The same species behaves differently by cultivar group, so the planner resolves size, timing and support against the group rather than the species.
Giant Single-Stem Sunflower — stalk, spacing in row 45 cm18 in, mature height 200–350 cm79–138 in, mature spread 45–60 cm18–24 in, days to maturity 80–110
Dwarf Or Branching Sunflower — stalk, spacing in row 40 cm16 in, mature height 40–120 cm16–47 in, mature spread 30–45 cm12–18 in, days to maturity 60–90
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.
how many vines it can carry — estimated
Rules that apply to Sunflower
Promising
Sunflower residue suppresses germination of some small-seeded species.
costly if ignored → warn
Mechanism: Sunflower tissue and roots release allelochemicals that persist in the soil and inhibit the germination and early growth of sensitive plants. Small-seeded, direct-sown neighbours are the ones at risk: a small seed has little reserve to push through the inhibition, whereas a transplant is already past the germination window and largely escapes.
Evidence: confirmed against 1 source in the research-based literature.
the source
'The Allelopathic Activity of Aqueous Extracts of Helianthus annuus L. ... on Germination, Development, and Physiological Indices of Pisum sativum L.', PMC10180669, pmc.ncbi.nlm.nih.gov/articles/PMC10180669 (read 2026-07-11): 'If a sensitive test plant is exposed to allelopathic compounds, its seed germination and seedling growth are reduced'; 'Aqueous extracts of various organs of sunflower significantly suppress the growth of weeds ... and agricultural plants.' Effect was concentration-dependent (stimulatory at low concentration, inhibitory at 75%), consistent with the 'modest' effect_size. NOTE: this study used pea (large-seeded); the small-seeded/direct-sown scoping is the corpus's operationalization (small seeds have less reserve to overcome inhibition; transplants bypass the germination window), not a claim from this source.
remedy
Keep small-seeded direct sowings out of last year's sunflower ground, or set transplants into it instead of sowing seed.
Do not plant lettuce or its family in ground that held it within 3 years, and know that lettuce drop's fungi live on most other vegetables too.
costly if ignored → advise
Mechanism: Bottom rot (Rhizoctonia solani) and lettuce drop (Sclerotinia sclerotiorum and S. minor) survive in soil as mycelium and sclerotia for years and build where lettuce repeats; a three-year break holds bottom rot down. Drop's fungi infect alliums, brassicas, nightshades, cucurbits and legumes as well, so a lettuce rotation slows them without starving them, and only cereals are safe ground.
Evidence: confirmed against 4 sources in the research-based literature.
NC State Extension, 'Lettuce Drop', content.ces.ncsu.edu/lettuce-drop (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Sclerotinia sclerotiorum and S. minor have a wide host range infecting allium (onion, garlic), brassica (cabbage, broccoli), solanaceae (pepper, potato), cucurbits (cucumber, squash), lettuce, and sweetpotato crops'; 'Cereals (maize, wheat, sorghum) are less susceptible to these pathogens'; 'the broad host range of both pathogens significantly limits the efficacy of crop rotation.'
Utah State University Extension, 'Drop (Sclerotinia Rot)', extension.usu.edu/planthealth/ipm/notes_ag/veg-drop-sclerotinia (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'The sclerotia can survive dormant in the soil for extended periods'; 'S. sclerotiorum affects a wide range of hosts, including root crops, legumes, and solanaceous crops however, drop is specific to lettuce'; rotate 'with nonsusceptible crops such as grains or corn to reduce disease build-up.'
UMass Vegetable Notes 2024 vol. 36:2, umass.edu/agriculture-food-environment/vegetable/newsletters/vegetable-notes/vegetable-notes-2024-vol-362 (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): bottom rot - 'Rhizoctonia survives in the soil, as mycelium or sclerotia'; lettuce drop - 'It produces sclerotia that can survive in the soil for many years'; 'Its host range includes many vegetables such as tomato and beans.'
remedy
Plant lettuce and its family in different ground, or wait until 3 years have passed since this ground last held them. Where drop has shown, the break that counts is one of grains or corn, since most vegetables carry it, and dead plants go in the bin, not the compost: the sclerotia survive it.
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.
Another 108 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.
