Companion Planting Checker

Companion planting is best treated as a planning screen for how neighboring crops share a bed, not as a fixed chart of friends and enemies. A useful pair may work because the plants mature at different times, use different root zones, attract helpful insects, or give each other physical support. A risky pair may fail because the plants crowd the same space, share a disease family, need incompatible water, or put one crop in the shade when it needs full sun.

Good mixed beds usually come from visible mechanisms. Corn can support a climbing bean after the stalk is sturdy. A quick radish or lettuce crop can leave before cucumber or tomato canopies close. Flowering herbs can feed predators and parasitoids, but they do not guarantee pest control. Woody herbs such as rosemary, sage, and thyme often belong on a drier edge instead of in the wet center of a vegetable bed.

Three questions matter before a companion label is useful. Can the plants physically grow together at the planned spacing? Do they repeat a family, pest, or disease problem that should affect rotation? Is the helpful claim practical enough to change the layout, or is it only a traditional note that should stay secondary to water, light, and sanitation?

The common mistake is using a companion chart as the planting map. Charts rarely know the bed width, container size, cultivar, local pest problem, disease history, or whether the plants will touch for two weeks or four months. Tomato next to basil in a ventilated raised bed is a different decision from tomato, basil, and potato packed into a dense block where nightshade disease carryover matters.

A dependable companion plan starts with the strongest constraints first. Separate known conflict crops, keep disease-prone families visible in rotation records, match irrigation needs, then use flowers, herbs, and classic pairings as supporting choices. That approach is less tidy than a perfect compatibility chart, but it is closer to how a real bed behaves through a season.

How to Use This Tool:

Use the checker while sketching a bed, container group, guild, or border. It is most useful before planting, when moving one crop to a different row, edge, pot, or season is still easy.

1. Start with Starter sample if you want a known example such as Mixed risk screen, Three Sisters bed, Tomato border companions, Brassica strip with legumes, or Container herb edge. Switch to Custom crop list when your own list is ready.
2. Enter one crop, herb, or flower per line in Crops in the bed. You can paste text, drop a TXT or CSV file, use Browse, or press Normalize to turn comma-separated names into one-per-line input.
3. Read the crop status line before interpreting pairs. It reports the number of unique crops loaded, duplicate lines ignored, and unlisted crops that need local confirmation.
4. Set Planting context and Spacing model to match the real bed. Container, tight interplanting, and square-foot layouts make root-volume, shade, and close-neighbor cautions stronger than standard rows.
5. Choose Watering pattern. Use Single shared watering zone when every crop will receive the same irrigation, and use separate zones or dry edges only when the bed or containers can actually be managed that way.
6. Pick Priority lens for the concern you want sorted first: pest-support notes, disease and family carryover, or water and airflow. Then choose Evidence stance; conservative mode filters weak traditional helpful notes while leaving practical cautions visible.
7. Adjust Close-neighbor strictness toward strict when the bed is crowded, disease has appeared recently, containers are small, or plants will share roots for most of the season.

After the summary updates, open Pair Risk Ledger first. If it shows Needs input or an unlisted crop, fix the crop list or verify that crop before using Bed Layout Moves as a planting guide.

Interpreting Results:

The summary badge is a triage signal, not a full garden plan. Separate first means at least one pair should move to another bed, container, row block, or season. Rework layout means the plan may still work after spacing, rotation, or water changes. Use spacing care points to context-sensitive watch items. No major flags means no listed conflict was found for the recognized crop set.

Pair Risk Ledger carries the main explanation because each row shows the pair, signal, level, action, confidence, and reason. Crop Profile Ledger shows recognized names, plant family, water preference, height, root habit, and crop notes. Evidence Notes helps separate practical constraints from traditional claims, and Relationship Map is useful for spotting which crops are connected by useful or caution links.

A low-warning result should not be read as proof of success. Verify unlisted crops, confirm plant families when rotation matters, and compare the warning with the real bed. Wilt, mildew, aphids, poor fruit set, drying potting mix, and stems pressed together are stronger evidence than any static pair label.

Technical Details:

Companion planting is a combination of intercropping, rotation awareness, resource management, and pest-habitat planning. Intercropping can save space, cover soil, provide physical support, and increase plant diversity, but those effects are not automatic. A flowering border may feed predators and parasitoids without eliminating pests. A legume may fit near a brassica without replacing fertilizer planning. A tall crop may protect lettuce from heat while reducing fruiting in a sun-loving neighbor.

The scoring model favors concrete growing constraints over optimistic pair notes. Plant family, water demand, height, root habit, bed context, spacing intensity, and close-neighbor strictness can raise a warning even when a traditional companion pairing exists. Conservative evidence mode only filters traditional helpful claims; family repeats, water mismatch, shade, root-volume pressure, and fennel cautions still remain eligible to drive the result.

Rule Core:

Each unique crop is compared with every other recognized or unlisted crop. A pair can collect several signals, such as a specific pair note plus same-family or water mismatch. The highest severity controls the displayed level, while confidence text explains whether the signal is practical, traditional, filtered, or unknown.

Severity Boundaries:

The displayed level is ordered by severity, so stronger conflict signals override weaker useful notes. Boundary values are integer bands from zero to four.

Family and Evidence Handling:

Plant family is a practical shortcut for rotation and disease planning because related crops can host similar pests and pathogens. Nightshades, brassicas, cucurbits, and alliums start from stronger disease-family caution than lower-risk family repeats. The warning becomes stronger when disease and rotation are the priority lens or when close-neighbor strictness is set above standard.

End-to-End Example Path:

For tomato, basil, potato, and sage in a tight container with one watering zone, the pair comparison first recognizes tomato and potato as nightshades and raises a disease-family conflict. Basil near tomato can remain useful, but it does not override the potato warning. Sage adds a water-fit problem because it prefers drier conditions than many fruiting crops. The final summary favors the strongest separate or caution findings, while the useful basil note remains visible as a lower-risk row.

Limitations and Privacy Notes:

Companion planting results are planning cautions for home gardens. They are not proof of pest control, yield gain, disease prevention, or crop safety in a specific yard.

• The recognized crop set is limited. Unlisted crops should be checked against seed packets, nursery notes, local extension guidance, or local grower experience.
• The result does not know cultivar resistance, exact spacing, bed orientation, soil test values, planting date, previous disease history, current pest counts, pruning plans, or real root volume.
• Flower and beneficial-insect notes should support integrated pest management, not replace scouting, sanitation, barriers, resistant varieties, or label-following pesticide decisions.
• Crop names and local TXT, CSV, or MD files are read in the browser for the current session. The checker does not require a garden address, account, or server lookup to produce the ledgers.

When the result conflicts with direct evidence from the bed, trust the bed. Actual wilting, mildew, insect pressure, crowding, and dry containers deserve a stronger response than a generic companion label.

Worked Examples:

Tomato border with basil, marigold, carrot, lettuce, and potato. With raised-bed context, standard spacing, dry edges, pest-support priority, practical mode, and careful strictness, the summary can still flag separation because potato and tomato are both nightshades. Pair Risk Ledger keeps basil and marigold as useful or border-style notes, but Bed Layout Moves should push potato into a different rotation block.

Three Sisters-style bed with corn, bean, squash, sunflower, and lettuce. In an in-ground row with edge companions, the useful associations for corn, bean, and squash appear because support, living mulch, and growth habit can fit. The same output may warn about shade or root-volume pressure if the spacing is changed to tight interplanting. Use Relationship Map to see which crops create useful links and which ones need placement care.

Container herbs with pepper, basil, rosemary, sage, thyme, and cucumber. Container context, tight spacing, one watering zone, water-airflow priority, and strict close-neighbor settings can raise dry herb and cucumber pairings into stronger warnings. Pair Risk Ledger may point to water mismatch, while Bed Layout Moves recommends dry edges or separate containers.

Tatsoi added from a CSV list. A dropped file that includes tatsoi and lettuce can load successfully, but the crop status may report one unlisted crop. Crop Profile Ledger will mark tatsoi as unlisted, and Pair Risk Ledger keeps that pair neutral until you verify its family, spacing, and water needs. The corrective step is to use a recognized common name when one exists or confirm the crop locally.

FAQ:

Glossary:

Companion planting

Growing crops, herbs, or flowers near one another because a practical relationship may help the bed.

Intercropping

Growing two or more crops in the same space during the same season.

Plant family

A botanical group whose members may share pests, diseases, growth habits, or rotation concerns.

Allelopathy

Plant-produced chemicals that can suppress nearby germination or growth in some conditions.

Beneficial insects

Pollinators, predators, parasitoids, and decomposers that can support garden health.

Water zone

A part of a bed, pot group, or irrigation setup that can be watered differently from nearby plants.

Root volume

The usable soil or potting mix available for roots, water, nutrients, and plant support.

References:

• Companion planting in home gardens, University of Minnesota Extension, reviewed in 2026.
• Companion Planting, Oregon State University Extension Service, June 2021.
• Encouraging beneficial insects in the garden, Pacific Northwest Extension Publishing, 2025.
• Crop Rotation in the Vegetable Garden, Iowa State University Extension and Outreach, April 2025.
• Disease Management in the Home Vegetable Garden, University of Georgia Cooperative Extension, December 1, 2022.
• Allelopathy in the Home Garden, Penn State Extension, September 19, 2023.