Managing the Water Garden in Spring

The spring season is traditionally the time for introducing new aquatic plants to the pool. They then have all summer to become established, and if planted at this time will usually provide a display the first season.

Not only are there replacements to be made for occasional losses sustained during the winter, but a number of subjects will need dividing and replanting if they are to retain their vigour and keep within bounds. I like to lift and divide marginal plants in alternate years. However, it is not desirable to divide all the plants at one time as this leads to what one might call fat years and lean years — the newly replanted material looking rather sparse early on and then the entire pool appearing overcrowded. It is much more sensible to lift and divide half of the plants each year.

Waterlilies do not need attention so often. The third year of their establishment being an appropriate time to divide vigorous varieties, while some of the more restrained kinds will last for four or five years without attention. The need for division will be apparent in any case if the plants make a preponderance of small leafy growth in the centre of the clump. Especially if this is accompanied by diminishing flower size.

Submerged plants can often be left for a number of years without attention, although the stringy winter growth of species like Lagarosiphon major should be removed each spring to allow fresh growth to break from the bottom. If a basket of oxygenating subjects is not growing very well it is a good idea to shake out the soil and replant healthy young cuttings in fresh compost.

When dividing marginal plants treat them like herbaceous perennials. Tough rootstocks being separated by inserting two hand forks back to back and levering the plant apart. I like to replant material from the outer edge of the clump as this is full of the vigour of youth.

Waterlilies can be treated similarly, except that in most cases they will need to be separated with a knife and, therefore, care should be taken to see that any wounds are dressed with powdered charcoal to prevent infection.

On lifting a healthy mature waterlily, the rootstock will be seen to consist of a main rhizome, which was the one originally planted, together with a number of side branches. It is these side growths that should be retained as they are young and vigorous, the original rootstock being discarded. Each severed branch will produce a plant providing that it has a healthy terminal shoot.

It may be appropriate at this juncture to briefly discuss the propagation of aquatic plants. Although few gardeners will envisage growing more than one or two kinds, it is useful to know how they are increased.

By and large marginal plants are increased by division, although some of the more vigorous kinds like Mentha aquatica and Veronica beccabunga are easily reproduced from short stem cuttings pushed into a pot of wet mud. Butomus umbellatus produces large quantities of tiny bulbils along its rootstock. These can be removed and planted in a seed tray of wet mud until large enough to reintroduce to the pool.

Seed finds a place in the propagation of aquatics, but it must be sown fresh and in most cases not allowed to dry out. The seed of Pontederia cordata must be sown when still green if it is to germinate satisfactorily, while that of Aponogeton distachyus should only briefly leave the water. Calla, lysichitum and orontium seed must be well ripened before sowing and preferably put in before the winter, while that of Myosotis scorpioides can be sown at any time.

Waterlilies do not set seed freely and apart from Nymphaea tetragona and the closely allied Nymphaea pygmaea alba, are not normally increased in this manner. These two are quite readily reproduced this way, although once again the seed should not be allowed to dry out. Sown in pans of wet mud and covered with a little water they germinate freely, but are vulnerable to the choking growth of filamentous algae. So they need constant attention. As they grow stronger they can be potted and stood in an aquarium or bucket of water until large enough to be safely planted out.

This is not the most usual method of propagation, for waterlilies are commonly increased from eyes in the spring. These eyes occur with varying frequency along the rhizomes of mature plants. In some cases they are latent and look like those of a potato tuber, whereas others appear as smaller versions of the main growing point. Yet a third kind, common in Nymphaea tuberosa and its varieties, takes the form of brittle rounded nodules.

All are treated in the same manner irrespective of their appearance, being removed from the rootstock with a sharp knife. The wounds on both eye and rhizome being dusted with powdered charcoal. The eyes are potted into small pots in a heavy loam soil and placed in a shallow container with water just over their rims. They are then stood in a greenhouse or on the windowsill.

After three or four weeks, growth will have started and the tiny leaf stems will be lengthening. At this time the water level should be gradually raised until the leaves are floating with their stems fully extended. As further growth is made the level should be raised and the plants progressively potted until sufficiently robust to withstand the rigours of pond life.

Replanting and propagating are normal spring activities, but an equally important, and yet often overlooked, task is feeding. Once planted, most gardeners tend to think that aquatic plants will look after themselves. While this is broadly speaking true, far better results can be obtained by the judicious use of fertilisers. But applying fertilisers is not as easy as it sounds where a pool is concerned. Obviously one must consider the livestock, and it is well known that many fertilisers are toxic to fish and snails. Even ‘safe’ fertilisers must be used with care, for the majority will drastically increase the level of mineral salts in the water and thus create a dense green algal bloom.

Fortunately, manufacturers have come up with a system which curtails both of these problems, for it is now possible to buy small perforated plastic sachets of slow release fertiliser which can be pushed into the soil beside a waterlily or marginal plant and provide it with nourishment. The more traditional method of achieving the same end is by making what are known as bonemeal ‘pills’. These consist of balls of wet clay soil which are mixed with coarse bonemeal and then pushed into the compost beside each aquatic plant. The frequency with which this needs to be done is variable and depends upon the condition of the individual plant. However, I would think that the annual treatment of most subjects would be sufficient.

03. August 2011 by Dave Pinkney
Categories: Aquatic Plants, Water Features, Water Garden Plants | Tags: , , | Comments Off on Managing the Water Garden in Spring

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