Raising Aquatic Plants from Seed

As with many land plants, a considerable number of aquatic plants can also be raised from seed. For the most part, these are natural species, although strains of one or two subjects, such as iris and mimulus, can also be increased in this way to advantage. Named cultivars rarely come true and must be increased vegetatively.

 

WATERLILIES

Although there is currently a trend towards growing some of the large-flowered waterlilies from seed, their progeny will always differ from the parents, being mostly inferior and rather difficult to grow to flowering size. Recent developments in techniques and knowledge of the behaviour of a select group of hardy waterlilies has spawned this unfortunate trend, for rarely will the expectations of the pool owner be realized. For a high-quality, large-flowered waterlily, vegetatively propagated stock is essential.

The only waterlilies that justify raising from seed, and indeed, apart from occasional division, cannot be increased in any other way, are Nymphaea ‘Pygmaea Alba’ and its progenitor, Nymphaea tetragona. Nymphaea ‘Pygmaea Alba’ is a very small cultivar that, after flowering, often produces greenish-white fruits, which are filled with viable seeds. If gathered and sown immediately, they ripen and germinate very freely. The trick is to gather the seed when it has developed sufficiently and before it is distributed in the water. To ensure that all the seeds are captured from the rupturing fruits, place a tiny muslin bag over each developing pod.

Waterlily seeds are very small and embedded in a sticky gelatinous material. If you try to separate the jelly from the seeds, damage can occur. It is far better to sow both the seeds and jelly together, spreading the sticky mess as evenly as possible over the surface of the compost. For waterlilies, a good soil-based compost is preferable to any of the wide range of soil-less types that are available. Spread the seed over the surface of the compost, using a pair of tweezers, then lightly cover them with more compost. Spray the pots of seeds gently from overhead with water to settle the compost. Then stand the pots in a warm light position in a bowl or aquarium, making sure that the water level is just above the level of the compost.

Seedlings should start to appear after three weeks. They will have translucent, more or less lance shaped leaves that, at first glance, look as if they could be an aquatic liverwort. At this stage, they are very vulnerable to becoming entangled in fine filamentous algae. At the first sign of trouble, administer one of the reputable algae controls. If algae is allowed to accumulate, then killed, it must be removed promptly so that it does not begin to rot and damage the seedlings.

As the waterlily seedlings develop, the water level should be raised very slightly. As soon as the first two or three floating leaves come to the surface, the plants can be pricked out. They should be lifted in small clumps, washed thoroughly to remove any compost, then gently teased apart. Prick the seedlings out into a seed tray or pan, then immerse it in a container of water so that the water level is approximately 2.5cm (1in) above the surface of the compost. As the floating leaves of the seedlings grow and develop, the water level can be raised gradually. After a season, the seedlings will be large enough to plant out in their permanent positions.

 

SOWING SEEDS

1. Always use a good clean seed compost and thoroughly soak before sowing seeds. Spread the seeds sparingly over the surface of the compost. Crowded seedlings are subject to disease.

2. Cover the seeds with a thin layer of compost or sand. The use of sand is particularly beneficial for waterlily seeds and helps to discourage the proliferation of troublesome algae.

3. Stand the pot in a container of water. For waterlilies fill the container so that the water level is just above that of the compost. With bog and marginal plants it should be just below compost level.

 

OTHER AQUATICS AND BOG PLANTS

Seed is the readiest means of raising many of the most popular marginal aquatics and the vast majority of bog garden plants. For the most part, it should be remembered that only the species will come true from seed, although there are notable exceptions, such as the Primula japonica cultivars ‘Postford White’ and ‘Miller’s Crimson’. While the majority of marginal and bog plants can be increased by vegetative means, raising from seed is generally the most reliable and economic method, especially when more than two or three plants are required.

Almost without exception, marginal subjects and deep-water aquatics, such as the water hawthorn (Aponogeton distachyos), should be sown when the seed is fresh and sometimes still green. Indeed, seed of the water hawthorn should not be allowed to leave the water until ready for sowing, otherwise it will perish.

Sow the seed of true aquatics in a wet, heavy loam compost, although plants that will have floating leaves, such as water hawthorn, appreciate a few centimetres of water over their crowns once through the soil. Initially, it is inadvisable to submerge freshly sown seeds, as they are often light and will float right out of the compost. A thin layer of silver sand will help hold them down and also assist in preventing the rapid development of algae on the surface of the compost. Once the seeds germinate and the seedlings have become established, they can be pricked out and treated rather like young waterlily plants.

Bog garden plants are dealt with in a similar manner, although some gardeners raise young plants in the open ground. However, this can be fraught with difficulty, for the seed is often fine, the soil conditions likely to be quite hostile, and watering difficult to control. Raising plants in seed trays or pans, within the protection of a cold frame, is much easier to manage, and the quality of plants produced is almost inevitably better.

The seed of most bog garden plants is best sown fresh, which is fine if you can collect your own, but a bit of a problem if you have to depend upon the garden centre. Collected seed is usually sown during mid to late summer, while packeted seed has to wait until the spring. In some cases, seed that is not fresh requires chilling to break its dormancy; this is quite common with primulas.

Sow the seed in a good soil-less seed compost in clean pans, in the same manner as when fresh. Then place the containers in the freezer, allowing them to remain there for a couple of weeks before bringing them out into the light and warmth. Germination will follow quite rapidly. Once the seeds of bog garden plants have germinated and the seedlings have been pricked out, they should be grown on in individual pots for the remainder of the season, ready for planting out in the following spring.