Seed Start Guide - Complete Indoor Seed Starting Resource

Master the Art of Starting Seeds Indoors

Starting seeds indoors gives you a significant head start on the growing season, access to rare varieties, and the satisfaction of growing plants from the very beginning. Whether you're growing tomatoes, peppers, herbs, or flowers, this guide covers everything you need to know.

                What You'll Learn
                Essential equipment for successful seed starting
Step-by-step germination techniques
Optimal timing for different plants
Common problems and how to fix them
Hardening off and transplanting seedlings

            

Why Start Seeds Indoors?

Cost Savings

A packet of seeds costs $2-4 and produces dozens of plants. Nursery transplants cost $3-6 each. Starting your own seeds can save hundreds of dollars per season.

Variety Selection

Nurseries carry limited varieties. Seed catalogs offer thousands of options including heirlooms, rare cultivars, and the newest hybrids.

Timing Control

Start seeds on your schedule. Get transplants ready exactly when your garden needs them, not when the store happens to stock them.

Healthier Plants

Your seedlings avoid exposure to diseases and pests common in commercial greenhouses. You control the growing conditions from day one.

Essential Seed Starting Equipment

Success in seed starting depends on having the right equipment. Here's what you need and why each piece matters.

1. Seed Starting Containers

Seeds need containers with drainage holes and cells sized appropriately for the plants you're growing. Options include cell trays, peat pots, soil blocks, and recycled containers.

Cell Trays (Recommended for Beginners)

Plastic cell trays are the most popular choice. They're reusable, affordable, and come in various cell sizes. A 72-cell tray works well for most vegetables and flowers. Look for trays that include a solid bottom tray to catch water and a humidity dome for the germination phase.

The SOLIGT 60-Cell Seed Starter Kit is excellent for beginners because it includes everything: cell trays, bottom trays, humidity domes, and even heat mats. Check price on Amazon

Cell Sizes Guide

Small cells (1"): Lettuce, herbs, onions, celery
Medium cells (1.5"): Tomatoes, peppers, brassicas, most flowers
Large cells (2"+): Cucumbers, squash, melons, sunflowers

2. Seed Starting Mix

Regular garden soil is too heavy for seed starting. Seed starting mix is specifically formulated to be light, sterile, and moisture-retentive while providing good drainage.

What to Look For

Sterile: Free from weed seeds, pathogens, and insects
Fine texture: Allows tiny roots to penetrate easily
Good drainage: Prevents waterlogging and root rot
Moisture retention: Stays evenly moist without drying out too quickly

The Espoma Organic Seed Starter contains mycorrhizae to promote root growth and is OMRI-listed for organic gardening. For a budget-friendly option, Hoffman Seed Starter Soil provides consistent results. See Espoma on Amazon

3. Heat Mat (Highly Recommended)

Most seeds germinate best at soil temperatures between 70-85°F (21-29°C). Room temperature (65-70°F) is often too cool for optimal germination, especially for warm-season crops like tomatoes, peppers, and eggplants.

How Heat Mats Work

A seedling heat mat is a waterproof electric pad that sits under your seed trays. It raises the soil temperature 10-20°F above ambient room temperature. This dramatically improves germination rates and speeds up the process.

Crop	Optimal Soil Temp	Days to Germinate (with heat)	Days Without Heat
Tomatoes	75-85°F	5-7	10-14
Peppers	80-90°F	7-10	14-21+
Eggplant	80-90°F	7-10	14-21
Basil	70-75°F	5-7	10-14
Lettuce	65-70°F	3-5	7-10

The VIVOSUN Seedling Heat Mat is UL-certified and fits standard 10"x20" trays. For precise temperature control, the VIVOSUN Heat Mat with Thermostat Combo lets you dial in exact temperatures. Check current price

Pro Tip: Once seeds germinate and you see green sprouts, remove the heat mat. Seedlings grow best at slightly cooler temperatures (60-70°F) than seeds need for germination.

4. Grow Lights (Essential)

Adequate light is the #1 factor in growing strong, healthy seedlings. A sunny windowsill rarely provides enough light, resulting in weak, leggy plants that struggle after transplanting.

Why Windowsills Fail

Winter sun is low and weak (low light intensity)
Days are short (insufficient duration)
Light comes from one direction (seedlings lean and stretch)
Glass filters out beneficial light wavelengths

Light Requirements

Seedlings need 14-16 hours of light daily. Position lights 2-4 inches above the tops of seedlings and raise them as plants grow. LED grow lights are the most energy-efficient option and produce minimal heat.

Types of Grow Lights

LED panels: Most efficient, full spectrum, low heat
T5 fluorescent tubes: Good coverage, affordable, moderate heat
Shop lights (LED): Budget option, works well for seedlings

For small setups, the GooingTop LED Grow Light clips onto shelves and has a built-in timer. For a dedicated seed starting station, the iGrowtek 2ft Grow Light with Stand provides excellent coverage for multiple trays. See on Amazon

5. Other Helpful Supplies

Spray bottle: For gentle watering of tiny seeds and new sprouts
Plant labels: Mark varieties and sowing dates
Waterproof marker: For writing on labels
Small fan: Provides air circulation and strengthens stems
Timer: Automates grow light schedule (many lights have built-in timers)

Step-by-Step Seed Starting Guide

Step 1: Prepare Your Containers

If reusing containers from previous years, wash them with a 10% bleach solution (1 part bleach to 9 parts water) to kill any pathogens. Rinse thoroughly and let dry.

Fill containers with moistened seed starting mix. The mix should be damp like a wrung-out sponge—wet enough to hold together when squeezed, but not dripping. Fill cells to the top, then gently press down to eliminate air pockets, leaving about 1/4" of space at the top.

Step 2: Sow Seeds at the Correct Depth

The general rule: plant seeds at a depth of 2-3 times their diameter. Tiny seeds like lettuce and basil should be barely covered or surface-sown. Larger seeds like squash and beans go 1" deep.

Seed Size	Planting Depth	Examples
Tiny (dust-like)	Surface sow, press in	Lettuce, petunias, snapdragons
Small	1/8" - 1/4"	Tomatoes, peppers, basil
Medium	1/4" - 1/2"	Brassicas, chard, beets
Large	1/2" - 1"	Squash, cucumbers, beans

Place 2-3 seeds per cell to ensure at least one germinates. You'll thin to the strongest seedling later.

Step 3: Cover and Provide Warmth

Cover trays with humidity domes or plastic wrap to maintain moisture during germination. Place on a heat mat if you have one, especially for warm-season crops.

Most seeds don't need light to germinate—they need warmth and moisture. Once seeds sprout, remove the humidity dome immediately and move trays under grow lights.

Step 4: Provide Light

As soon as you see green sprouts emerging, get them under lights. Position LED grow lights 2-4 inches above seedlings. Set timers for 14-16 hours of light per day.

Raise lights as plants grow, maintaining the 2-4 inch distance. If seedlings stretch and become leggy, lights are too far away or on for too few hours.

Step 5: Water Properly

Overwatering is the #1 killer of seedlings. Water when the surface of the soil looks dry and lighter in color. Water from the bottom by adding water to the tray and letting cells absorb it for 10-15 minutes, then pour off excess.

Alternatively, water gently from above with a spray bottle or watering can with a fine rose. Avoid flooding cells or splashing soil onto leaves.

Step 6: Thin Seedlings

Once seedlings have their first set of true leaves (the second set that appears, after the initial seed leaves), thin to one plant per cell. Use scissors to snip extra seedlings at the soil line rather than pulling them out, which can disturb roots of the keeper.

Step 7: Fertilize

Seed starting mix contains few nutrients. Once seedlings have true leaves, begin feeding weekly with a diluted liquid fertilizer at 1/4 to 1/2 strength. Organic fish emulsion or liquid kelp work well.

Step 8: Pot Up if Needed

If seedlings outgrow their cells before it's time to transplant outdoors (roots coming out the bottom, plants becoming rootbound), move them to larger containers. 3-4" pots give plants room to continue growing indoors.

Step 9: Harden Off

The week before transplanting, gradually expose seedlings to outdoor conditions. Start with 1-2 hours in a sheltered, shady spot. Increase sun exposure and time outdoors daily. By day 7, seedlings should tolerate full sun and overnight temperatures.

This process toughens cell walls and prepares plants for wind, temperature fluctuations, and intense sunlight they'll experience in the garden.

Step 10: Transplant

Transplant on a cloudy day or in late afternoon to reduce transplant shock. Water seedlings well before removing from containers. Dig holes slightly larger than the rootball, set plants at the same depth they were growing (except tomatoes, which can be buried deeper), and water in well.

When to Start Seeds: Timing Charts

Timing is calculated backwards from your last frost date. Find your last frost date at your local extension service or weather data, then count back the specified number of weeks.

Example: If your last frost date is May 15th and tomatoes need 6-8 weeks indoors, start tomato seeds between March 20th and April 3rd.

Vegetables

Vegetable	Weeks Before Last Frost	Germination Temp	Days to Germinate
Tomatoes	6-8 weeks	75-85°F	5-10
Peppers	8-10 weeks	80-90°F	7-14
Eggplant	8-10 weeks	80-90°F	7-14
Broccoli	6-8 weeks	65-75°F	5-10
Cabbage	6-8 weeks	65-75°F	5-10
Cauliflower	6-8 weeks	65-75°F	5-10
Lettuce	4-6 weeks	60-70°F	3-7
Kale	4-6 weeks	65-75°F	5-10
Onions	10-12 weeks	65-75°F	7-14
Cucumbers	3-4 weeks	75-85°F	3-7
Squash/Pumpkins	3-4 weeks	75-85°F	5-10
Melons	3-4 weeks	80-90°F	5-10

Herbs

Herb	Weeks Before Last Frost	Germination Temp	Days to Germinate
Basil	4-6 weeks	70-75°F	5-10
Parsley	8-10 weeks	65-70°F	14-28
Cilantro	4-6 weeks	60-70°F	7-14
Oregano	6-8 weeks	65-70°F	7-14
Thyme	8-10 weeks	65-70°F	14-21
Rosemary	10-12 weeks	70-75°F	14-28

Flowers

Flower	Weeks Before Last Frost	Germination Temp	Days to Germinate
Marigolds	4-6 weeks	70-75°F	5-7
Zinnias	4-6 weeks	70-80°F	5-10
Cosmos	4-6 weeks	70-75°F	5-10
Petunias	10-12 weeks	70-80°F	7-14
Snapdragons	8-10 weeks	65-75°F	7-14
Impatiens	10-12 weeks	70-75°F	14-21
Sunflowers	2-3 weeks	70-80°F	7-10

Troubleshooting Common Problems

Leggy Seedlings

Symptoms: Tall, thin, stretched stems; pale color; plants fall over

Cause: Insufficient light or lights too far away

Solution: Move lights closer (2-4 inches from tops). Increase light duration to 14-16 hours. Add a small fan to strengthen stems. Consider upgrading to brighter lights.

Damping Off

Symptoms: Seedlings fall over at soil line; stems appear pinched or rotted at base

Cause: Fungal disease from overwatering, poor air circulation, or contaminated soil

Solution: Use sterile seed starting mix. Don't overwater. Remove humidity domes after germination. Provide air circulation. If reusing containers, sterilize with diluted bleach solution.

Seeds Not Germinating

Possible causes:

Old seeds: Seeds lose viability over time. Test old seeds by placing 10 in a damp paper towel—if fewer than 5 sprout, buy fresh seed.
Wrong temperature: Use a heat mat for warm-season crops. Check optimal germination temperatures.
Planted too deep: Review proper planting depths. Small seeds need only light covering.
Too wet or too dry: Maintain consistent moisture like a wrung-out sponge.
Seeds need light: Some seeds (lettuce, petunias) need light to germinate—don't cover them.

Yellowing Leaves

Cause: Usually nutrient deficiency, as seed starting mix has limited nutrients

Solution: Begin feeding with diluted liquid fertilizer once true leaves appear. Use 1/4 strength and increase gradually.

Mold on Soil Surface

Cause: Too much moisture and not enough air circulation

Solution: Scrape off mold. Let soil surface dry between waterings. Remove humidity domes. Add a fan for air movement. The mold is usually harmless but indicates conditions that can lead to damping off.

Seedlings Not Growing

Cause: Temperatures too cold, insufficient light, or rootbound

Solution: Maintain temperatures of 60-70°F for most seedlings. Ensure adequate light. Check if roots are circling the bottom of cells—if so, pot up to larger containers.

Quick Reference Checklist

Before You Start

Calculate your planting dates based on last frost
Gather containers, soil, lights, and heat mat
Clean and sterilize any used containers
Organize seeds and create a planting schedule

Sowing Day

Moisten seed starting mix before filling containers
Plant seeds at correct depth (2-3x seed diameter)
Label everything with variety and date
Cover with humidity dome and place on heat mat

After Germination

Remove humidity dome immediately when sprouts appear
Move to grow lights (2-4 inches above seedlings)
Remove heat mat after germination
Water from bottom when soil surface dries

Growing On

Thin to one seedling per cell after true leaves appear
Begin fertilizing at 1/4 strength weekly
Pot up if roots outgrow containers
Run a fan briefly each day to strengthen stems

Before Transplanting

Harden off over 7-10 days
Water well before transplanting
Transplant on cloudy day or evening
Water in thoroughly after planting