Seed starting shelf guides

Short, sourced answers for lighting seedlings on a shelf. Each page links the university extension pages or manufacturer pages it uses.

Start with the main guide or the calculator. A plain list of every page is on the site map.

Sizing and cost

• How Many Grow Lights Does a 4-Foot Wire Shelf Need?
  A 4-foot wire shelf 24 inches deep usually takes two 4-foot light bars. Table of fixtures and watts for common shelf depths, and the rule behind it.
• How Many Seed Trays Fit on a Wire Shelf?
  How many 1020 seed trays fit on common wire shelf sizes, and how to count them for your own shelf.
• What Does a Seed Starting Shelf Cost to Run?
  Electricity cost of seed starting lights: kWh and dollars for a season at three electricity prices, and the formula to use your own numbers.
• Grow Lights for a Narrow 24-Inch Shelf
  How many 2-foot light bars a 24-inch shelf needs for common depths, with total watts and the number of 1020 trays that fit.

Light: distance, hours and type

• How Far Above Seedlings Should Grow Lights Hang?
  University extension sources give different hanging heights for seedling lights. Here is what each one says, why they differ, and how to choose.
• How Many Hours a Day to Run Seedling Lights
  Extension sources mostly say 14 to 16 hours a day for seedlings, with some saying 12 to 14. Here is where they differ and why.
• Can You Use Regular Shop Lights for Seedlings?
  Extension sources say plain shop lights work for most home seed starting if they are bright enough and hang close. Here is what to look for.
• Leggy Seedlings: Is It the Light?
  Tall, thin seedlings are most often a light problem. What Utah State Extension says to do about it and how to tell the cause.
• Cool White vs Red-Blue Grow Lights for Seedlings
  What extension sources say about light colour for seedlings: Oklahoma State on cool white versus red and blue, Iowa State on full-spectrum shop lights, Illinois on mixed tubes.
• LED vs Fluorescent for Seed Starting
  What extension sources say about LED and fluorescent lights for seedlings: hanging distance, tube life, hours and cost, without picking a winner for you.

Setup

• How to Set Up a Seed Starting Shelf, Step by Step
  A step by step setup for a seed starting shelf with lights, using university extension guidance for hours, distance, domes and airflow.
• How to Hang Grow Lights on a Wire Shelf
  Ways to hang a light bar from a wire shelf so you can raise it as seedlings grow, and what to check before you start.
• How to Link Several Grow Light Bars to One Timer
  How many linkable grow light bars can share one plug and one timer, how to add up the watts, and what to check on the timer.
• Can You Start Seeds on a Bookshelf?
  What you need to start seeds on a bookshelf: depth for the tray, height for the light, and how many 2-foot light bars it takes.

Heat and airflow

• Seedling Heat Mat and Thermostat: Do You Need One?
  What Utah State Extension says about heat mats, and what a thermostat controller does, using one manufacturer's published ranges as an example.
• Fan for Seedlings and Damping-Off
  Utah State Extension says a small fan at plant level helps prevent damping-off. What it says about how much air and how often.

Seed starting basics

• Seed Germination Temperature Chart for Common Vegetables
  Soil temperature for germination of 13 common vegetables, from Alabama Cooperative Extension: minimum, best range, maximum and the single best temperature.
• Do Seeds Need Light to Germinate?
  Which seeds germinate in light, which in darkness and which in either, from NC State Extension's list, and how to sow each kind.
• Seed Starting Mix vs Garden Soil
  What extension sources say to use for starting seeds in containers and why garden soil is not recommended.
• How to Water Seedlings: Bottom Watering and Moisture
  What NDSU and Oklahoma State Extension say about watering seedlings from the bottom, how long to leave water, and keeping soil moist but not soggy.
• When and How to Fertilize Seedlings
  NDSU and Oklahoma State Extension disagree on when to start fertilizing seedlings. What each says and how to choose.
• Window Light vs Grow Light for Seedlings
  Cornell suggests a sunny windowsill. Oklahoma State says most windows give too little light. What each says and how to decide.
• Seedling Problems: Leggy, Purple, Wilting and Leaning
  Common seedling problems and the causes extension sources give: weak light, cold wet soil, overwatering, damping-off and one-sided light.
• Seed Starting Containers: Trays vs Individual Pots
  What Oklahoma State and Cornell Extension say about seed trays, individual pots and moving seedlings to larger containers.

Timing and crops

• When to Start Seeds Indoors: Frost-Date Calculator
  Pick a crop and enter your average last frost date to get an indoor start date, using the weeks-before-last-frost chart from NC State Extension.
• Hardening Off Seedlings: How Long and How
  How long hardening off takes and how to do it, from Oklahoma State, Utah State and Cornell Extension, including where they differ.
• How Long Do Seedlings Stay Under Lights?
  Weeks from sowing to setting out for common crops, from Cornell Extension, and what that means for shelf time and electricity cost.
• Grow Lights for Tomato Seedlings
  Timing, temperature and lighting for tomato seedlings under shelf lights, with figures from NC State, Cornell, Alabama and Saskatchewan Extension.
• Starting Peppers Under Lights
  Timing, germination temperature and lighting for peppers started indoors, with figures from NC State, Cornell and Alabama Extension.
• Growing Lettuce and Herbs Under Shelf Lights
  Lettuce needs light to germinate and grows cool. What NC State, Cornell, Alabama and UConn Extension say about lettuce, basil and parsley under lights.

Buying guides

• Best Grow Lights for a 4-Foot Wire Shelf
  Three 4-foot light fixtures that suit a 4-foot wire shelf, compared on the specifications each manufacturer publishes. Not tested.
• Best T5 LED Grow Light Bars for Seed Starting
  Two Barrina T5 LED bar sizes compared on published specifications: 4-foot 20 W and 2-foot 10 W, with how many each shelf needs. Not tested.
• Best Small Grow Light for a Single Seed Tray
  What size light a single 1020 seed tray needs, with a 2-foot LED bar as the example and the published specifications.
• Best Seedling Heat Mats With Thermostat
  Seedling heat mats compared on published specifications: sizes, wattage, controllers and temperature range. What each page states and what it leaves out.
• Best 1020 Seed Trays and Humidity Domes
  How 1020 seed trays and humidity domes fit together, with the published size of one extra-strength dome and what to check on any tray set.
• Best Timers for Seed Starting Grow Lights: What to Check
  How to choose a timer for seed starting lights: the rating to match, the hours to set, and mechanical versus digital. A buying checklist, not a ranked list.
• Best Wire Shelving for Seed Starting: What to Check
  How to choose wire shelving for seed starting: size, shelf gap, depth and weight, with a table of how many lights and trays each shelf size takes.
• Best Small Fans for Seedlings: What to Check
  How to choose a small fan for seedlings: Utah State Extension's guidance on gentle airflow, plus what to check on a fan listing.
• Seed starting shelf gear shortlist
  Light bars and a heat mat, with manufacturer specifications.