When it comes to planting perennials, not all neighbors play nice. Some pairings are a recipe for disaster — turning your garden from a dream into a battlefield.
In this guide, we’ll spill the dirt on 24 perennials you should never plant side by side. Avoid these combos, and your garden will thank you!
1. Mint and Any Other Garden Plants
Mint’s aggressive spreading habit makes it a garden bully that will quickly overtake neighboring plants. Its underground runners (rhizomes) extend rapidly in all directions, stealing nutrients and water from everything nearby.
Always plant mint in contained spaces or dedicated pots sunk into the ground. Even other herbs like thyme and oregano will be smothered by mint’s unstoppable growth. Many gardeners have learned this lesson the hard way, watching helplessly as their mint patch becomes a mint invasion.
2. Sunflowers and Potatoes
Sunflowers release chemicals into the soil that inhibit potato growth – a phenomenon called allelopathy. When planted too close together, your potato harvest will suffer dramatically with smaller tubers and reduced yield.
The tall sunflowers also create too much shade for sun-loving potatoes. Keep these plants in separate garden beds, with sunflowers on the north side of your garden to prevent unwanted shadows. Many gardeners report potato plants becoming stunted and yellowish when grown near sunflower roots.
3. Foxglove and Vegetable Gardens
Foxglove contains powerful cardiac glycosides that make every part of the plant highly toxic. When planted near vegetables, there’s risk of accidental contamination during harvesting or from rain splashing soil between plants.
Children might not distinguish between the plants when helping in the garden. Foxglove’s dramatic spires look stunning in ornamental borders, but keep them far from anything edible. Some gardeners create dedicated “toxic plant” sections safely away from food crops and play areas.
4. Black Walnut Trees and Most Perennials
Black walnut trees produce juglone, a natural toxin that prevents many plants from growing nearby. This chemical warfare extends through the entire root zone, which can spread 50-60 feet from the trunk!
Perennials like peonies, baptisia, and most vegetables will wither and die when planted in juglone-rich soil. Only juglone-resistant plants like hostas and daylilies stand a chance. The effect persists even after tree removal, as juglone remains in soil from decomposing roots and fallen leaves.
5. Peonies and Aggressive Growers
Peonies develop slowly and resent competition from fast-spreading perennials like yarrow, bee balm, or ornamental grasses. These gentle giants need their space to thrive and may take years to recover if crowded.
Once established, peonies can live for decades but remain vulnerable to being overshadowed. Plant them with equally mannerly companions like iris, lily-of-the-valley, or small spring bulbs. Garden veterans know the heartbreak of watching a treasured peony decline after an aggressive neighbor moves in.
6. Delphiniums and Tomatoes
Delphiniums and tomatoes both fall victim to the same diseases, particularly early blight and powdery mildew. Planting them together creates a disease superhighway that can quickly devastate both plants.
Their soil and water requirements also clash dramatically. Delphiniums prefer cool, consistently moist conditions, while tomatoes need warmer soil and hate wet foliage. Garden planning should separate these incompatible plants by significant distances. Experienced gardeners maintain distinct growing zones for ornamentals versus vegetables.
7. Lavender and Water-Loving Plants
Mediterranean lavender demands dry, well-drained soil and bright sunshine. Planting it alongside moisture-lovers like astilbe, ligularia or hostas creates impossible growing conditions for both plants.
The constant moisture needed by bog-loving perennials will cause lavender roots to rot. Meanwhile, the dry conditions lavender craves will leave water-hungry neighbors parched and struggling. Garden designers group plants with similar needs to avoid daily watering dilemmas. Creating dedicated xeriscaping zones keeps drought-tolerant plants like lavender properly maintained.
8. Rue and Basil
Rue contains natural compounds that actively inhibit basil’s growth and flavor development. When planted together, basil plants become stunted and produce fewer leaves with diminished essential oils.
Rue also attracts black swallowtail butterfly caterpillars that will happily munch on nearby basil plants. Keep these herbs in separate garden areas to avoid conflict. Interestingly, rue has similar negative effects on many brassicas like cabbage and broccoli, making it a troublesome companion for many edible plants.
9. Tall Perennials and Short Sun-Lovers
Towering perennials like hollyhocks, delphiniums, and Joe-Pye weed create dense shade that can kill sun-worshipping plants like coreopsis, gaillardia, and lavender. The shadow effect becomes more pronounced as the season progresses.
Always position tall plants on the north side of garden beds where their shadows won’t cover smaller neighbors. Height differences of just 2-3 feet can mean the difference between thriving plants and sad, leggy specimens stretching desperately for light. Garden designers use plant height as a fundamental planning tool.
10. Phlox and Other Mildew-Prone Plants
Garden phlox is notoriously susceptible to powdery mildew, which spreads rapidly through air currents to infect other vulnerable perennials. Planting it near bee balm, coreopsis, or zinnias creates a mildew breeding ground.
The fungal spores multiply exponentially when host plants grow in close proximity. Space these mildew-magnets far apart and ensure good air circulation with proper spacing. Modern mildew-resistant phlox varieties like ‘David’ can help, but separation remains the safest strategy for preventing garden-wide outbreaks.
11. Fennel and Most Vegetables
Fennel secretes compounds that inhibit the growth of many common garden vegetables and herbs. Beans, tomatoes, peppers, and kale all struggle when planted near this licorice-scented perennial.
Even other herbs in the same family, like dill and cilantro, will show stunted growth near fennel. Designate a separate area for fennel, ideally at least 3-4 feet from other plantings. Some gardeners maintain dedicated fennel patches specifically to attract beneficial insects like swallowtail butterflies, while keeping it isolated from edible crops.
12. Aggressive Spreaders Near Delicate Perennials
Vigorous spreaders like obedient plant, gooseneck loosestrife, and common yarrow will quickly overrun and smother more delicate perennials. Their underground rhizomes infiltrate neighboring plant root systems with ruthless efficiency.
Contain these garden thugs with buried barriers or dedicated beds where their spreading habit won’t threaten less aggressive plants. Many gardeners have watched in dismay as prized specimens disappeared beneath waves of spreading perennials. Some aggressive plants require annual root pruning to keep them in check.