heads when grown near sunflowers.
Strawberries
Strawberries need consistent moisture and partial shade but sunflowers create the wrong kind of shade. The kind that is too dense and sporadic.
The allelopathic compounds also seem to affect strawberries runners and fruit. Most importantly strawberries are perennial crops that need stable growing conditions. Sunflowers though are annuals that disturb the soil each season. So it’s no shock they don’t get along.
Herbs (Basil, Oregano, Thyme)
Most herbs prefer well draining soil and can’t handle the way sunflowers will compete for water.
The allelopathic compounds (yep, I keep going on an out them but they’re so important) also affect the essential oil production in herbs. You end up with a weaker flavor and less potent medicinal properties. Basil is particularly sensitive and will often develop weak stems and poor leaves if it’s near sunflowers.
Small Annual Flowers
Petunias, impatiens, begonias and other small annual flowers simply can’t compete with the size and how much resources sunflowers demand. They’ll be overshadowed literally and figuratively, often becoming leggy and only producing a few blooms as they stretch desperately for sunlight.
Good Companion Plants for Sunflowers
So now I’m done scaring you with what not to plant with your sunflowers lets get a bit more positive.
These are a few plants that work brilliant planted together with sunflowers:
Corn is probably the best companion for sunflowers. They’re both tall and love the sun, as well as having similar water and nutrient needs. They don’t compete directly since corn is a grass and sunflowers are broadleaf plants. And as a bonus they look great when grown together.
Nasturtiums work wonderfully as ground cover beneath sunflowers. They’re tough enough to handle some shade and actually benefit from the protection that larger plants give them. To top that all off nasturtiums are also edible and attract insects that benefit the whole garden.
Zinnias and other tall annual flowers are great for creating beautiful displays with sunflowers. Choose variations that grow to at least 3 or 4 feet tall so they don’t get completely overshadowed.
Marigolds are another excellent choice. They are tough enough to handle sunflower competition and will help keep pests away that can upset the sunflowers.
Tips for Successful Companion Planting
Plan for mature size
A small sunflower seedling will, assuming you do everything right, become around 10 foot tall. So plan your spacing beforehand and remember that sunflowers cast shade in all directions as the sun moves.
Consider timing
Plant companions that have similar germination and grow at the same sort of rate. Or if not possible then stagger your planting times so everything reaches maturity when it should.
Test your soil regularly
Sunflowers are heavy feeders so you’re going to have to to monitor the soil more frequently when growing them with companions.
Water deeply but less often
This encourages the roots to grow more deeply in both sunflowers and their companions. This will also reduce any direct competition in the surface of the soil.
Provide adequate spacing
I recommend at least 6 feet between sunflowers and any other plants. And make sure any smaller companions are in place where they get the morning sun.
Final Thoughts
If you’re a big fan of sunflowers then learning what not to plant with them is going to be valuable. It will save you from years of disappointment, as well as helping you to create better garden designs.
Something good to remember is that sunflowers aren’t trying to be garden bullies. They’re just doing what they’ve evolved to do. They’re allelopathic because it helps them survive in nature and compete so strongly for resources because they need so much fuel for their huge flower heads and stalks.
Instead of fighting against their nature, work with it. Give your sunflowers the space they need, choose companions wisely and you shouldn’t have any problems.