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Irrigation, Soil & Fertility TimingPotting Soil and Seed Starting Mix Calendar
The mix you start seeds in and the mix you pot them up into are different jobs, and both need to be ready before the seed trays come out.
Key points
- Use a fine starting mix for seeds and a nutrient-bearing potting mix later.
- Count back from your last frost date to find sowing dates.
- Moisten mix before filling trays and store extra supplies dry.
- Clean containers and fresh mix reduce damping-off.
- Harden off for about a week or two before transplanting.
What is the difference between seed starting mix and potting soil?
A seed starting mix is fine, light and low in nutrients so that tiny seeds can emerge and roots can move through it easily, while holding steady moisture. Potting mix is coarser and generally contains more nutrients or is meant to be fed, which suits larger plants growing in containers for longer.
Garden soil is a poor choice in containers. It compacts, drains badly and can bring weeds, pests and disease.
Light is the most common limit indoors. Seedlings stretch toward weak light and become leggy. A bright window rarely suffices, so many growers use grow lights placed close to the plants on a timer.
When should you start seeds?
Count backward from your average last spring frost date using the weeks to transplant listed on the seed packet or in your state extension guide. Frost dates vary by location and year, and USDA Plant Hardiness Zones do not give them.
For example, if your last spring frost usually falls around a certain date and a crop needs six to eight weeks indoors, subtract that range to find your sowing window. Treat the result as a starting point and adjust for the weather you actually get.
Temperature matters. Some seeds germinate best with warm soil, and others need cooler conditions. Heat mats help, but should be removed once seedlings emerge, following the maker's instructions.
When should you buy or mix your supplies?
Buy mix, trays and containers in late winter, before the start date, since popular items sell out. If you mix your own, gather ingredients and blend them a week or two ahead. Moisten the mix before filling trays, because dry peat-based material resists water.
Store unused bags sealed and dry, and keep them off cold floors. Old or damp mix can harbor mold and fungus gnats.
Label each tray with crop, variety and sowing date. When transplanting, those labels help you track which varieties performed best.
How do you reduce seedling disease?
Damping-off is a common seedling problem and is driven by wet, cool, poorly drained conditions and contaminated containers. Start with clean trays, fresh mix, good drainage and moderate watering. Avoid reusing mix that held diseased plants.
Provide light close to the seedlings and good air movement. Extension publications describe cleaning containers before reuse, and you should follow any label directions for products you choose.
Reusing mix is possible but needs care. Old mix may carry disease or lose structure. If you reuse it, screen out roots, refresh it with compost and watch for problems.
When do you pot up and harden off?
Move seedlings into larger containers when they have developed their first true leaves and are crowding. Then begin hardening off about a week or two before planting out, with gradual exposure to sun, wind and cooler temperatures.
Transplant after the danger of frost has passed for tender crops, and watch the forecast. Keep notes on sowing and transplant dates so next season's plan comes from your own results.
Sterile mixes may lack beneficial microbes, and garden compost may carry unwanted material. Pick what suits your seeds and your risk tolerance.
What goes in a homemade mix?
Common ingredients include peat or coir for water holding, perlite or vermiculite for air, and compost for nutrients and biology. Proportions vary by use. Seed mixes are lighter and finer, and potting mixes are coarser and richer.
Choose ingredients from reputable sources and avoid garden soil. Compost used in a seed mix should be well finished and sieved. Test a small batch with seeds before filling every tray.
Organic growers should check that mixes meet certification standards. Labels and certifiers can confirm.
How do you water and feed seedlings?
Water from below or with a gentle sprinkle, and let the surface dry slightly between waterings. Over-watering is the most common cause of weak seedlings.
Seedlings in a nutrient-poor starting mix need a light feeding once true leaves appear, following the product label. Move plants to a richer mix when you pot them up. Keep them under bright light, close to the plants, to prevent stretching.
Keep a simple sowing log with dates, germination results and transplant dates, so you can adjust next year.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.