Watch the episode · 4:31You check your seedlings in the morning, and one has come up with its seed leaves still locked inside the shell. Growers call it helmet head. It looks alarming, but it has simple causes, and a safe fix. To understand both, you need to know how the shell is meant to come off in the first place.
The short answer ▶ 0:23
Most often the seed was sown too shallow, so there was no soil to hold the shell back. A dry shell stays hard and grips. Some seeds are simply more prone to it. And the fix: soften the shell, give it time, and only then ease it off.
- 01Too shallow
nothing holds the shell
- 02Too dry
the shell stays hard
- 03The seed
some are more prone
- 04The fix
mist, wait, then ease
How the shell should come off ▶ 0:40
A cannabis seed starts by splitting its shell and sending a root down. Then the stem just below the seed leaves grows into a tight hook. The hook pushes up through the soil, dragging the seed leaves behind it, still folded inside the shell. As it pulls, the soil grips the shell and the seed leaves slide out. The empty shell usually stays below the surface.
Source · Shahriari et al. (2013); U. Minnesota Horticulture text
Some plants leave nothing to chance. Cucumber seedlings grow a little peg at the base of the stem that pins the shell down. Grown in space, with no up or down, they made pegs on both sides. Cannabis has no peg. It relies on the soil.
Source · Yamazaki et al., npj Microgravity (2016)
Why it sticks ▶ 1:22
There is no study of helmet head in cannabis, so the clearest test comes from peppers, which come up the same way, pulling their seed leaves out of the shell. In a study in Iran, five pepper varieties were sown on the surface, 1 cm deep, or 2 cm deep.
Source · Shahriari, Khajeh Hosseini & Hasheminia (2013)
On the surface, more than one seedling in eight (13.5%) came up still wearing its shell. At 1 cm it was one in two hundred (0.5%). At 2 cm, the same.
Source · Shahriari et al., Iranian J. Field Crops Res. (2013), CC BY 4.0
| 1 CM deep | 2 CM deep | |
|---|---|---|
| Came up | 67% | 59% |
| Days to come up | about 16 | about 18 |
But deeper isn't better. At 2 cm fewer seedlings came up, and they took about two days longer. 1 cm was the sweet spot. Hemp trials sow at about the same depth: a North Dakota trial used half an inch, just over 1 cm.
Source · NDSU Langdon REC hemp trial (2017)
That is why growers so often see helmets after the paper towel method (see the best way to germinate seeds). A sprouted seed gets tucked in just under the surface, with almost no soil above it to hold the shell.
Moisture matters too. A shell that dries out stays hard and keeps its grip. Growers report helmet head most when the top of the mix dries out before the seedling is through.
Even the way the seed lies can matter. In a muskmelon study, seeds sown root down, root up or on their side shed their shells differently. And seeds pre-soaked in a controlled way, called priming, kept their shells less often.
Source · Nascimento & West, HortScience (1998)
Average over all depths and soil firmness levels
And some seeds are simply more prone to it. In the pepper study, the rate ranged from 3% in one variety to 8.5% in another.
Source · Shahriari et al. (2013)
What to do ▶ 2:48
First, don't rush. Growers find many shells slip off on their own within a day or so, as the seed leaves swell and push them open.
If it's still stuck, soften it. Mist the shell with clean water a few times a day, or keep the seedling humid under a cover for a while. A soft shell lets go far more easily.
Once it's soft, ease it off gently from the edge, with clean fingertips or fine tweezers. Never pull it straight off: the seed leaves can come away with it. Sometimes a thin, clear skin still holds the leaves together. Mist that too, and it usually splits as they open.
Why be so careful? The seed leaves are the seedling's first food store and its first solar panels. In soybean studies, losing one seed leaf made little difference, but losing both stunted the seedlings.
Source · Soybean cotyledon removal studies (e.g. Thomas & Costa 1993)
How to prevent it ▶ 3:41
Sow about 1 cm deep, about half an inch. Keep the top of the mix moist until the seedlings are up. If a seed is already sprouted, plant it with the root pointing down. And be patient. For the other early-life problems worth avoiding, see 5 seedling mistakes.
- 01Sow ~1 CM
about half an inch
- 02Keep it moist
the top of the mix too
- 03Root down
if pre-sprouted
- 04Be patient
many shells drop alone
The soil holds the shell while the seed leaves pull out, which is why seedlings sown on the surface got stuck far more often. If one does, soften the shell, then ease it off; and if a seed never came up at all, read why seeds don't sprout.
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Sources
- Shahriari, R., Khajeh Hosseini, M. & Hasheminia, S.M. (2013). "Effects of Different Sowing Depths and Soil Compactions on Emergence and Initial Growth of Pepper Varieties (Capsicum annum L.)." Iranian Journal of Field Crops Research 11(2): 226-233. doi:10.22067/gsc.v11i2.26132 (CC BY 4.0)
- Yamazaki, C. et al. (2016). "The gravity-induced re-localization of auxin efflux carrier CsPIN1 in cucumber seedlings: spaceflight experiments for immunohistochemical microscopy." npj Microgravity 2: 16030. doi:10.1038/npjmgrav.2016.30
- Nascimento, W.M. & West, S.H. (1998). "Priming and Seed Orientation Affect Seed Coat Adherence and Seedling Development of Muskmelon Transplants." HortScience 33(5): 847-848.
- NDSU Langdon Research Extension Center (2017). Hemp seeding date trial (sown at one-half inch).
- Thomas, A.L. & Costa, J.A. (1993). "Growth of soybean seedlings as affected by cotyledon shading and its reserves." Pesquisa Agropecuária Brasileira 28(8): 925-929; and Ramana et al., "Role of Cotyledons in Regulation of Physio-Biochemical Components of Soybean," Indian Journal of Agricultural Research.
- University of Minnesota Libraries, Horticulture open textbook, "Introduction to Seed Germination" (epigeal emergence and the hypocotyl hook).
Every claim in this article comes from the episode script, which is checked against these papers. Prices on Seedscanner are dated observations; this page is not growing or legal advice.
