Lily of the Nile is so easy to propagate by division of congested old plants that not many of us bother to grow it from seed. No one wants to leave the prominent but less than appealing seed pods out in the garden long enough to turn brown and ripen after the blue or white flowers are gone anyway. Besides, only the most basic old fashioned varieties reliably produce genetically similar seed, and even these often revert between blue and white. Yet, collecting seed for propagation is still an option for those who do not mind the risk of genetic variation.
The natural variation of flower color among seedlings of some plants can actually make gardening a bit more interesting. No one really knows if naturalized four o’ clocks will bloom white, yellow, pink or red until they actually bloom. The few types of iris that produce viable seed almost always produce seedlings with identical flowers, but oddities sometimes appear. (Most bearded iris have serious potential for genetic variation, but do not often produce viable seed.)
Cannas are likewise likely to produce seedlings that are indistinguishable from the parents. However, seedlings of many of the fancier cultivars are often variable. Their seed are very hard so should be scarified before sowing. However, I find that I get so many canna seeds that even if less than half germinate without scarification, there are too many anyway! (Scarification involves scratching or chipping the hard seed coat to promote germination. It can be as simple as rubbing the seeds on a file or sand paper, but should not be so aggressive that it damages the seed within.)
African iris are just as easy to divide as lily of the Nile are, and are as easy to grow from seed as naturalized four o’ clocks are. The difference is that they lack genetic variation, so are always indistinguishable from their parents. The only problem is that they are so easy to propagate that they can soon dominate the garden.
If seed capsules have not been groomed from the various perennials and annuals that can be grown from seed, or if they have been left out in the garden intentionally so that they can ripen, this would be a good time to collect them for their seed. (Four o’ clocks should have been collected earlier though.) Seeds from certain trees, such as silk tree, redbud and the many specie of pine, can likewise be collected. Most seeds prefer to be sown about now to chill through winter, since cold winter weather actually promotes germination when weather warms in spring. However, seeds for annuals and frost sensitive perennials, like cannas, that might germinate early and get damaged by frost, should probably wait until the end of winter to get sown.