There’s something quite satisfying about planting lily bulbs.
Sure, there are guided rules for how deep you should plant a lily bulb, but on the extreme ends of those rules, a slight knowledge of lily roots goes a long way. You should even consider ignoring the rules and experiment yourself.
In planting depth, there are usually two ends of the spectrum: too shallow or too deep. Too shallow, and the bulb roots from the stem of the lily won’t have the support. The bulb will want to anchor itself to the soil, but the stem roots will be unconfined and therefore the plant will be wobbly and even potentially die. Too deep, and the danger is that the bulb itself will go unanchored. If the base of the lily bulb is below the optimal depth, there is a good chance that the plant will die and won’t reach to the surface. In the example you gave above of a 5cm bulb, if you dig a hole that is 15cm deep, that’s already risky. From the surface of the soil, that hole actually goes 10cm deeper than the optimal depth.
In general rules, it is safe to claim that lily bulb is approximately 3x its height, however, do consider that lilies are known to adapt and do okay even if a hole is a bit shallow or deeper than the example guidelines.
Lilies each have their own set of requirements, making it a little more complicated. In most cases, Asiatic lilies will do fine with the “three times the height of the bulb” rule, while Oriental lilies (the ones with the especially big, fragrant flowers), usually need to be planted deeper. With their larger bulbs, some need to be planted deeper than 20cm. Trumpet lilies can also be planted even deeper. The Madonna lily is the odd one out. She likes to be planted more shallowly, with just 2-3 cm of soil above her nose, because in autumn she makes a little rosette of leaves that sits at ground level.
These preferences also depend on soil type. If your soil is heavy clay that holds a lot of moisture, you ought to plant a bit shallower to avoid the soil over the winter being cold and damp. If your soil is sandy and coarse, it drains quicker and you can plant deeper without the risk of rot. Heavy soil can be improved with grit or compost before planting. The reason is because it isn’t that lilies are very picky, it is that bulbs that are too wet getting squishy instead of producing flowers.
Make sure the planting holes are spacious enough so that the bulbs will fit in comfortably. A good standard is to make the holes three times wider than the bulbs. If the water does not drain well, toss in some coarse sand or grit. People plant bulbs with bonemeal because they swear by it, but honestly, it does not make that much of a difference to me if there is bonemeal or not, so do what you want.
How you want the lily bulb to look when it grows is how you determine the spacing. If you want a more natural look, plant the bulbs 15-20 cm apart. For a more formal look, plant the bulbs 30 cm apart and will give the bulbs room to grow for the first few years.
It’s true that the depth of the planting is not one of the main factors that contribute to the growth of the lilies, but it is one of the factors that is easy to do correctly. Planting too shallow may lead to the growth of weak stems that may not be able to withstand the full flowering of the plant. If the plant is too deep, then the plant could be too weak to produce full flowering due to the plant working too hard to reach the surface of the soil. You can use the three times rule to help you determine the starting point for the planting depth and then you can adjust it depending on the soil type and the kind of lily you are cultivating.
One other thing is that you should not plant the bulbs in the summer. The best time for planting the bulbs is in spring and the bulbs should be planted when they are in a dormant stage which usually occurs in the early part of the spring. If you plant the bulbs in a soil that is reasonably good, and you ensure the correct planting depth, then you should be good to go.
