Growing a healthy garden in Pennsylvania means knowing which plants don’t play nice together. Some plants release chemicals that harm their neighbors, while others compete for the same nutrients or attract pests.
Understanding these toxic plant relationships can save you time, money, and the heartbreak of watching your garden struggle.
1. Tomatoes And Black Walnut Trees
Black walnut trees produce a chemical called juglone that’s deadly to tomato plants. When planted near these trees, tomatoes will wilt, yellow, and eventually die.
The toxic zone extends as far as the tree’s canopy reaches, plus about 50 feet beyond. Even after removing a walnut tree, the soil remains toxic for years, so find a different spot for your tomato patch.
2. Fennel And Beans
Feathery fennel might look innocent, but it’s actually a garden bully. When planted near beans, fennel releases compounds that stunt their growth and reduce your harvest significantly.
The aromatic oils that make fennel tasty in cooking work as natural herbicides in the soil. Keep at least 3-4 feet between these plants, or better yet, give fennel its own dedicated space away from most vegetables.
3. Cabbage And Strawberries
Cabbage and strawberries make terrible neighbors in Pennsylvania gardens. Cabbage plants release chemicals that inhibit strawberry growth, leading to smaller berries and weaker plants.
Meanwhile, strawberries can attract pests that damage cabbage. Keep these plants in separate garden beds to avoid disappointing harvests. Consider planting strawberries near borage or spinach instead for better results.
4. Sunflowers And Potatoes
Those cheerful sunflowers cast a dark shadow on potato plants. Sunflowers release chemicals that inhibit potato growth and can reduce your spud harvest by up to 30%.
Additionally, both plants are heavy feeders competing for the same nutrients. Pennsylvania gardeners should keep sunflowers at least 5 feet away from potato patches. For best results, plant your potatoes on the opposite side of your garden from these towering beauties.
5. Onions And Beans
Onions and beans create a toxic relationship in the garden. Onions release compounds that slow bean growth, while beans can’t effectively fix nitrogen when planted near alliums.
Pennsylvania gardeners often notice stunted bean plants with yellowing leaves when these vegetables are neighbors. Keep all onions, garlic, and other alliums at least 3 feet away from your bean rows for the best harvest from both plants.
6. Allelopathic Goldenrod And Native Wildflowers
While goldenrod adds beautiful yellow blooms to Pennsylvania gardens, it’s secretly waging chemical warfare. It releases compounds that prevent germination and growth of many native wildflowers.
The roots exude substances that remain active in soil for months. If you love goldenrod, plant it in contained areas away from your delicate native plant collection. Consider butterfly weed or purple coneflower as friendlier alternatives for pollinator gardens.
7. Cucumber And Sage
Cucumber vines wither when planted near aromatic sage. The essential oils in sage that repel certain garden pests unfortunately also inhibit cucumber germination and growth.
Pennsylvania gardeners often notice stunted cucumber plants with fewer fruits when these plants share space. Keep sage with other Mediterranean herbs like rosemary and thyme, and plant your cucumbers at least 4 feet away for better yields.
8. Corn And Tomatoes
Corn and tomatoes battle for nutrients when planted as neighbors. Both are heavy feeders that deplete the soil quickly, leading to stunted growth and reduced harvests for both crops.
Even worse, corn attracts tomato hornworms while tomatoes can harbor corn earworms. Pennsylvania gardeners should keep these popular garden staples on opposite sides of the vegetable patch. Try planting beans between them as a buffer that adds nitrogen back to the soil.
9. Brassicas And Strawberries
Broccoli, cauliflower, and other brassicas release chemicals that stunt strawberry growth. When planted nearby, strawberry plants produce fewer runners and smaller fruits.
The soil-borne compounds can persist for months in Pennsylvania’s clay-heavy soils. For sweeter berries and better brassicas, keep these plants in separate garden beds at least 3 feet apart. Consider planting strawberries near borage or spinach instead for improved flavor and yield.
10. Mint And Vegetable Gardens
Mint’s aggressive nature makes it a garden menace when planted near vegetables. Its roots release chemicals that inhibit germination and growth of many common garden plants.
Left unchecked, mint will quickly take over a Pennsylvania garden bed. Always plant mint in containers, even when buried in the ground. This contains those spreading roots and prevents the essential oils from affecting nearby tomatoes, peppers, and leafy greens.
11. Asparagus And Garlic
Asparagus spears struggle when garlic grows nearby. The sulfur compounds that give garlic its distinctive flavor and aroma inhibit asparagus growth and can reduce yields by up to 25%.
Since asparagus is a perennial that stays in place for decades in Pennsylvania gardens, it’s especially important to keep garlic and other alliums away from established asparagus beds. Plant onion family crops at least 4 feet away from your asparagus patch.
12. Allelopathic Tree Of Heaven And Garden Plants
The invasive Tree of Heaven (Ailanthus altissima) is a serial plant killer in Pennsylvania landscapes. It produces toxins through its roots, bark, and leaves that prevent nearby plants from growing.
These chemicals can persist in soil for years, creating dead zones in gardens. Remove these trees completely, including all roots, before attempting to garden nearby. Consider raised beds with fresh soil if you must garden near where these trees once stood.
13. Peppers And Fennel
Pepper plants produce significantly fewer fruits when grown near fennel. The aromatic compounds in fennel that give it that distinctive licorice flavor act as growth inhibitors to pepper plants.
Pennsylvania gardeners often notice stunted pepper plants with pale leaves when these plants share space. Keep fennel in a dedicated herb garden at least 5 feet away from your pepper patch. Your pepper harvest will thank you with more abundant fruits.
14. Potatoes And Tomatoes
Though related, potatoes and tomatoes make terrible garden neighbors. Both are susceptible to the same diseases, creating a playground for pathogens that can quickly spread throughout your garden.
Late blight, which caused the Irish Potato Famine, can devastate both crops when planted close together. Pennsylvania’s humid summers create perfect conditions for disease spread. Keep these nightshade relatives at opposite ends of your garden to reduce disease pressure.
15. Allelopathic Sunflowers And Beans
Those cheerful sunflowers have a dark secret – they’re toxic to bean plants. Chemicals released from sunflower roots inhibit bean germination and stunt their growth.
Pennsylvania gardeners often notice poor bean harvests when these plants share space. The allelopathic effect can persist in soil even after sunflowers are removed. Plant your beans at least 5 feet away from sunflowers, or better yet, in a completely different garden bed.
16. Roses And Grapes
Roses and grapevines battle each other when planted too close together. Both are susceptible to many of the same fungal diseases, creating a perfect storm for pathogen spread in Pennsylvania’s humid climate.
Black spot and powdery mildew jump easily between these plants. Additionally, both are heavy feeders competing for the same nutrients. Keep roses and grapes at least 10 feet apart in your landscape to prevent these problems.
17. Allelopathic Rye And Vegetable Seedlings
Rye cover crops protect soil but can harm vegetable seedlings. Rye releases compounds that inhibit germination and growth of many common garden plants like lettuce and spinach.
Pennsylvania gardeners using rye as a winter cover crop should wait 3-4 weeks after turning it under before planting vegetables. This allows the allelopathic chemicals to break down in the soil. For spring plantings, terminate rye cover crops early to avoid stunting your vegetable seedlings.
18. Tall Trees And Acid-Loving Plants
Maple and oak trees create surprisingly toxic conditions for acid-loving plants like blueberries, azaleas, and rhododendrons. Their fallen leaves break down into alkaline compounds that neutralize soil acidity these plants need.
Additionally, the dense shade reduces flowering and fruiting. Pennsylvania gardeners should plant acid-lovers at least 20 feet from large deciduous trees. Regular applications of acidifying fertilizers can help counteract the alkalizing effect of tree leaf decomposition.