Challenging conventional wisdom: Sowing Depth for Chilli Seeds

I like to challenge the accepted truths on the internet. Let's face it, if nobody challenges them, how do we know they're right?
Usually I'm wrong, and the "conventional wisdom" is right. But so what? Occasionally I find something to be a myth, and that's why I do this.

So, this post is about sowing depth. The internet says:

- "2 to 3mm"
- "don't sow too deeply"
- "three times the diameter of the seed"
- "5mm deep"

These have all come from either reputable seed suppliers, or influential blogs. Now that's a range between vague and bizarre. Three times the diameter of the seed, for some seeds that could be over a centimetre, which is a lot deeper than 2 to 3 mm!

I suspect that this vast range of accepted sowing depths is due to one simple reason: It doesn't really matter. If you think about it, a chilli seed is a pretty big thing in the seed world compared to lettuce or radish. Some can be almost as large as apple pips. So they probably have enough energy to pop up from well over a centimeter under the soil.

And let's consider nature. Who eats chillis in the wild? The animals who can't taste the heat [pretty much humans are the only ones who get a kick from eating things we're not supposed to!]. Birds can't taste capsaicin, so there's your number one seed distributor right there. What do birds do? Well, they don't bury the seeds at three times the diameter, unless of course you're talking about in "guanu". Blech.

In a related test, all of these seeds were simply scattered on the soil, not buried at any depth, and produced a 100% germination rate. Note you can see more of the root (the white furry bit) but otherwise they're thriving!

So, I believe if I test this over a range between:
- dropping them on the soil uncovered and
- burying them 1.5inches underground
I would expect all of them to grow. But if I had to guess which would fail, my money is that the deeper ones would be less reliable, or at least, weaker when they hit the surface.

So let's test it.

Thanks to my super chilli plant of 2013, I have a LOT of identical and reliable seeds at my disposal. These things sprout if you look at them the wrong way. Germination time is around 4 days too, so this should make a fairly short experiment, a week should do it!

Obviously this is entirely unscientific, and not intended to be a controlled experiment. It's just an attempt to get some answers, given the vast range of advice out there.


Let the experiment begin

Left to Right: 1) Dropped on top of soil, 2) Buried 0.5cm, 3) Buried 1/2 inch, 4) Buried 1.5 inch

So now we kick back and wait for them to germinate.

Update 1:

As you can see, three have popped up pretty quickly. The fourth one is the deepest, the 1.5 inch (I've rotated the tray, it's the reverse of the photos above.)

What's interesting is that in spite of the differences in sowing depth, all the seedlings are pretty much the same height above the soil at the same time. I can't explain this, but that's science for you!

I predict we'll see the fourth one in the next day or two anyway.

Update 2:

All plants were transplanted to pots simultaneously and were all fine. It seems that sowing depth has little or no effect on chilli plants.

Bear in mind that while I tried to conduct this experiment in a controlled and fair manner, it was not scientific or statistically significant.

Caveat:

I have learned that there may be one reason to sow seeds deeper, although it's hearsay at this point. If you've ever had a large number of seeds suffer from a condition known in the growing community as "helmet head", you'll know that while it often resolves, sometimes a seedling can die. It's a problem where the seedling gets its first leaves stuck inside the original seed pod. If it can't get these out, it can't photosynthesize or grow and it will die. 

I've heard that burying seeds reduces the chance of this happening. However, personally Ive found it not to happen often at all, maybe only one seedling in 50, perhaps because I grow in a high humidity environment of a heated propagator which softens the seed pods.