Why do different vendors suggest such different sowing depths?

Or better yet, what are the factors that might encourage you to sow peas (for example) deeper or more shallowly than some one else would?

I'm thinking about simple annuals and crops, not fussy perennials like:
"sow on surface" (needs light)
"barely cover"
"cover thinly"
" sow 1/6th inch deep"

For example, different seed packets or seed catalogs gave the varying sowing depths suggested below. The range for any given crop is a factor of 3 to 4!
Peas: ½" , ¾", 1" 1 ½"
Bok Choy: or ¼" to ½" deep OR ½" to ¾" deep
Lettuce: ⅛" deep, ¼" , ½"
Swiss Chard: ¼", ½", ¾"

I understand why peas are planted deeper than lettuce: lettuce e seeds & embryos are tiny and lack the resources to launch a 2" stem thick enough to push aside lots of soil. The pea root is so big it might NEED a weight of soil so it doesn't push the whole pea right to the surface.

But I see advice e to sow lettuce an where from ⅛ to ½ inch deep, and pea vendors suggest anything from ½" to 1 ½".

I'm guessing that soil type might be a factor, but this is pure guessing:
1. maybe plant shallower in heavy clay because it tends to crust and seeds can't break through
2. maybe plant deeper in dry climates & sandy soil so they don't dry out as fast
3. maybe plant shallower in cold soil because the seedling has less vigor for pushing to the surface
4. maybe plant deeper when day-night swings are large so the seedling doesn't chill at night
5. maybe plant pathogens in the soil vary widely so that some regions need to plant very shallowly to get the seedling's top out of the soil ASAP.

6 Maybe different vendors and different gardeners just have different opinions for no good reason, Yet I would think that farmers and market growers would care A LOT about what works better and then "everyone would know the answer" instead of such a huge range of advice.

7. Maybe different cultivars of the same crop have very different depth requirements and I just missed that while compiling notes.

If 1 or 2 are the reason, I would guess that anyone with a misting irrigation system could sow very shallowly because the surface layers would never be too dry or too wet.

If 4 were the reason, a floating row cover would allow much shallower sowing.

Sometimes, I think, also determined by whether they germinate best in light or dark. Though, in practice, that doesn't always seem to matter much with most seeds.

Karen

kqcrna said:Sometimes, I think, also determined by whether they germinate best in light or dark. Though, in practice, that doesn't always seem to matter much with most seeds.

Karen

Most packages for salpiglossis say to surface sow and cover or place in darkness until germination. I have found covering them with soil is more successful.

Exactly. But I have to say, with wintersowing, my seeds tend to migrate to the surface anyway. After being buried in feet of snow, then snow melt, monsoon spring rain, repeated freezing and thawing, the seeds almost always seem to be visible on the soil surface by spring germination time. But they do sprout most of the time.

Trudi's reasoning for that is "for every day, there's also a night". I can't argue with that logic.

Karen

This is a question I've wondered about myself. Clearly the larger the seed the deeper the sowing depth .. I'm thinking Rick may be onto something about germinating seeds pushing themselves out of the soil. I also think, and this is pure speculation, that perhaps it might have to do with the tap root growing deep enough and anchoring what turns out to be top heavy plants.

Another though is perhaps depth might be related seed coat type .. clearly those with harder seed coats would require more time for the coat to soften and crack so planting at a deeper depth gives more time for the coat to soften and also leech out any germination inhibitors in the seed coat.

>> those with harder seed coats would require more time for the coat to soften and crack

After losing 90% of my first 2-3 sowings of snow peas every year, I read a suggestion to Deno-spout peas on paper towels! Then plant the sprouted peas as soon as a few millimeters of root have emerged. I'm going to try that with early snow peas and snap peas this year.

Maybe it would also be a smart way to start "scratch and soak" seeds like lupins.

I have a healthy respect for microorganisms. It's often a close race between seeds rotting and rooting.

I just sowed a few sugar & spice sweet peas this morning (needed some examples & pictures for a workshop)
I had nicked them last Sunday and rolled them in a damp paper towel and then put it into a plastic bag. They were ready to sow today, so 5 days. I held the seed with tweezers & lightly cut into it with a exacto(shipping) knife.