Sources

HarvestMath is a calculation and reference tool, not a source of original horticultural research — the expertise behind every number here belongs to the university extension services and government agencies listed below. Every figure, formula, and reference range used on this site traces to a published source, and this page lists all of them: what they say, where the link goes, and which HarvestMath page uses it.

For how sources are selected, checked, and kept current, see our methodology page.

Soil & Amendments

• Clemson Cooperative Extension's "Soil Texture Analysis: The Jar Test"

  Clemson Cooperative Extension (HGIC)

  Cited on: Soil Type Calculator

• Oregon State University Extension's research on coffee grounds and soil health

  Oregon State University Extension

  Cited on: What Can (and Can't) You Compost? The Complete List

• "How Much Compost, Soil or Mulch?"

  Texas A&M AgriLife Extension (Travis County)

  Cited on: Compost Calculator

• EPA's Composting At Home guidance

  U.S. Environmental Protection Agency

  Cited on: What Can (and Can't) You Compost? The Complete List

• Beware the Mulch Volcano

  University of Vermont Extension

  Cited on: 8 Mulch Mistakes That Are Secretly Hurting Your Plants

• "Pine Needle Mulch — the Myths and Legends"

  N.C. Cooperative Extension (Lee County Center)

  Cited on: Mulch Calculator

Fertilizer & Nutrients

• Reducing Fertilizer Use with a More Accurate Soil Test

  USDA Agricultural Research Service (ARS)

  Cited on: 8 Fertilizing Mistakes That Are Hurting Your Garden and Lawn

• Fertilizer Recommendations Guide

  USDA Natural Resources Conservation Service (NRCS)

  Cited on: NPK Calculator

• Understanding Your Soil Test Report

  University of Maryland Extension

  Cited on: How to Read and Understand Your Soil Test Results

• University of Minnesota Extension's quick guide to fertilizing plants

  University of Minnesota Extension

  Cited on: 15 Homemade Fertilizers You Can Make at Home (and How to Mix Them)

• University of Wisconsin Extension's guide to using wood ash in the home garden

  University of Wisconsin Extension

  Cited on: 15 Homemade Fertilizers You Can Make at Home (and How to Mix Them)

Watering & Irrigation

• "Rainwater, Storm Water & Graywater"

  Colorado Division of Water Resources

  Cited on: Rain Barrel Calculator

• "Rainwater Collection in Colorado"

  Colorado State University Extension

  Cited on: Rain Barrel Calculator

• Rain Barrel Diverter Kit Installation guide

  Fulton County, Georgia (Government)

  Cited on: How to Build a Rainwater Collection System for Your Garden

• Utah State University Extension's Center for Water-Efficient Landscaping

  Utah State University Extension

  Cited on: Drip Irrigation Calculator

Spacing & Planting

• "Using Growing Degree Days to Manage the Home Garden"

  Iowa State University Extension

  Cited on: Growing Degree Days Calculator

• "Degree Days for Common Insect Pests"

  University of Wisconsin Extension

  Cited on: Growing Degree Days Calculator

• GDDTracker

  Michigan State University Extension

  Cited on: Growing Degree Days Calculator

• "Using growing degree days to predict germination"

  West Virginia University Extension

  Cited on: Growing Degree Days Calculator

• Bagworm fact sheet

  University of Massachusetts Amherst Extension

  Cited on: Growing Degree Days Calculator

• Clemson Cooperative Extension's guide to bush and pole-type snap beans

  Clemson Cooperative Extension (HGIC)

  Cited on: How to Grow Beans from Seed (Bush, Pole, and the Nitrogen Myth)

• Clemson HGIC's Tomato Diseases & Disorders factsheet

  Clemson Cooperative Extension (HGIC)

  Cited on: How to Grow Tomatoes from Seed: A Complete Guide

• Cornell Cooperative Extension's "Spacing Landscape Plants"

  Cornell Cooperative Extension of Rockland County

  Cited on: Plant Spacing Calculator

• disease-resistant pepper varieties guide

  Cornell Vegetable Program

  Cited on: How to Grow Peppers from Seed: Bell, Hot, and Sweet Varieties

• "How to Determine Plant Quantity for Planting Beds"

  Iowa State University Extension

  Cited on: Plant Spacing Calculator

• Iowa State University Extension's Septoria Leaf Spot guide

  Iowa State University Extension

  Cited on: Why Are My Tomato Leaves Turning Yellow? A Diagnostic Guide

• LSU AgCenter's Expected Vegetable Garden Yields

  LSU AgCenter

  Cited on: Vegetable Yield Calculator

• NC State Extension's average first and last freeze dates resource

  NC State Extension

  Cited on: Frost Date Calculator

• New Mexico State University Extension's guide to nitrogen fixation by legumes

  New Mexico State University Extension

  Cited on: How to Grow Beans from Seed (Bush, Pole, and the Nitrogen Myth)

• Oregon State University Extension's guide to growing your own cucumbers

  Oregon State University Extension

  Cited on: How to Grow Cucumbers from Seed (and Why Yours Aren't Fruiting)

• Oregon State University Extension's guide to planting by soil temperature

  Oregon State University Extension

  Cited on: How to Grow Carrots from Seed (and Actually Get Them to Germinate)

• Pacific Northwest Pest Management Handbooks' tomato blossom-end rot page

  Pacific Northwest Pest Management Handbooks (OSU / WSU / University of Idaho)

  Cited on: Growing Tomatoes in Raised Beds: Spacing, Depth, and Blossom End Rot

• Penn State Extension's beginner vegetable gardening resources

  Penn State Extension

  Cited on: How to Start a Vegetable Garden: A Complete Beginner's Guide

• University of Connecticut Home Garden Education Center's guide to blossom-end rot

  UConn Home Garden Education Center

  Cited on: Growing Tomatoes in Raised Beds: Spacing, Depth, and Blossom End Rot

• "Pecan Trees for the Home or Backyard Orchard"

  University of Georgia College of Agricultural & Environmental Sciences

  Cited on: Plant Spacing Calculator

• Illinois Extension's cucumber growing guide

  University of Illinois Extension

  Cited on: How to Grow Cucumbers from Seed (and Why Yours Aren't Fruiting)

• University of Maine Cooperative Extension's guide to spacing trees in an orchard

  University of Maine Cooperative Extension

  Cited on: Plant Spacing Calculator

• University of Maryland Extension's How to Start a Vegetable Garden

  University of Maryland Extension

  Cited on: How to Start a Vegetable Garden: A Complete Beginner's Guide

• University of Maryland Extension's Key to Common Problems of Tomatoes

  University of Maryland Extension

  Cited on: Why Are My Tomato Leaves Turning Yellow? A Diagnostic Guide

• University of Maryland Extension's Vegetable Planting Calendar

  University of Maryland Extension

  Cited on: Seed Starting Calendar

• University of Maryland Extension's frost and freeze dates guide

  University of Maryland Extension

  Cited on: Frost Date Calculator

• University of Minnesota Extension's guide to growing beans in home gardens

  University of Minnesota Extension

  Cited on: How to Grow Beans from Seed (Bush, Pole, and the Nitrogen Myth)

• University of Minnesota Extension's guide to growing cucumbers in home gardens

  University of Minnesota Extension

  Cited on: How to Grow Cucumbers from Seed (and Why Yours Aren't Fruiting)

• University of Minnesota Extension's guide to growing peppers in home gardens

  University of Minnesota Extension

  Cited on: How to Grow Peppers from Seed: Bell, Hot, and Sweet Varieties

• University of Minnesota Extension's guide to growing tomatoes in home gardens

  University of Minnesota Extension

  Cited on: How to Grow Tomatoes from Seed: A Complete Guide

• University of Missouri Extension's "Starting Plants Indoors From Seeds" (G6570)

  University of Missouri Extension

  Cited on: Seed Starting Calendar

• University of Wisconsin Extension's guide to scarlet runner bean

  University of Wisconsin Extension

  Cited on: How to Grow Beans from Seed (Bush, Pole, and the Nitrogen Myth)

• Utah State University Extension's blossom end rot guidance

  Utah State University Extension

  Cited on: Growing Tomatoes in Raised Beds: Spacing, Depth, and Blossom End Rot

• Utah State University Extension's guide to growing carrots in your garden

  Utah State University Extension

  Cited on: How to Grow Carrots from Seed (and Actually Get Them to Germinate)

• Utah State University Extension's guide to growing lettuce in your garden

  Utah State University Extension

  Cited on: How to Grow Lettuce from Seed: The Cool-Season Crop That Breaks the Rules

• Succession Planting

  West Virginia University Extension

  Cited on: Succession Planting: How to Get More Harvests From the Same Space

• West Virginia University Extension's companion planting guide

  West Virginia University Extension

  Cited on: Companion Planting Chart: What to Grow Together (and What to Avoid)

Lawn & Landscaping

• Ask Extension's response on lawn overseeding rates

  Ask Extension (eXtension Foundation)

  Cited on: Overseeding Calculator

• "Seeding Rate Considerations"

  UMass Extension

  Cited on: Grass Seed Calculator

• Warm Season vs Cool Season Grasses

  University of Illinois Extension

  Cited on: Cool-Season vs. Warm-Season Grass: Which Type Do You Have?

• "Sod Installation and Maintenance" (Publication 2951)

  Mississippi State University Extension

  Cited on: Sod Calculator

• Turfgrass Producers International

  Turfgrass Producers International (TPI)

  Cited on: Sod Calculator

• "Turfgrass Establishment: Seeding" (W160-B)

  University of Tennessee Extension

  Cited on: Grass Seed Calculator

Indoor Plants

• Kansas State University Extension's guide to lighting options for starting seeds (LEDs vs. fluorescent)

  Kansas State University Extension

  Cited on: LED vs. Fluorescent Grow Lights: Which Should You Use?

• University of Illinois Extension's tips for repotting houseplants

  University of Illinois Extension

  Cited on: Pot Size Calculator

• University of Maryland Extension's guide to container vegetable gardening

  University of Maryland Extension

  Cited on: Pot Size Calculator

• Virginia Cooperative Extension's guide to calculating and using Daily Light Integral

  Virginia Cooperative Extension

  Cited on: DLI & Grow Light Calculator

Hydroponics & Greenhouse

• "What are the different conductivity scales? What do they mean?"

  Bluelab

  Cited on: Hydroponic Nutrient Calculator

• Purdue University's "Calculating Greenhouse Heating Requirements"

  Purdue University

  Cited on: Greenhouse Heater Calculator

Trees & Shrubs

• "How Do You Measure the Age of a Tree?"

  National Institute of Standards and Technology (NIST)

  Cited on: Tree Age Calculator

• "Measuring Tree Height using a Clinometer"

  Iowa State University (NRI Grazing Land / Center for Survey Statistics and Methodology)

  Cited on: Tree Height Calculator

• Tree Measurement Guidelines

  Texas A&M Forest Service

  Cited on: Tree Height Calculator

Government Datasets

Beyond individual citations, two calculators on this site are built directly on official government datasets rather than just linking out to further reading:

• USDA Plant Hardiness Zone Map

  USDA Agricultural Research Service (ARS)

  The basis for HarvestMath's 826-point USDA hardiness zone dataset — each reference point was checked directly against these official 2023 zone boundaries (ARS/PRISM Climate Group data) via a point-in-polygon lookup. Powers the Hardiness Zone Finder, Frost Date Calculator, and Seed Starting Calculator.

  Cited on: USDA Hardiness Zone Finder

• USDA NRCS's Soil Texture Calculator

  USDA Natural Resources Conservation Service (NRCS)

  The USDA soil texture triangle classification system this site's Soil Type Calculator implements to turn sand/silt/clay percentages into a texture class.

  Cited on: Soil Type Calculator

Something look wrong?

If a link is broken, a source has been updated, or you think a figure on this site should cite something it doesn’t, tell us. This page is only as trustworthy as it is accurate.