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Companion Planting Checker
Check whether two crops make good or bad companions, see why, and size the main crop's plant count for your bed area using each pairing's spacing.
Companion Planting Checker
Chlorova · updated for 2026
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Click “Calculate” to see your results.
Companion planting verdicts blend field experience with a few well-studied effects — marigold nematode suppression and allium scent masking are supported, while many folklore claims are weakly evidenced and results vary with climate, soil and pest pressure. Plant counts here are simple area-divided-by-spacing math for the main crop and ignore the companion's own footprint, so treat them as a starting layout rather than a precision stand estimate. Where a pairing is marked avoid, the mechanism is real enough that separating the crops is worth the space.
Companion planting advice is usually a wall of folk claims with no way to separate the useful pairings from the myths. This checker narrows it to one question you can actually act on: do these two crops help, ignore, or harm each other, and roughly how many of the main crop fit your bed? Pick a pairing to read a verdict, the reasoning behind it and a spacing-based plant count sized to your area, so you decide what to interplant with a bit of evidence rather than a garden-center anecdote.
Checking a Pairing in Seconds
- Choose the two-crop pairing from the dropdown menu.
- Read the verdict to see if they help, tolerate or harm each other.
- Scan the reason line for the mechanism behind that verdict.
- Enter your real bed area in square feet as the second number.
- Note the main-crop spacing pulled from the pairing data.
- Read the plant count as how many of the main crop fit the bed.
- Treat an Avoid verdict as a signal to split the pair across beds.
- Re-check both crops' light and water needs before finalizing the plan.
How the Verdict and Count Are Reached
The verdict and the explanation are not calculated but looked up, keyed to the pairing you select, because companion effects come from research and observation rather than arithmetic. Each pairing carries its own recommended in-row spacing for the main crop, and the plant count is genuine math: one square foot holds 144 divided by the spacing squared, so the calculator multiplies that density by your bed area and rounds up so you never plant a fraction of a seedling.
Because the spacing differs by pairing, the same bed returns a very different count — a tomato-and-basil block at 24 inches fits far fewer main plants than a carrot-and-onion row at three inches. Blank or non-numeric area input returns dashes, so an unfinished plan cannot masquerade as a finalized spacing.
Worked Example: A Four-by-Eight Bed
- You pick Tomato + Basil and enter a 32 square foot bed.
- The verdict returns Excellent companions with the pest-mask reasoning.
- The pairing data sets main-crop spacing at 24 inches.
- Density is 144 divided by 24 squared, one plant per four square feet.
- Multiplying by 32 square feet gives eight tomato plants.
- The ceiling keeps it a whole eight rather than a fractional stand.
- Now switch to Carrot + Onion on the same 32 foot bed.
- Three-inch spacing gives sixteen plants per square foot.
- The count jumps to over five hundred carrots, as roots are crowded.
- That contrast is the point: spacing, not the pairing, drives the number.
Verdicts and Their Basis
A plain read of the four verdict levels this tool returns, ordered from the pairings backed by controlled trials down to the folklore that does not hold up. Treat Excellent and Good as worth planning around, Neutral as space-dependent, and Avoid as a genuine clash rather than superstition.
| Verdict | What to do |
|---|---|
| Excellent | Plant them together on purpose |
| Good | A helpful, low-risk pairing |
| Neutral | Fine if spaced or split in the bed |
| Avoid | Separate beds; the clash is real |
Key Terms
- Companion planting
- Growing crops near each other for a claimed pest, pollination or shading benefit.
- Trap crop
- A plant grown to pull pests away from the crop you actually want to protect.
- Nitrogen fixation
- Soil bacteria converting atmospheric nitrogen into plant food, as beans do near corn.
- Root-knot nematode
- A microscopic worm that galls roots; marigold residues are shown to suppress some species.
- In-row spacing
- The distance between plants in the same row, which sets how many fit a bed.
What the Trials Actually Show
- Only a handful of companion claims survive controlled trials, and French marigold suppression of root-knot nematodes is among the best documented, working through root residues rather than scent.
- Scent-masking pairings like carrot and onion are plausible and cheap to try, but they reduce pest landing rather than guarantee control, so still scout for damage.
- The allium-bean aversion is the folklore most worth respecting, since onion-family exudates genuinely interfere with the nitrogen-fixing bacteria beans depend on.
- Many 'beneficial' lists confuse plant competition with companionship: two heavy feeders marked friendly may simply shade and out-compete each other in a small bed.
- The sturdiest gains in home gardens come from flowers and herbs that feed pollinators and predators along the edges, not from cramming a supposed companion into every square foot.
Common Mistakes and Fixes
- Planting a companion too close and turning a benefit into root competition for water.
- Trusting a folklore pairing over a documented one when the two conflict in the same bed.
- Ignoring the companion's own footprint and overestimating how many main plants fit.
- Using an Avoid pair and reading a poor stand as bad luck instead of the predicted clash.
- Assuming one interplanted herb replaces real pest scouting and threshold decisions.
- Forgetting that sun and water needs must match before a verdict of Good means anything.
When to Use This and When Not To
Use this checker when planning an interplanting or deciding whether a new pairing is worth the space, especially for pest-prone crops like tomatoes, brassicas and beans. It speaks to companionship and rough layout only, so for exact row counts and yields use the dedicated spacing and yield calculators, and let a real pest observation, not the verdict alone, drive any spray or removal decision.
Companion Planting Checker: Frequently Asked Questions
Is companion planting actually backed by science?
Partly. Marigold nematode suppression, some scent masking and pollinator-attracting border plantings have real support, while many popular pairings rest on folklore passed around seed catalogs. This tool flags which category each pairing falls into so you can tell evidence from anecdote rather than treating every friendly-list claim as equally true.
Why does the plant count change so much between pairings?
Because the count is driven by each pairing's recommended spacing, not by the two crops being friendly. A 24-inch tomato fits far fewer plants per bed than a three-inch carrot, so the same square footage returns wildly different numbers.
What does an Avoid verdict really mean?
It means the pair has a documented or strongly observed downside, such as alliums disrupting bean nitrogen fixation or dill stunting brassicas. The safe move is separate beds rather than assuming your garden will be the exception.
Does the tool account for both crops' space?
No, the count covers the main crop only and ignores the companion's own footprint, so it is a starting layout rather than a final spacing plan. Leave room for the second crop or interplant it along edges and gaps.
Can companion plants replace pest control?
They reduce pest landing and add habitat but rarely eliminate pressure, so keep scouting and act on thresholds. Treat a Good verdict as a helpful layer, not a substitute for watching the plants. A flowering border that draws hoverflies and parasitic wasps usually does more for pest control than one crowded in-row companion.
Sources & Further Reading
- Companion Planting: Research vs. Folklore — University of Minnesota Extension. Basis for separating trial-supported pairings from anecdotal ones.
- Marigolds and Root-Knot Nematode Management — USDA Agricultural Research Service. Supports the marigold nematode-suppression verdict.
- Interplanting and In-Row Spacing for Vegetables — Cornell University Cooperative Extension. Source of the per-crop spacing values used in the plant-count math.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated