In hypogeal germination, the cotyledons remain under the ground. In this kind of germination, while the epicotyl or stem of the plant that grows above the cotyledon, continues growing, the length of the hypocotyls or the stem that appears below the cotyledon, remains the same. Thus, the epicotyls force the plumule to rise above the ground.
Compared to the seeds those have epigeal germination, the seeds that have hypogeal germination are much bigger. In hypogeal germination, the cotyledons remain under the ground and, hence, they are much less susceptible to grazing or night-frost. According to evolutionary strategy, plants that have hypogeal germination usually produce lesser number of seeds, but all of them have a greater chance of survival.
It has been found that plants that have hypogeal germination comparatively require lesser external nutrients for growth and development. As a result, these seeds can germinate even in soils poor in nutrient content. Seeds that show hypogeal germination also require less sunlight and, hence, one can find them in the midst of dense forests, when plants compete against each other for sunlight.
In addition, plants showing hypogeal germination also have a relatively slow growth, particularly during the initial phase. They require more time to develop between two inundations in areas that are frequently flooded. Then again, plant that have emerged from hypogeal germination are relatively more resilient when there is flooding. Following the slow growth during the initial stage, these plants develop more rapidly compared to those that have grown from epigeal germination.
Lily seeds that are categorized as hypogeal germinators usually have a comparatively slower germination rate. Unlike epigeal germinators, these seeds give rise to small cotyledons that remain inside the seed coat and under the surface of the soil. The tissue emerging from the seeds is nourished inside a tiny bulb that develops at the seed's side. Following this stage, the tissue needs to be kept in a cold storage for some period prior to the emergence of the leaves above the ground. Alternatively, when the conditions are natural, such as in winter, they may be left in the ground.
Several lilies having this type of germination, for instance the Oriental species as well as their hybrids, do not produce seeds that germinate in the first year of their sowing. In fact, this appears to be their natural protective approach - always keeping some seeds in reserve in the case predators or weather obliterate the first set of bulbs. Amateur or home lily growers may want to sow the seeds in big containers with a view to allow them to remain viable for two years to complete germination of one batch of the seeds, especially if they are from any rare lily species.
Lilies that consistently show immediate hypogeal germination include Lilium dauricum and Lilium brownii var. australe. These types of lilies produce copious amounts of leaves in just one season. Therefore, seeds of these lilies can be successfully sown at the same time when you sow the seeds of lilies belonging to the epigeal germinator category. In fact, Lilium humboldtii has demonstrated this pattern of germination as well.
This pattern of germination mainly applies to the Oriental lily species as well as their numerous hybrids. The seeds of these lilies are sown in a moist medium towards the end of July or beginning of August in plastic bags. This sowing medium may comprise sphagnum peat, vermiculite or a combination of both. The level of moisture in the medium is of importance. Precisely speaking, the medium ought to be moist, but never soaked or saturated with water. You should ensure that no water drips out when you squeeze the medium in the hand. This allows good aeration, which is necessary for good germination of the seeds, in addition to the suitable development of the roots. Both these processes require adequate amounts of oxygen.
Subsequently, these plastic bags are placed in perforated boxes or trays and stored in a place away from light at a temperature ranging between 15°C and 21°C (60°F and 70°F) for incubation. Subject to the lily variety, it may take anything between three and four months for the bulblets to develop. After the formation of sound, solid bulblets as well as roots, remove the trays/ boxes to a room having a cooler temperature - about 10°C/ 50°F, and allow them to remain there undisturbed for about three weeks. Following this period, the trays/ boxes are put in cold storage having a temperature of around 1°C/ 34°F and allowed to remain there for no less than 12 weeks. This duration is called the vernalization period.
Like in the case of epigeal germinators, the pre-germinated seeds of lilies showing delayed hypogeal germination pattern are planted in the next spring. However, there is one exception. It is best to sow the hypogeal bulblets in April - about a couple of months after the seeds of epigeal germinators are sown. During this period, the temperature of the soil as well as the light conditions will be most suitable for their immediate growth. If the conditions are favourable, the minute lily bulblets will generate true leaves just after a week of sowing them.
It is worth noting here that you can also germinate the seeds of the western American lilies as well as their hybrids, Lilium monadelphum, and several other hypogeal germinators as well as their species from Europe following the procedure discussed above. However, generally, seeds of these hypogeal-germination species plus their hybrids need cooler temperatures. The ideal temperature during the initial stage of their germination is 10°C (50°F).
On the other hand, you can also sown seeds of hypogeal germinator lilies in beds, flats, trays or even in pots towards the end of July or during August in any dependable soil mix. In this case, you need to place the containers in a frame, cold greenhouse or any other site that is well protected. The seeds will germinate under normal conditions, sleep through winter and emerge in the subsequent spring. It has been found that most Western American as well as European lily species and even their hybrids respond excellently to this germination procedure.
Usually, it is never advisable that you plant the Oriental lilies, which bloom late in the season, in outdoor beds. These plants essentially require some sort of protection during their initial days. However, even planting these lilies outdoors may be successful provided you allow the seedlings to remain down for at least two years.
Harvesting of the bulblets should be undertaken only when their tops have become completely senesced. After harvesting, pack the bulblets in dried out sphagnum peat and put them in cold storage at 1°C/ 34°F temperature till you plant them in the following spring.