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.
Well established · suboptimal if ignored → advise
How it works
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.
What to do about it
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.
How much it matters: Not quantified by the source read. It gives the depth and the reason, not a yield or root-mass difference against a plant set at its pot line - so no figure is stated here rather than one being estimated.
The evidence
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.
Where this rule stops
TOMATO ONLY, and the two exclusions are deliberate rather than incomplete. The same page tells you to set a PEPPER so the soil reaches its cotyledons - a real published depth - but gives no reason for it, and rooting along a buried stem is a tomato trait rather than a general one. A grade-A rule states its mechanism, so pepper waits for a source that gives it one rather than borrowing this one. EGGPLANT is named in the page's title and given no setting depth at all; reading the title as though it were the text is how a claim reaches a crop nobody wrote about.