Stop burying your seeds.
Most seeds that never come up were sown too deep. The twice-the-width rule, and which seeds want light.
As a rule of thumb, sow a seed at about twice its own width. Depth varies between plants, so where the packet gives a figure, follow that first, and very fine seed such as lettuce wants only a dusting or none at all. Keep a shallow sowing damp until the seedlings are through. A seed that never came up was often just covered with too much soil.
Every spring it plays out in gardens across the country. You sow a row, you wait, and the seedlings never come. When the bed is scratched back a week or two later, the seeds are often still there, swollen, sometimes with a pale shoot that gave up a centimetre short of daylight. Often they were not dead. They were simply too deep.
It is one of the most common sowing mistakes there is, and the easiest to fix, because the correction is smaller than most people expect. Not a new technique, not a product. Less soil.
How deep should a seed go?
About twice its own width is a reliable rule of thumb for almost everything in a spring packet. Depth varies between plants, so where the packet gives its own figure, follow that first. Look at the seed in your palm, imagine stacking two of them, and that is roughly the cover it wants.
Here is how that works across a spring packet:
- Lettuce: a millimetre or two of cover, or simply pressed onto the surface.
- Basil: a light cover of about three millimetres.
- Carrot and rocket: a shallow furrow five to ten millimetres deep, filled with sieved mix.
- Beetroot, silverbeet, spinach, coriander: about a centimetre.
- Zucchini, pumpkin, sunflower: about two centimetres.
- Beans and corn: two to three centimetres, roughly a fingertip to the first knuckle.
The packet gives the exact figure. The rule is for the moment you have lost the packet, or you are looking at a bed you sowed last weekend and wondering.
Why too deep is worse than too shallow
A seed carries a fixed store of energy, and the shoot has to reach light before that store runs out. Every extra centimetre of soil is more work, and deeper soil is also cooler and wetter, which slows the seed down and gives rot a longer run at it. Sow a lettuce seed three centimetres down and it can germinate perfectly well and still never see the sun.
Too shallow has its own risk, drying out, but that is a risk you can manage with a watering can. Too deep cannot be managed after the fact. So when in doubt, sow towards the shallow end of the range on the packet and keep the seed zone evenly moist.
Which seeds want light
Lettuce is the famous one. Its seed germinates better with light reaching it, which is why the old advice is to sow it on the surface and press it in, or cover it with the thinnest sprinkle of mix or vermiculite so it stays put while the light gets through. Many very fine seeds behave in a similar way, which is one more reason the smallest seed in the packet gets the least soil on top.
If your lettuce sowings have been patchy, sowing depth is worth ruling out, though warmth plays a part too, since lettuce seed can go dormant in hot soil. Try a row pressed onto damp mix with nothing over it and compare.
Sowing fine seed without losing it
Fine seed is fiddly, and the fix for fiddly is not to bury it deeper so it "stays". A few habits make it easy:
- Mix a pinch of seed with a spoon of dry sand so you can see where it lands and sow it thinly.
- Draw the row with a finger or the edge of a ruler, no deeper than the first line of your fingernail.
- Sieve a little mix or sifted compost over the top rather than raking soil back.
- Firm the row gently with the flat of your hand. Seed needs to touch soil to draw water from it.
- Water with a fine rose, the sprinkler head on a watering can, or a spray bottle, never a jet, and keep the surface damp until the seedlings show.
On a hot Perth or Adelaide afternoon a shallow sowing can dry in a couple of hours. A square of shade cloth laid over the row keeps a surface sowing cooler while still letting light through. Damp hessian holds moisture over a covered sowing such as carrots, as long as you check it daily and lift it as the first green appears.
Big seed goes deep, and likes it
Beans, corn, pumpkin and sunflower show the same rule at the other end of the scale. Their seed is large, its energy store is generous, and a little depth does it a favour: cooler, more evenly moist, and harder for the birds that treat a freshly sown bean row as a buffet to reach. Push each seed in to the first knuckle and close the hole over it.
These are warm-season crops, so they want warm soil as well as depth. Soil temperature is a separate correction and gets its own lesson. Given both, they are among the easiest seeds to bring up. Not every large seed wants heat, though: broad beans, for one, are sown into cool soil, so the packet still comes first.
A note for every zone
This one does not change with your postcode. The rule is the same in Brisbane, Sydney, Melbourne, Adelaide, Perth, Canberra and Hobart. The main thing that shifts is how fast the surface dries between waterings, and the dry spring winds of Perth and Adelaide and the humid mornings of Brisbane sit at opposite ends of that. Adjust the watering to suit. Soil type matters more than postcode here: in light, sandy soil you can sow a touch deeper than in heavy clay, where the surface holds moisture and warms slowly.
Grow it: seeds for this lesson
Carrot Heirloom Mixthe classic fine seed. Sow it thin in a shallow furrow, cover with sieved mix, keep it damp for the two to three weeks it usually takes.See the seeds
Lettuce Salad Bowl Greena loose-leaf lettuce that germinates better with light, so sow it shallow and follow the depth on the packet. Pick it as cut-and-come-again through spring.See the seeds
Basil Large Sweet Italianfine seed that wants warmth and a light cover of about three millimetres. Sow once the nights are mild.See the seeds
Bean Bush Jadethe contrast. Two to three centimetres deep, once the soil has warmed.See the seeds
Get the depth right and a packet goes a long way. Sowing depth is one of the first things worth checking when a row disappoints, alongside moisture and soil temperature. If a sowing has already failed and you want to run down the other suspects, our longer list of reasons seeds do not germinate covers water, temperature, age and pests one at a time, and the What to Sow Now page tells you what to sow, month by month, where you live.
Frequently asked questions
Should I soak seeds before sowing?
Most seed does not need it. Large seed such as peas, beans and corn can be soaked before sowing to soften the coat, and the right length of soak varies by variety, so follow the time on your packet rather than a fixed rule. Sown into warm, damp soil they come up within about a week either way, so treat soaking as optional.
How do I know if I sowed too deep?
Wait out the germination days on the packet, then scratch back a small section of the row. Swollen seed with a pale, bent shoot that has not reached the surface is the tell. Re-sow a shallow row beside it; the deep ones may still arrive late.
Does the rule apply to punnets and trays?
Yes. If anything it matters more, so keep fine seed near the surface of the punnet just as you would in a bed.
Why do my carrots come up in patches?
Usually uneven depth and uneven moisture. Carrot seed is slow, usually two to three weeks in spring and sometimes sooner in warm soil, and any part of the row that dries out during that time simply stops. Sand-mixed seed, a sieved cover and even moisture go a long way.
How long should seeds take to come up?
It depends on the soil temperature more than the date. In spring soil, lettuce and rocket usually show in one to two weeks, faster when it is warm, beans in about a week once the soil is warm, and carrot in about two to three weeks. Cold soil stretches all of those.
Do I need seed-raising mix?
For fine seed a fine, sieved mix or sifted compost gives the even surface and moisture that tiny seed needs. Big seed is happy in ordinary garden soil.
