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.
days to maturity — estimated
ph range — estimated
Rules that apply to Parsnip
A good hunch
Alliums and carrot family are unrelated to nightshades/cucurbits/brassicas and can break those families' rotation cycles — but they are not "low-pressure" and need their own rotation.
suboptimal if ignored → advise
Mechanism: They host none of the big-three families' soilborne pathogens, so following those crops with an allium or apiaceous crop breaks the host cycle. But they are not inherently low-pressure: allium white rot (Sclerotium cepivorum) is allium-specific, its sclerotia persist 15+ years, and it cannot be eradicated once established; apiaceous crops carry their own soilborne diseases.
Evidence: confirmed against 1 source in the research-based literature.
the source
UMass Extension, 'Alliums, White Rot', ag.umass.edu/vegetable/fact-sheets/alliums-white-rot (read 2026-07-11): 'Sclerotium cepivorum is specific to allium species'; sclerotia 'may remain viable in soil for 15 years or more without allium hosts'; 'Currently, there is no way to eradicate white rot from a field where it has become established.'
Do not apply fresh high-N fertilizer to root crops.
suboptimal if ignored → advise
Mechanism: Excess N drives foliage at the expense of root bulking. (Forking is a separate problem — caused by rocks, compaction, or coarse/fresh organic matter in the root path, not by nitrogen.)
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
UMN Extension, 'Growing carrots and parsnips in home gardens', extension.umn.edu/vegetables/growing-carrots-and-parsnips (read 2026-07-11): 'Excessive nitrogen fertilization can also contribute to lots of leaf growth at the expense of root growth'; 'Forked, or split, roots may result from rocky, heavy, or compacted soil.'
Univ. of Illinois Extension, 'Carrots', extension.illinois.edu/gardening/carrots (read 2026-07-11): 'excess organic debris such as woodchips worked into the soil just before planting also may affect root penetration, causing forked and twisted roots'; hairy roots 'are caused by a viral disease known as "aster yellows."'
Do not plant the carrot family in ground that held it within 3 years.
costly if ignored → warn
Mechanism: The carrot leaf blights (Alternaria dauci, Cercospora carotae and the bacterial blight) and celery's Septoria late blight survive on crop debris and die as it decomposes; a break of two to three years starves them. Alternaria radicina, the black-rot species, infects celery, celeriac, dill, fennel, parsley and parsnip as well, and can persist in soil for up to eight years, which is why the family rotates as one.
Evidence: confirmed against 5 sources in the research-based literature.
the 5 sources
UMass Extension, 'Carrot, Alternaria Leaf Blight', umass.edu/agriculture-food-environment/vegetable/fact-sheets/carrot-alternaria-leaf-blight (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'Practice relatively long rotations of 3-4 years'; 'A. radicina infects celery, celeriac, caraway, dill, fennel, parsley, and parsnips'; 'The fungus survives in the soil on crop debris but dies when the residue decomposes'; A. radicina 'can persist in soil for up to 8 years.'
Cornell Vegetables, 'Carrot Leaf Blight Diseases and their Management', vegetables.cornell.edu/pest-management/disease-factsheets/carrot-leaf-blight-diseases-and-their-management (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'it is important to practice a minimum of a 2-year rotation out of carrots to allow time for decomposition of crop residue'; 'All three pathogens can be transmitted through infected or contaminated seeds and can survive on crop residue, which can act as a source of inoculum in subsequent cropping years.'
Clemson HGIC, 'Crop Rotation', hgic.clemson.edu/factsheet/crop-rotation (fetched in-container 2026-09-02; read by the maintainer 2026-09-02): 'a crop should only be grown in the same spot a maximum of once every three years'; its family table puts carrot, parsley, celery, cilantro, fennel and dill in the parsley family (Apiaceae).
remedy
Plant carrot-family crops in different ground, or wait until 3 years have passed since this ground last held them. Clear and turn in the old tops promptly either way, and buy clean seed: all three carrot blights ride on seed.
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.
A direct-sown root crop left unthinned grows crowded roots that stay small - the seedlings stand far denser than they can mature, compete for the same soil, and the roots twist and intertwine instead of sizing up.
suboptimal if ignored → advise
Mechanism: Root crops are sown as seed in place and germinate much denser than they can mature. Where the seedlings stand too close, their swelling roots run out of room and compete for space, water, and nutrients, so they stay undersized and are deformed - twisting and intertwining against their neighbours. Thinning the stand while the seedlings are young leaves each remaining root the room to develop. (Distinct from FORKING, which the sources attribute to rocky or compacted soil and root-knot nematodes, not crowding - so this rule claims small-and-intertwined, not forked.)
Evidence: confirmed against 2 sources in the research-based literature.
the 2 sources
UMN Extension, 'Growing carrots and parsnips in home gardens', extension.umn.edu/vegetables/growing-carrots-and-parsnips (read 2026-08-21): 'You must thin out both carrots and parsnips to allow room for the roots to develop properly'; 'When the largest seedlings are three to four inches tall, thin by pulling up extra seedlings so that the remaining plants are about two to four inches apart'; 'If the plants develop healthy green tops, but limited root growth, it is usually because the plants are crowded.'
Univ. of Illinois Extension, Home Vegetable Gardening - 'Carrots', extension.illinois.edu/gardening/carrots (read 2026-08-21): 'Twisting and intertwining result from seeding too thickly and inadequate thinning of seedlings'; 'Thin the seedlings when they are about one inch tall to no more than three seedlings per inch for finger carrots ... and one seedling per 1 to 2 inches for larger varieties.'
remedy
Thin while the seedlings are young - about 2.5 cm (one inch) tall - to the crop's spacing, pulling or snipping the extras rather than leaving the row solid: roughly one seedling every 2.5-5 cm for a full-size carrot, wider for parsnip and the larger varieties. The pulled thinnings of carrot are edible, tops and all.
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.
