The general rule of thumb for planting bulbs is to plant two to three times as deep as the bulbs is tall.
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But what do you do with lily bulbs that come in various sizes that grow quite large after planting such as tiger lily that would grow even without any soil on top of the bulbils?

I don't always follow the rule of thumb for these reasons. :-D My bulb planting depth depends on the space available and whether there are any unseen bulbs in the space. And since I planted bulbs in any space I had, chances are there is something there already. So I carefully insert the tip of a trowel, wiggle it a bit and bury the lily bulb. As a result, nowadays, my lilies are not planted as deep as the rule of thumb suggests. :-D

From my experience, the height and girth of a tiger lily depends more on the spacing than its planting depth. But if space allows, I don't see why you should not plant the bulbs according to the recommended depth. Make sure you give each bulb enough space for it to grow to its potential.

Out of all the discussions I see about Tiger Lily planting, I have yet to see a single one that takes int consideration that you have to accommodate for the root system that grows above the bulb. Why is that?

Do all lilies have this double-root system?
It looks as if I have to place them 6" down to allow for this but I have yet to see this depth mentioned.

I usually prepare the soil to 12" down for these.
Add some bonemeal then some compost.

I am going to venture to say that the second root system will grow where it needs to be and the deeper it can be, the taller the stalk can grow. Shallow planted bulbs grow shorter stalks.
Has anyone else seen this?

The looser the soil, the deeper the bulb (6 inches in friable soil).

Good news: lily roots pull the bulb to optimal depth. I have read lilies are the only bulb that has this capability.

Quick search on retractable roots didn't pull up info I wanted. You might have better luck.

Double tiger lilies were my only lily (have Asiatics and Orientals) to produce bulbils along the stems. And if I remember correctly, it was only with skimpy water that stem roots appeared.

As I understand it, all bulbs produce offspring towards the preferred depth. Over time, they seek the best growing depth.

P.S. Why would you fry soil?

Friable simply means easily crumbled - or, if you weren't being facetious - workable.

Stem roots appearing above the bulb and sometimes showing at ground surface are pretty common and doesn't hurt the plant at all. If one is digging and resetting/transplanting, 5-6" deep is a good depth and should keep stem roots below the surface if you feel this is necessary.

I've never known a tiger lily bulb yet that didn't work it's way down to where it wants to be - even the little bulbils that fall on the surface naturally around the mother plant do this. Been growing them for years and tend to let them come up where ever they will since I have the luxury of lots of space. Crowded or not crowded, mine tend to be 3-4' tall

Yes, I know "friable", I was just having fun.

I did know some one who barbecued it once to get sterile soil. He was not allowed to use the oven anymore since he used it to pulverize clam shells and stunk up the house for days.....