Free Gardening & Crops tool
Free Gardening & Crops toolCompanion Planting Planner
The Companion Planting Planner determines mutually beneficial and antagonistic vegetable pairings to optimize bed space and reduce insect pest pressure. Companion planting advice ranges from research-backed to folklore, and most gardeners cannot tell which is which. This planner checks up to four crops against the pairing charts published by NCAT and Virginia Cooperative Extension, flagging conflicts, confirming listed companions, and calling everything else neutral instead of pretending every pair has a verdict.
Companion Planting Planner: result
| Crop | Good with | Avoid with |
|---|---|---|
| Asparagus | Tomato, Basil | Onion |
| Tomato | Asparagus, Basil, Carrot, Onion | Beans, Cabbage, Fennel |
| Basil | All crops | None |
| Carrot | Lettuce, Onion, Peas, Tomato | Fennel |
| Onion | Cabbage, Carrot, Lettuce | Beans, Peas |
| Beans | All crops | Onion |
| Peas | Carrot, Radish | Onion |
| Cabbage | Onion, Spinach | Beans, Strawberry, Tomato |
| Lettuce | Carrot, Radish, Strawberry | Onion |
| Radish | Lettuce, Peas | None |
| Spinach | Beans, Strawberry | Cabbage |
| Strawberry | Beans, Lettuce | Cabbage |
| Squash | None | None |
| Fennel | None | Almost everything |
At a glance
- What it does: Checks up to four crops against the NCAT and Virginia extension pairing charts before a bed is committed.
- Who it is for: Written for gardeners who want charted conflicts separated from repeated folklore.
- Typical result: Returns a verdict for the plan plus a good, bad or neutral line for every pairing in it.
How to use the companion planting planner calculator
- Choose up to four crops from the lists. Every pairing in the plan is checked in both directions, because charts are not always symmetric. prefill →
- Read the verdict row first: one conflicting pair turns it red, because that combination is listed as bad for each other. prefill →
- Treat neutral results honestly. Most pairings simply have no reliable guidance either way, which is not the same as danger.
- Consult the full pairing table for substitutes when a conflict appears in your layout.
- Set spacing, light and watering first; pairings are a refinement on top of those fundamentals, never a replacement.
Companion Planting Planner: the formula & its source
Each pair of selected crops is tested against both entries in the chart:
conflict ⟺ A lists B as bad ∨ B lists A as bad
companion ⟺ no conflict ∧ (A lists B as good ∨ B lists A as good)
Anything else is neutral: no reliable guidance in either chart rather than a warning. The verdict row then summarises all pairings:
verdict = conflicts ∪ companions ∪ neutrals over every pair
| Symbol | Meaning | Unit | Typical range |
|---|---|---|---|
c1–c4 | crops chosen for the plan, three required and one optional | select options | any listed crop |
pair | one combination tested in both directions | count | 1–6 per plan |
conflict | either source chart lists the pair as a bad neighbour | flag | yes or no |
companion | no conflict and a chart lists the pair as good | flag | yes or no |
neutral | neither chart carries an entry either way | flag | most pairs |
verdict | summary line across every pairing in the plan | text | pass or conflict |
Integrate polyculture companion plants into modular bed layouts following our intensive raised bed companion spacing guide.
Companion Planting Planner: worked example
Worked example: tomato, basil and carrot. Three crops give three pairings. Basil is charted as good with nearly everything, so tomato + basil and basil + carrot both pass; tomato's own list includes carrot, so tomato + carrot passes too: all 3 pairings listed as companions. Swap in cabbage, however, and the verdict flips. Cabbage lists tomato as a bad neighbour, so one conflicting pair turns the whole plan red. Basil alone cannot outvote it.
Example B: four crops with one clash
Example B: a fourth crop spoils the plan. Keep tomato, basil and carrot, which pass all three of their pairings, then add cabbage in the optional fourth slot. Cabbage lists tomato as a bad neighbour, and because one conflicting pair is enough, the verdict for the whole plan flips red even though most of the six pairings are harmless or helpful. The output lists the clash beside the passing pairs rather than averaging them, which matches how the charts are meant to be read: a single bad combination is worth relocating, and no amount of good company outvotes it.
What the two charts actually contain
Two published charts sit behind this planner: the NCAT companion planting table and the matching table in Virginia Cooperative Extension's SPES-620. Each lists crops that are conventionally described as good neighbours and crops described as bad ones, and neither is written as a complete matrix, so most combinations simply carry no entry. The planner tests every selected pair in both directions because the lists are not always symmetric. A crop may name another that does not name it back. Underneath the entries sit a few recognisable mechanisms: strong aromas that confuse flying insects away from a host plant, a sacrificial crop that draws a pest off the real one, tall growth that shades a shade-tolerant neighbour, legumes that fix nitrogen for heavy feeders nearby, and root chemicals that suppress what grows beside them. Coverage varies just as much as content: one crop may carry half a dozen listed neighbours while another carries a single line, and rows the charts never reached are simply absent rather than disapproved. The planner mirrors that behaviour by reporting the entries it found in either direction and leaving every other combination unlabelled.
How to read a neutral pairing
Most rows in the output come back neutral, and that is the honest state of the evidence rather than a gap the planner failed to fill. Neutral means neither chart carries an entry for the pair in either direction: there is no reliable guidance saying it helps and none saying it hurts. Treat it as permission to plant the two together on spacing, light and season grounds alone. Silence is not a warning, and it is not an endorsement either. The distinction matters when a bed plan is tight: a charted conflict is worth acting on because something specific was observed, while a neutral result gives you no reason to rearrange a layout that already works. Gardens run on light, water, spacing and timing long before any chemistry between neighbours becomes visible. A plan of neutral pairs and a plan of confirmed companions can look identical on the page, and the difference only surfaces over a season of watching: the first records that nothing went wrong, the second records that the charts predicted nothing that went wrong either.
Where the advice holds and where it slips
The literature behind these pages is mostly traditional rather than replicated, and Cornell horticulture guidance is explicit that few companion planting recommendations are research-proven. Pest confusion and trap cropping have real studies behind them; long lists of compatible vegetables are largely compiled observation. Fennel is the consistent negative on most lists (its root chemicals inhibit many neighbours), while entries such as basil appear as a general companion in both source charts and therefore pass every pairing here. Use conflicts as hypotheses worth testing across a couple of beds instead of rules enforced over the whole plot. Quantity also escapes the chart: one basil plant does not turn a tomato row insect-proof, and what flies in from surrounding fields decides as much as what you planted beside it. Claims about repellent aromas deserve the same scepticism, because a strong smell in your hand rarely means a repellent dose of it drifting through open air by the time a landing insect arrives. Where a chart entry also matches a mechanism you can name, trust it a little further than one you cannot.
Pairings come after spacing and season
Order of work matters more than any verdict. Set spacing first, because a crowded bed with perfect companions usually yields less than a neutral pair given room; then check light and watering, then confirm the crops share a season. Only after those are settled does a pairing chart earn a vote, and it votes on arrangement rather than membership. Rotate families between years and keep tools and stakes clean, since soil-borne disease and carried-over pests outrank any neighbour effect. When a conflict does appear in a layout you like, use the chart's substitute list rather than deleting the crop: swap the offending neighbour, split the plan across two beds, or accept the clash deliberately in a season where you will watch the row closely for whatever shows up. Keep a short note of what you planted where and what you saw. Two seasons of the same rotation make conflicts and successes far easier to attribute than any single-year verdict, and the note costs one line per bed.
What your companion planting planner result means
The mechanisms behind the real pairings fall into a few families: pest confusion (strong aromas hide host plants from flying insects), trap cropping (a sacrificial plant draws the pest away), physical facilitation (a tall crop shading a shade-tolerant one), and soil effects like legumes fixing nitrogen for heavy feeders nearby. The chart encodes the accumulated version of those observations.
Equally important is what the neutral verdict means: absence of evidence, not evidence of harm. Gardens run on light, water, spacing and season long before chemistry between neighbours shows up. A neutral pair planted with proper spacing will usually out-yield a "perfect" pair crowded together.
How to read the output:| Result range | What it means | What to do next |
|---|---|---|
| all pairings pass | no charted conflict anywhere in the plan | plant on spacing and light grounds |
| one conflicting pair | a crop names another as a bad neighbour | swap that crop or split the plan |
| companions listed | the charts describe the pair as good | keep normal spacing anyway |
| mostly neutral | few entries exist in either direction | treat as permission, not endorsement |
Different companion combinations alter nitrogen uptake dynamics; balance soil fertility using our vegetable nutrient requirement calculator.
Common mistakes with the companion planting planner
- Reading a neutral pairing as a warning: Neutral means neither chart has an entry, so plant the pair on spacing and light grounds instead of treating silence as danger.
- Expecting one herb to protect a row: A single basil plant does not make a tomato row insect-proof; planting quantity and incoming pest pressure still decide the outcome.
- Letting pairings override spacing and season: Crowded ideal companions usually yield less than a neutral pair given light, water and correct spacing, so fundamentals come first.
- Treating the chart as pest control: Rotation, resistant varieties, hygiene and monitoring still do the main work; companions only ease pressure at the margins.
Limits of the companion planting planner calculator
Most of this literature is traditional rather than replicated: Cornell is explicit that companion-planting claims are often anecdotal. Use conflicts as hypotheses to test on a couple of beds, not as rules enforced across the whole plot. And remember fennel's allelopathy (root chemicals that inhibit many neighbours) is one of the more consistent negatives on the chart.
Pairings say nothing about quantity: a single basil plant does not turn a tomato row insect-proof, and pest pressure also depends on what is flying in from surrounding fields. Rotation and hygiene beat companions for soil-borne disease every time.
Agronomy note from Dr. Dinesh A. Nagegowda, PhD: Scientific companion planting operates on validated ecological mechanisms: root exudate allelopathy, nutrient complementarity (e.g., nitrogen fixation by legumes via Rhizobium symbiosis), and volatile terpene emission that deters pests or attracts parasitoid beneficial insects. In our intercropping trials, pairing heavy-feeding Solanaceae with nitrogen-fixing cover crops optimizes soil nutrient cycling without synthetic inputs.
Forecast cumulative seasonal harvests across polyculture beds with our home garden vegetable yield calculator.
Sources
- NCAT/ATTRA: Companion Planting and Botanical Pesticides, Table 1 (used for: the first pairing chart the planner checks every selected crop against)
- Virginia Cooperative Extension SPES-620: Companion Planting, Table 1 (used for: the second independent pairing chart, checked in both directions)
- Oregon State Extension EC 871: Vegetable Gardening in Oregon (used for: bed spacing and cultural fundamentals that pairings are applied on top of)
Companion Planting Planner: frequently asked questions
Is companion planting scientifically proven?
In parts, yes. Pest-confusion and trap-crop effects have real research behind them; most chart entries are traditional observations. Cornell notes few recommendations are research-proven, which is why this planner marks unsupported pairs neutral.
What does a neutral pairing mean?
Neither chart lists the pair as good or bad. There is no reliable guidance either way. It is permission to plant them together on spacing and light grounds alone.
Why is basil good with everything?
Basil is listed as a general companion in both source charts, so it passes every pairing in this planner. Treat that as tradition rather than proof: the chart reflects compiled folklore plus a few small studies.
Do companion plantings replace pest control?
No. They are a modest cultural control that reduces pressure at the margins; monitoring, rotation, resistant varieties and timely intervention still do the heavy lifting.
Found a number that does not match its source? Report a mistake. Corrections are published in place and the page date is updated.
Something on this page not matching its cited source? Report it.
Dr. Dinesh A. Nagegowda, PhD: Plant & Molecular Biology Editor
Dinesh is a plant molecular biologist and Chief Scientist at CSIR-CIMAP whose research runs through functional genomics, transcriptomics, metabolomics, and biosynthetic pathways in plants. He writes and…