The same species behaves differently by cultivar group, so the planner resolves size, timing and support against the group rather than the species.
Bush Tomato — bush, spacing in row 60 cm24 in, mature height 90–120 cm35–47 in, mature spread 60 cm24 in, days to maturity 60–80
Vining Tomato — vine, spacing in row 60 cm24 in, mature height 180–300 cm71–118 in, mature spread 60 cm24 in, days to maturity 70–90
Which of these numbers are soft
Every number above carries a confidence tier. These are the ones the corpus does not stand behind — estimated from general knowledge, or contested between sources. They are shown because hiding them would be the dishonest half of showing the rest.
ph range — estimated
sowing depth — estimated
night temperature it stops setting fruit — contested
Rules that apply to Tomato
Promising
Do not plant nightshades in ground that held nightshades within 3 years.
costly if ignored → warn
Mechanism: Alternaria solani (early blight) and Septoria lycopersici overwinter on tomato debris and solanaceous weeds; a 2-3 year break plus debris removal starves this inoculum. Fusarium has a very narrow host range, so rotating away from the host is effective. Verticillium is the exception: microsclerotia persist ~10 years and it infects 300+ host families, so a garden-scale rotation does not control it.
Evidence: confirmed against 5 sources in the research-based literature.
the 5 sources
Clemson HGIC, 'Tomato Diseases & Disorders', hgic.clemson.edu/factsheet/tomato-diseases-disorders (read 2026-07-11): 'tomatoes and related vegetables, such as potatoes, peppers, and eggplants, should not be planted on the same land more than once in three years.'
NC State Extension, 'Septoria Leaf Spot of Tomato', content.ces.ncsu.edu/septoria-leaf-spot-of-tomato (read 2026-07-11): 'The fungus is known to overwinter on infected tomato debris and solanaceous weed hosts'; 'Rotate away from tomato for 2 or more years.'
NC State Extension, 'Verticillium Wilt of Tomato and Eggplant', content.ces.ncsu.edu/verticillium-wilt-of-tomato-and-eggplant (read 2026-07-11): 'The pathogen can survive in the soil without a host for up to 10 years in the form of structures called microsclerotia'; 'affects more than 300 host families.'
Plant nightshades in different ground, or wait until 3 years have passed since this ground last held them. Clear and remove the old debris either way — the inoculum overwinters on it.
Do not plant a juglone-sensitive species within the root zone of Juglans. The sensitive list is the hazard's own, and it is wider than the nightshades - it also carries apple and cabbage.
fatal if ignored → block
Mechanism: Juglans roots exude hydrojuglone, oxidized to juglone, a respiration inhibitor. nightshades are highly sensitive: wilt and death.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
UW-Madison / Wisconsin Horticulture (Joy, Hudelson & Jull, rev. 2024-02-28), 'Black Walnut Toxicity', hort.extension.wisc.edu/articles/black-walnut-toxicity (read 2026-07-13): 'Vegetables such as tomato, potato, eggplant and pepper ... are particularly sensitive to juglone'; 'The toxic effects of a mature black walnut tree can extend 50 to 80 feet from the trunk of the tree, with the greatest toxicity occurring within the tree's dripline.'
Iowa State Univ. Extension & Outreach, Yard and Garden FAQ, 'What plants are sensitive to the juglone produced by black walnuts?', yardandgarden.extension.iastate.edu (read 2026-07-13): juglone-sensitive plants include 'members of the tomato family (tomato, potato, eggplant, pepper).'
remedy
Plant it outside the walnut's root zone — the whole drip line and beyond. There is no soil treatment for juglone.
Tomato fruit set fails when night temperatures persistently exceed ~21C (70F); high day temperatures (>29-32C) compound it.
costly if ignored → warn
Mechanism: Sustained high day (>29-32C / 85-90F) and night (>21C / 70F) temperatures make pollen non-viable ("tacky") and impair its release; flowers go unfertilized within their ~50-hour window and abscise.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
Univ. of Missouri IPM, 'Understanding Tomato Fruit Set' (Missouri Produce Growers Bulletin), ipm.missouri.edu/MPG/2013/4/Understanding-Tomato-Fruit-Set (read 2026-07-11): 'When daytime temperatures exceed 85 degrees F or nighttime temperatures exceed 70 degrees F, pollination suffers due to pollen becoming "tacky" and non-viable.'
UMD Extension (J. Brust), 'Expect Poor Fruit Set in Tomatoes This Week', extension.umd.edu/resource/expect-poor-fruit-set-tomatoes-week (read 2026-07-11): 'Daytime highs of 90° F and above and nighttime lows only getting down to 70° F ... will cause blossom drop and fruit abortion in tomatoes.'
remedy
Wait it out — fruit set resumes on its own once nights cool, and the plants make up the gap. Do not reach for shade cloth: it cools the day, and this failure is driven by the night temperature, which Missouri IPM says shading will not touch. If hot nights are normal where you garden, buy a heat-set cultivar next season.
A tomato transplant goes into the ground DEEPER than it grew in its pot - buried up to its first leaves - rather than at the soil line it came out of.
suboptimal if ignored → advise
Mechanism: A tomato's stem forms roots along whatever length of it is buried in moist soil. Setting the plant deep therefore converts stem into root, and the plant ends up with a larger root system than the one that came out of the pot.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
Utah State Univ. Extension, 'Tomato, Pepper & Eggplant: Planting & Spacing', extension.usu.edu/vegetableguide/tomato-pepper-eggplant/planting-spacing (fetched in-container 2026-08-03, READ AND CONFIRMED BY THE MAINTAINER 2026-08-03): the instruction and its scope - 'The stem of a tomato transplant may be buried in soil up to the first leaves (or more if the plant is spindly)'. The same page's PEPPER sentence, quoted here so the exclusion above is auditable rather than asserted: 'Transplants should be set so the soil level reaches the cotyledon leaves or the first true leaf.' The page gives EGGPLANT no setting-depth sentence of any kind. [read]
NOT A SOURCE FOR THE MECHANISM, and recorded so it is not re-sought: no page read for this rule states WHY a buried tomato stem roots. The mechanism above is the standard horticultural account of adventitious rooting, and it is asserted on the strength of the instruction it explains rather than on a page that states it outright; a source stating it directly would strengthen the pointer list rather than change the rule.
remedy
When you set it out, bury the stem up to the first leaves - deeper still if the plant is leggy from being started indoors. This is the one seedling you plant lower than it was growing.
Late blight comes back from last year's potatoes, not from the soil: tubers left in the ground carry it through winter as volunteers, so ground that grew potatoes last season needs its volunteers pulled before this year's tomatoes or potatoes go in.
suboptimal if ignored → advise
Mechanism: Phytophthora infestans needs living tissue to overwinter. Infected tubers left in the ground, in a compost heap that does not freeze, or in a cull pile survive and sprout as volunteers that carry the blight into the new season; tomatoes do not carry it, because frost kills the whole plant and the seed is clean.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
UMass Extension, 'Late Blight Management for Fall, Winter and Spring', umass.edu/agriculture-food-environment/home-lawn-garden/fact-sheets/late-blight-management-for-fall-winter-spring (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Potato tubers that are infected with late blight and don't freeze or decay during the winter can carry the pathogen over the winter to next spring' - 'Left in the ground at harvest, down several inches in the soil'; 'Disposed of in a compost pile that does not fully decompose and does not freeze'; 'Tomatoes will not carry late blight over the winter, because freezing kills the whole plant'; 'Tomato seed, even from fruit that was infected with late blight, will not carry the pathogen'; 'In the spring (April - June), inspect last year's potato plot and any compost or cull piles for volunteer potato plants that might come up. If you find potato plants, pull them out and put them in the trash or destroy them.'
UMN Extension, 'Late blight of tomato and potato', extension.umn.edu/disease-management/late-blight (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'P. infestans can overwinter in Minnesota if protected in potato cull piles'; 'The host range is typically limited to potato and tomato, but hairy nightshade (Solanum physalifolium) is a closely related weed that can readily become infected'; 'Control volunteer potato plants, as infected plants can grow from infected tubers.'
remedy
From April to June, walk last year's potato ground and any compost or cull pile, pull every volunteer potato and put it in the bin, not the compost. Then plant.
A specialist pest or pathogen finds and builds on a continuous single-family host patch more readily than one broken by non-host families.
suboptimal if ignored → advise
Mechanism: Resource concentration - a specialist locates and holds its host more efficiently at higher host density and continuity, so an unbroken run of one family across adjacent beds is an easier target than the same family separated by a non-host (associational resistance).
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
Root 1973, 'Organization of a plant-arthropod association in simple and diverse habitats: the fauna of collards (Brassica oleracea)', Ecological Monographs 43:95-124 (abstract read by the maintainer 2026-08-21): proposes 'the resource concentration hypothesis, which states that herbivores are more likely to find and remain on hosts that are growing in dense or nearly pure stands; that the most specialized species frequently attain higher relative densities in simple environments.' This is another rule here's claim and mechanism verbatim.
Elmstrom, Andow & Barclay 1988, 'Flea beetle movement in a broccoli monoculture and diculture', Environmental Entomology 17(2):299-305 (read by the maintainer 2026-08-21): the empirical test on a brassica specialist (the flea beetle Phyllotreta cruciferae) - immigration ~1.3x faster into monocultures than into dicultures, emigration ~2x faster from dicultures - reported as 'strong, direct support for the hypothesis that host plants are harder to find and easier to lose in vegetationally diverse habitats than in monocultures.'
remedy
Where one family fills adjacent beds in a single season, breaking the run with a non-host family between them lengthens the distance the specialist must travel to the next host. Disclosed, never docked: a continuous single-family block can be the right call anyway - a wind-pollinated crop like corn sets better massed - so the app names the trade-off and leaves the choice to the gardener.
Cold-hardy annuals can be sown or transplanted before the last spring frost — hardy crops about three weeks before, half-hardy about two weeks before. Tender crops wait for the frost itself.
suboptimal if ignored → advise
Mechanism: Cold hardiness. Hardy crops tolerate hard frost and half-hardy crops tolerate light frost, so they can occupy the ground weeks before frost-tender crops are allowed in. The offset is measured from the last spring frost.
Evidence: confirmed against 4 sources in the research-based literature.
the 4 sources
University of Illinois Extension, 'Time Your Vegetable Plantings by Cold Hardiness' (2018-03-26), extension.illinois.edu/blogs/ilriverhort/2018-03-26-time-your-vegetable-plantings-cold-hardiness: very hardy vegetables 'withstand freezing temperatures and hard frosts without injury' (planted earliest in spring); frost-tolerant vegetables 'can withstand light frosts, but not freezing temperatures' (planted a couple of weeks later). LLM-summarised from a read of the page; it establishes the CATEGORIES but states no week offsets.
Colorado State Univ. Extension, GardenNotes #720, 'Vegetable Planting Guide', extension.colostate.edu/resource/vegetable-planting-guide (read 2026-07-25): hardy vegetables go in 'As early as 2-4 weeks before the date of the average last spring frost'; semi-hardy 'As early as 0-2 weeks before the date of the average last spring frost'; tender from seed 'around the date of the average last spring frost'; very tender 'Typically 2+ weeks after'. This is the source that states the offsets in weeks.
Where this rule sits in that range: hardy at 3 weeks is mid-range of CSU's 2-4. Half-hardy at 2 weeks is the EARLIEST EDGE of CSU's 0-2 - the most aggressive planting CSU sanctions, not the middle of it. The rule is inside published guidance at both ends, but it leans early on the half-hardy side and a gardener following it in a cold spring is taking that boundary.
Do not import an offset from another publication's hardiness scheme without checking the category names: CSU has no 'very hardy' tier, so a '4-6 weeks before frost' figure written against a four-tier scheme does not describe CSU's 'hardy'. Categories are not comparable across sources.
Long-season warm crops are started indoors several weeks before the last frost and set out after it — tomato and tomatillo about six weeks ahead, pepper and eggplant about eight. Basil, six.
suboptimal if ignored → advise
Mechanism: A frost-tender crop with a long days-to-maturity cannot mature if it is only direct-sown after the last frost — the frost-free window is too short. A head start under cover buys the weeks the season lacks; the seedling is set out only once frost has passed — the transplant date is still frost-gated.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
University of Minnesota Extension, 'Starting seeds indoors', extension.umn.edu/planting-and-growing-guides/starting-seeds-indoors: seed-starting chart gives tomato '5-6 weeks' and pepper and eggplant '9 weeks' before the outdoor planting date (warm crops set out after the last frost). Read 2026-07-19 (LLM-summarised); read by the maintainer 2026-07-25, which is the read that promoted this rule. Note what the corpus's per-crop values do with the sources: tomato 6 sits at the top of Minnesota's '5-6 weeks' and inside Illinois's '6-8'; pepper and eggplant 8 sit BELOW Minnesota's '9 weeks' and at the top of Illinois's '6-8'. They are a reconciliation of two published ranges, not a figure either source prints.
Over-feeding a tomato with nitrogen grows a tall, dark-green, leafy plant that flowers late and sets little fruit. The plant pours its resources into vines and foliage instead of blossoms, so flowering is delayed or sparse and yield drops. This is a vegetative-versus-reproductive imbalance the gardener causes by fertilizing, not the heat that makes already-open flowers fail to hold - keep the two apart.
suboptimal if ignored → advise
Mechanism: High available nitrogen biases the plant toward vegetative growth: abundant nitrogen keeps it building leaves and stems, and that leaf-and-stem growth takes priority over the switch into flowering, so flower initiation is delayed and fewer flowers form. The visible result is lush dark foliage with reduced and later fruit set - growth spent on the plant rather than the crop.
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
UGA Extension, 'Troubleshooting Cultural Problems in Tomatoes' (C1089), fieldreport.caes.uga.edu/publications/C1089 (read 2026-08-22): 'applying too much nitrogen can cause excessive growth and hinder flowering'; 'Too much nitrogen is also a common problem, and can be identified when the plant is tall and dark green, with excessive growth and no blooming.'
Univ. of Missouri Extension, 'Watering and Fertilizing Tomatoes in a High Tunnel' (G6462), extension.missouri.edu/publications/g6462 (read 2026-08-22): 'Too much nitrogen creates excessive vine growth, twisted foliage, delayed flowering and lower yield.'
remedy
Go easy on nitrogen, especially before fruit set; do not feed a tomato a nitrogen-heavy fertilizer at planting and hold off feeding again until flowers and small fruit have appeared. Let a soil test, not a guess, say what the bed actually needs - the engine does not invent NPK numbers.
Another 103 rules in the corpus apply by situation rather than by species — rotation intervals, frost windows, sun, spacing and soil. Read the full evidence page.
A garden-planning corpus: every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.
