September 29, 2026
Which seed starting soil to use and when to sow
For germinating seeds, buy a seed-starting mix and keep it evenly damp. It is milled fine and carries little fertilizer, so seeds make close contact and new roots push through easily. Potting soil suits transplants and larger containers, once seedlings hold their first true leaves. The date to sow is set by your yard's real last frost, and that can sit weeks off the published date.
Seed-starting mix
A seed-starting mix is built for one job, getting a seed to sprout and stand up. It skips the heavy ingredients so nothing crusts over a small seed or burns a first root.
- Texture. Fine peat or coir with perlite or vermiculite, and little to no compost or bark.
- Moisture. It wicks water evenly, then dries fast in shallow cells.
- Best for. Sowing through the first pair of true leaves.
All-purpose potting soil
Potting soil grows plants that already have roots and leaves. It holds a bigger reserve of water and nutrients, which helps a filling container and hinders a bare seed.
- Texture. Coarser, with bark and compost, sometimes a slow-release fertilizer.
- Moisture. It holds more water for longer and drains slower.
- Best for. Potting up seedlings and running container plants.
Once seedlings carry true leaves, move them into a potting soil that holds moisture so you water less often as roots fill the pot.
Why seedlings dry out so fast
Fine particles pull water up to the surface, where a warm, bright windowsill evaporates it. A small cell holds a thimble of water against a large open face, so the top can go bone dry within a day. Bottom-water by setting trays in a shallow pool for a few minutes, which soaks the mix from below and leaves the surface crumbly rather than caked.
Peat and coir turn water-repellent once they fully dry out, so a dry cell can shed water off the top while staying parched inside. Wet the mix before you sow, and never let it dry all the way. The same trap hits garden beds, and we cover it in why water runs off a dry bed.
When to start seeds indoors
Seed packets give a lead time in weeks before your last frost. That instruction only works when you count back from your yard's real last frost, which the published regional date often misses. Our own garden shows the gap plainly. The regional date lands on April 28, but our station taught Siltwell an offset near 7°F below the forecast, and that pushes our effective last frost out to May 20.
On a free account you enter a microclimate offset by hand in Settings, and Siltwell shifts your frost dates by it. With a connected station on Pro, Siltwell learns the offset for you after about 14 days of overlapping outdoor temperature readings. Either way the shifted last frost is what you count back from. You can check how far your area runs off the model with the microclimate checker, and we explain the gap in why your last frost date is probably wrong.
One limit worth stating: the mix controls moisture and seed contact, and it cannot warm cold soil. Many warm-season seeds stall below roughly 60°F regardless of the mix, so a heat mat matters as much as the recipe. For cool-season sowing timed to real lows, see sowing winter seeds by microclimate.
Which one to buy
Buy a bag of seed-starting mix for the trays and a bag of potting soil for potting up, and use each for its job. Then set your microclimate offset, read your shifted last frost, and count back the packet's weeks to fix each sowing date.
Common questions.
- What is the best seed starting mix?
- A fine-milled mix of peat or coir with perlite or vermiculite, holding little to no compost or fertilizer. The fine texture puts the seed in close contact with damp material, and the lean recipe keeps salts away from tender first roots. Reserve richer, coarser potting soil for potting up after the first true leaves appear.
- Can I use potting soil for seeds?
- You can sow in potting soil, but germination gets less reliable. The bark chunks bury small seeds at uneven depths, and any slow-release fertilizer can stress young roots. If it is all you have, sift out the coarse pieces and sow shallow.
- Why do seedlings dry out so fast?
- Seed-starting mix is fine and lean, so it drains and evaporates quickly, and small cells hold little water against a large exposed surface. A warm, bright spot speeds that loss further. Bottom-water so the mix stays evenly damp without a crusted, dry top.
- When should I start seeds indoors?
- Count back the weeks on the seed packet from your yard's real last frost, not the regional published date. Our own beds run colder than the April 28 regional date, and Siltwell's learned offset moves our effective last frost to May 20, about three weeks later. Starting by the published date would push transplants outdoors while frost was still likely.
Related guides
How fertile soil changes how often you water
Learn what makes soil fertile, how organic matter builds it, and why a richer bed holds water and dries slower than a lean one.
How raised bed soil mix decides how often you water
How the ratio in a raised bed soil mix sets pore size, drying rate, and how often each bed needs water.
How to mix potting soil with compost
How much compost to mix into potting soil, why it holds more water, and why a richer pot hides the dry-down from your finger.
Free planting tools
- When to plant shows the last date to sow each crop before your fall frost.
- Microclimate checker shows how much colder or warmer gardens near you ran than the forecast low.
- GDD estimator adds up a typical season's heat and shows which crops can ripen where you are.
