Making your own seed starting mix is one of the simplest ways to ensure strong, healthy seedlings. A good mix supports fast germination, steady moisture, and early root development. After more than 30 years of starting seeds in Sonoma Valley—indoors under lights, outdoors in raised beds, and in containers—I’ve tested dozens of homemade blends. The best mixes all have the same qualities: fine texture, good drainage, good moisture-holding, and freedom from disease.

Below are several reliable recipes, including the well-known University of California seed starting mix, professional-style blends, and the homemade formula I use for most of my seedlings.

What Makes a Good Seed Starting Mix

Seed starting mix is different from potting soil. A good mix should be:

Lightweight — fine enough for young roots to push through
Sterile — free from pathogens that cause damping-off
Moisture-retentive — holds water evenly
Well-draining — seeds can rot if the mix stays soggy
Low fertility — seedlings don’t need heavy nutrients early on

Over the decades, I’ve found that keeping mixes simple with just a few key ingredients produces the most consistent results.

The University of California Seed Starting Mix

The UC mix is widely respected for its simplicity and reliability. It was developed for consistent germination and disease prevention.

University of California Mix (UC Mix)

1 part fine sand
1 part peat moss

This mix is:

Sterile (when made with new sand and peat)
Fast-draining
Excellent for germinating seeds prone to rot

My experience with the UC mix:

I use UC mix when I want maximum drainage, especially for onions, leeks, herbs, and cool-season crops that prefer not to sit in wet media. Seeds germinate evenly and roots stay clean and well-branched.

If you want a bit more moisture-holding ability, mix in 10–20% vermiculite.

My Preferred Starting Mix (30+ Years of Use)

This is the mix I rely on for most vegetable starts—tomatoes, peppers, brassicas, basil, lettuces, and more. It balances moisture and airflow beautifully.

My Standard Seed Starting Mix

1 part peat moss or coconut coir
1 part vermiculite
1 part perlite

Why it works:

Coir/peat retains moisture without compacting
Vermiculite holds water and keeps the mix fluffy
Perlite improves drainage and airflow

Tips from experience:

Moisten the mix before filling trays—it should feel like a wrung-out sponge.
Compress lightly; don’t pack tightly. Seedlings need air pockets.
Bottom-water after sowing to keep the surface from crusting.

Cornell Seed Starting Mix

Another widely used recipe from Cornell University, ideal for vegetable and flower starts.

Cornell Peat-Lite Mix

3 parts peat moss
1 part perlite
1 part vermiculite

Cornell developed this mix for commercial greenhouse propagation. It retains moisture well and is excellent for small seeds like lettuce, kale, and herbs.

John Innes Seedling Mix (UK Standard)

A classic British recipe known for stability and balanced nutrients.

John Innes Seedling Mix

2 parts loam
1 part peat or coir
1 part sharp sand
Plus a pinch of lime (to neutralize peat)

This mix is heavier and not sterile unless pasteurized. I use it occasionally for large seeds like squash, cucumbers, and melons when starting in pots—but rarely for fine indoor seedings.

Recipe: Simple Peat-Free Seed Starting Mix

If you prefer peat-free gardening, this blend performs very similarly.

Peat-Free Mix

2 parts coconut coir
1 part compost (sifted)
1 part perlite or pumice

Note from experience:

Compost must be very fine and fully mature. Sift it through ¼-inch hardware cloth to remove sticks and clumps.

How to Sterilize Your Homemade Mix

If using compost, garden soil, or sand from outdoors, sterilizing prevents damping-off.

Two easy methods:

Oven Method
Spread mix on a baking tray (2–3 inches deep) and heat at 180–200°F (82–93°C) for 30 minutes.

Solarization (best for larger batches)
Place moist mix in a black plastic bag, seal, and set in full sun for 4–6 weeks.

I rarely need sterilization when using store-bought components, but it’s essential when using native soil or compost.

How to Use Homemade Seed Starting Mix

Pre-moisten the mix until evenly damp.
Fill trays or pots loosely—don’t compact.
Tap containers to settle the media instead of pressing it down.
Sow seeds at recommended depth (often very shallow).
Mist the surface, then bottom-water after germination.
Keep warm (70–80°F for warm crops) until sprouts appear.
Provide strong overhead light to prevent legginess.

With consistent moisture and good airflow, most vegetable seeds germinate in 5–10 days.

Troubleshooting Your Homemade Mix

Mix dries out too fast

Add more:

Peat or coir
Vermiculite

Mix stays too wet

Add more:

Perlite
Sharp sand

Seedlings look yellow

This can happen after 2–3 weeks; the mix is intentionally low in nutrients.
Use a diluted liquid fertilizer (¼-strength) once true leaves appear.

Final Thoughts

Making your own seed starting mix puts you in control of quality, texture, and moisture. After decades of experimenting in my Sonoma Valley garden, I always return to simple recipes—UC mix when I want extra drainage and my own 1:1:1 coir/vermiculite/perlite mix for most seedlings.

With the right blend, seeds germinate more reliably and seedlings develop strong, healthy root systems that transplant beautifully into the garden.

Seed Starting Mix Comparison Chart

Mix Name	Ingredients	Best For	Pros	Cons	My Notes from Experience
University of California (UC Mix)	1 part fine sand; 1 part peat moss	Onions, leeks, herbs, cool-season crops; seeds that dislike excess moisture	Very fast draining; sterile; simple	Dries quickly; low moisture retention	Excellent for fine seeds; great in humid greenhouses. I add 10–20% vermiculite for extra moisture.
My Standard Mix (1:1:1)	1 part peat or coir; 1 part vermiculite; 1 part perlite	Tomatoes, peppers, brassicas, lettuce, herbs—most vegetables	Balanced moisture + drainage; very lightweight; consistent germination	Requires separate fertilizing after 2–3 weeks	My go-to mix for 30+ years. Produces compact, sturdy seedlings.
Cornell Peat-Lite Mix	3 parts peat; 1 part perlite; 1 part vermiculite	Small seeds; lettuce; kale; herbs; broccoli	Holds moisture well; very uniform; commercial-quality	Peat-heavy; can stay wet in cool rooms	Best when starting many trays at once; reliable for cool-season crops indoors.
John Innes Seedling Mix	2 parts loam; 1 part peat or coir; 1 part sharp sand	Large seeds; squash; cucumbers; melons; outdoor starts	Nutrient stable; good structure	Heavy; not sterile unless pasteurized	Excellent for pot-grown seedlings; anchors roots. I use it mostly outdoors.
Peat-Free Homemade Mix	2 parts coir; 1 part sifted compost; 1 part perlite or pumice	Organic gardens; eco-focused gardeners; general seed starting	Sustainable; good water retention; inexpensive	Compost must be perfect; sterilization needed	Works well when compost is fine and mature. Sift well to avoid crusting.
Simple Coir-Vermiculite Mix	2 parts coir; 1 part vermiculite	Lettuce, herbs, small-seeded crops	Very fine texture; great for tiny seeds	Needs perlite added for drainage	Great for indoor germination trays where fine texture is critical.
Sand-Coir Mix (Desert-Style)	1 part washed sand; 1 part coir; 1 part perlite	Heat-loving crops, heavy clay soils	Fast draining; reduces rot	Requires frequent watering	Helpful for peppers and chilies in warm climates like Sonoma.

Seed Starting & Propagation Learning Hub

Start here — Seed Starting Basics: A Complete Beginner-to-Advanced Guide for Indoor and Outdoor Seed Starting

Seed Starting Fundamentals

Seed Starting Guide
Starting Plants from Seed
Vegetable Seed Starting Indoors and Out
How to Start Seeds Indoors Step-by-Step
Starting Vegetables Indoors: 15 Varieties for Success
Vegetable Seed Germination
How to Read a Seed Packet

Planning & Timing

Seed Starting Tools, Supplies & Setup

Seedling Care, Troubleshooting & Success Tips

How to Make Your Own Seed Starting Mix

What Makes a Good Seed Starting Mix