Plants that are hard to grow in garden ponds

Most garden ponds are constructed using some sort of hard surface, usually a pond liner, concrete or a preformed pond. This is a very different environment to the wild, as any substrate will be shallow. For some plants this isn’t a problem, but there do seem to be some plants that are difficult to grow – generally these are the ones that like to grow on deep silt. I’d be interested to hear from anyone who has grown any of the following in a garden pond long-term (I haven’t).

– Water plantain Alisma plantago-aquatica
– Arrowhead Sagittaria sagittifolia
– Water starworts Callitriche spp.
– Flowering rush Butomus umbellatus

Water violet Hottonia palustris also seems to like rather deep sediment, though for me the problem is always that it gets eaten by water snails. Any thoughts on how to keep this beautiful native going are most welcome.

Small (or smaller) pondweed

My photo shows some shoots of one of the fine-leaved pondweeds (it’s either small pondweed Potamogeton berchtoldii or least pondweed Potamogeton pusillus). It’s the thing with the long narrow leaves. I’m quite surprised to see it come up here on the edge of the crowfoot bed. These species both do best in disturbed muddy habitats such as ditches or stock watering ponds and so are not ideal candidates for garden ponds. I had a small patch struggling along at the margin for quite a while, but it was killed off by the drought – or so I thought. It now seems that it has spread either by seed or turions and has been stimulated to grow by rising water levels. [Turions are the resting bodies of waterplants]. Jeremy Biggs has got it growing well in his Oxford pond, but that’s quite early succession. I wonder if it needs a certain amount of disturbance – such as was provided by the drought – to keep going?

Small pondweed and least pondweed are two very similar species that differ in some minor botanical details. One day I’ll get around to identifying it properly.

The Drought Ends

We’ve had the driest start to the year since 1929 in Britain, and Wales has been no exception. We’ve had very little serious rain (by which I mean a week or so of wet weather) since the snow melted in January, and in mid June the spring that feeds our water supply dried up. A combination of this and the World Cup has distracted me from posting for a while. Since July started the weather has broken though, and we’ve had a couple of weeks of changeable weather including some good downpours.

The pond had been feeling the heat too. Water levels dropped steadily over the drought and by mid June levels were approaching a 6″ below the winter maximum. The photograph on the left gives an impression of this (though it’s surprisingly hard to take a good photo of this sort of thing). Water levels were getting so low that the upper ‘shelf’ of the pond was starting to become exposed, and one corner of the pond was dry altogether. Unlike many people, I didn’t have the option of turning on the hosepipe (and even if I did I don’t think I would have used it) so the pond just had to sit and wait.

It’s interesting to see what’s been happening during the dry period. The frogs seem to have been using the pond a lot at night. There have been absolutely no water quality problems such as algal blooms, and in fact the blanket weed may well have declined. Since there are so many different pondweeds in the water that’s probably not surprising – although low water levels concentrate the nutrients in the water, they also concentrate the aquatic plants which will be using them up. More significant is likely to be low oxygen levels, and any sensitive species such as mayflies may have struggled.

Since the recent rain, water levels have risen quite markedly. This picture shows the same piece of the pond, only shot from the other side (the stone at the bottom of this one is in the middle of the other picture). Much of the bare slate waste has now been covered by water. This change in water levels seems to provide an opportunity for plants and animals. Although the water level has risen, the vegetation hasn’t, so there is now a shallow layer of warm water above the whole pond. Most of the plants are growing upwards towards the new water surface but this process is likely to take a couple of weeks. In the meantime water beetles and the few remaining toad tadpoles are making use of the open water. The plants are growing back though, and it’s fascinating to see how the aquatics are switching from their stunted terrestrial growth forms to longer underwater shoots.

This photo shows this phenomenon for water milfoil. The yellow-green bases to the stems are the terrestrial sections; the bushy red growth is the new growth since it has rained.

Finally this week (and nothing to do with the pond), the dry weather has been very good for butterflies and moths. We’ve seen several species we don’t normally get including Small Pearl-bordered fritillary, but the highlight were four of this amazing caterpillar. It’s an Emperor Moth, Britain’s only native silkmoth. This one was eating my Tayberries but I don’t begrudge it a single leaf!

In Praise of Pondweeds

Ok, so here we go with the plants. And yes, I admit it, I really like pondweeds. Now, I hear you say, I’d already worked that out as this thing is about ponds. Of course you like pondweed. But I don’t just mean any old pond plant – I mean proper pondweeds, Potamogeton.

There are 21 different species of Potamogeton native to Britain. They interbreed with great frequency and thoroughness, so a further 27 hybrids have been found, and no doubt a few more remain to be discovered. How many of these are in general cultivation? Well, according to the 2006-7 edition of the Plant Finder (the most recent copy I have) there are two. Why this is I have really no idea. However, it is certainly true that the water garden industry generally grows a rather small and conservative range of plants, especially submerged species (or ‘oxygenators’ as they are misleadingly sold – in fact ponds do not depend on submerged plants for their oxygen). Potamogetons don’t seem to have come onto the radar. I think that’s a pity, because these plants have a quiet beauty, are great for wildlife and add a range of textures to the garden pond.

From a horticultural point of view, pondweeds come in three basic flavours: floating-leaved, submerged broad-leaved, and submerged fine-leaved. The floating-leaved and broad-leaved types both grow from perennial rhizomes (roots) that creep across the bottom of the pond and send up stems every so often. On the other hand, the fine-leaved types die back in winter to clusters of unattached resting buds called turions that disperse around the pond. It’s therefore probably best to think of most of the fine-leaved types as ‘annuals’ (there are a couple of exceptions).

Broad-leaved pondweed growing wild in a pond in South Wales

The floating-leaved group contains just three species, but one of these, the Broad-leaved pondweed Potamogeton natans, is very common and will be familiar to many people, even if they don’t know its name. Here’s a nice patch of it in a pond in South Wales.

Broad-leaved pondweed has a number of merits. It is easy to grow, it has attractive oval leaves that are usually a reddish colour, and it provides shade and a good resting place for damselflies to lay their eggs. However, it is also a vigorous plant that is liable to take over small ponds, including most garden ponds. Unless you are happy to have the surface covered, I can’t really recommend it for any but the largest pond, or possibly ornamental lakes.

Fortunately there are plenty of other species to choose from. I think my favourite, and so far the one that has thrived most in my garden pond, is the Various-leaved pondweed, Potamogeton gramineus. This is quite an uncommon species that naturally grows in rather shallow water in ponds, ditches and lake margins where conditions are not too nutrient-rich.

Various-leaved pondweed growing as the submerged form

Various-leaved pondweed (who comes up with these names?) has transparent submerged green or brown leaves with a prominent white midrib. Later in the summer, or when crowded by other plants it produces a crop of floating leaves that resemble a smaller version of P. natans. In the three years since I’ve planted it, it has romped happily around the pond; however it doesn’t have a dense habit and won’t shade out other species. I haven’t searched but I expect that the broad flattish leaves are ideal for newts to lay eggs in. In deeper water it seems to produce somewhat narrower leaves.

Floating leaf of Potamogeton gramineus

Like most of the broad-leaved pondweeds, Potamogeton gramineus is easy to grow. You need to obtain a piece of the plant with some rhizome attached to it (the bigger the piece of rhizome the better) and not too much leaf. Weight it down to the bottom of the pond with a stone or similar object (you can use a garden gnome if you wish!) so that the rhizome is in contact with some sand, fine gravel or whatever rooting material you have. Then just let it do its thing.

Now at this point I feel the need to mention two of my bugbears regarding pond plants. The first one is those funny little plastic mesh baskets that water gardens like you to buy to put plants in. Why is this considered attractive? On land containers tend to be an option of last resort when you don’t have space for a border, but for some reason water gardeners convince themselves that pond plants need to be confined. Now it’s true that pond plants tend to be quite vigorous, but unfortunately most of them are also quite capable of escaping the confines of a container after a few years. Much better to dispense with these environmentally unfriendly eyesores and just plant direct into the substrate.

My other bugbear is the perception (especially for ‘oxygenators’) that you can just throw a bunch of plants into the water and they will establish. Now, for the restricted range of these plants widely available to gardeners – things like Canadian pondweed or curly waterweed – this happens to be true, because they are so invasive that they will establish pretty much regardless of how they are treated. But the majority of plants – including native submerged species – need to be properly planted with the roots attached to the substrate. After all, you don’t plant your lettuces out by just throwing the plants on the surface of the soil, do you?

The final pondweed I’m going to write about today is the shining pondweed Potamogeton lucens. This one is really very attractive and is a beautiful contrast to the normally rather fine-leaved plants growing in ponds such as water milfoils. It has large, broad, translucent leaves that are much larger than various-leaved pondweed (up to 25cm long and 6.5cm wide, though in my pond they are more like 12 x 3cm).

Shining pondweed, Potamogeton lucens

The leaves are translucent green and have a netted appearance, and the new stems have a lushness that gives an almost tropical feel. In the wild this plant prefers still or slow-flowing, calcareous water and usually grows in at least 1m or so depth. Here in Wales this is a rare plant but it’s reasonably common in south-east England where suitable habitat is more frequent.

Although it prefers deep water in the wild, I planted my pieces in water only 10cm or so deep, from where it is spreading steadily into deeper water. Like most of the broad-leaved pondweeds it takes a year or so to settle down and start growing strongly in the pond. For me this species is reason enough to have an area of deeper water in the pond (50cm or more deep) where it can grow happily. According to one reference I have the stems of this species can reach 6m, so depth should not be a problem!

I have seen shining pondweed in cultivation in Holland, growing in a small pond in spite of the attentions of goldfish and a strong green colour to the water. If you can get hold of some I can recommend it. It should be planted in the same way as P. gramineus.

Here be Dragons…

The first things to arrive in a new pond are always the insects. If you build the pond during the warmer months, pond skaters will find it and land pretty much as soon as you start to fill it. They find ponds by looking for reflective surfaces, but I still think it’s pretty amazing as you don’t see them flying around. At least I never have. I was much more excited by the whirligigs though – those amazing silvery beetles that cruise around the pond and when disturbed swim rapidly round and round in circles. There are quite a few species in Britain and I’ve never attempted to ID ours. When I first started the pond there were up to 30 at a time – I counted them – but now there are only 5 or 6 – they need open water to thrive.

The most spectacular species though, were the dragonflies – or more specifically the broad-bodied chaser, Libellula depressa. This is a common enough dragonfly, but its’ behaviour never fails to entertain. The males are bright blue and highly territorial. In my case the pond is only big enough to support a single territory, but in the event it is apparently desirable real estate and hotly contested. One male takes up the dominant station, a patch of sedge at the pond margin, whilst a further three or four others position themselves in strategic locations in low bushes near the pond. The arrival of a female is the signal for frenetic aerial dogfights as all the males, with loud clattering of wings, pursue her in a line so that the result is a bit like a dragonfly version of the Red Arrows. The successful male grasps the female behind the head, mates with her in the heart shape that dragonflies make when mating, and then, still holding her so that other males cannot also mate with her, flies in tandem while she lays batches of eggs by dipping her abdomen repeatedly into the water. If you look carefully you can see the tiny eggs as they sink (broad bodied chasers are pretty fearless, so this is generally fairly easy with a little stealth).

Azure damselflies Coenagrion puella laying eggs. The blue male guards and holds onto the greener female.

Sadly don’t get many other dragonflies up here – it’s relatively exposed. There are a few damselflies – mainly large red damselfly Pyrrhosoma nymphula and azure damselfly Coenagrion puella. There’s also the odd other species of dragonfly, notably hawkers (Aeshna spp. – we’ve had common and migrant). There are also golden-ringed dragonfly Cordulegaster boltonii, the huge black and yellow species that is pretty common in the uplands. It’s really a stream species and I’ve found the larvae in our local stream, but last year I found a female laying eggs in the muddy margins of the pond. I don’t know how they did, but this summer I’ll have a look for the larvae. As far as I know there is no record of them breeding in standing waters.

I’ve not seen too many dragonflies yet this year, but it’s relatively early still and the cold spring will have delayed emergence. The garden has quite a number of young trees now so hopefully these will provide a more sheltered microclimate for these beautiful insects.

About the Pond

Hi there. Inspired by another pond blog, I’ve decided to write my own, to share the joys and pitfalls of having a garden wildlife pond.

I live with my wife and little girl in the top left hand corner of Wales [ (c) Oliver Postgate], in a cottage overlooking Anglesey. We live in a wind-swept location in ffridd country – that zone where field systems become patchy and give way to moorland. It’s an exposed location and gets its fair share of wind, as well as snow and ice. Not your typical location for a garden pond then!

Why did I want a pond? Well, I have always loved ponds and pond dipping, and used to regularly drag my parents off to various watery places to look for animals. My mother was a biology teacher and we had a microscope at home, so as well as seeing the standard tadpoles, newts and invertebrates, I got to see some of the smaller stuff – nematode worms, the little ‘freshwater sea anemone’ Hydra, rotifers, protozoans and larger algae such as the beautiful Volvox, which looks a bit like a slowly rotating bright green, partially transparent orange. I’m also a keen gardener and I’m always very frustrated by the relative narrowness of ‘water gardening’. Gardening is such a diverse thing with so many different styles and plants available, yet when it comes to water gardening the approach is very stereotyped, with the ideal seeming to be a cross between one of Monet’s waterlily paintings and a koi carp pond, fringed with large drifts of bog primulas. This is supposed to be created by planting everything in neat little plastic baskets. Now I’ve nothing against water lilies or bog primulas and I’m sure many people get a lot of pleasure from koi (I doubt that plastic baskets really give anyone any pleasure). But is that really all there is?

I wanted to create something using native pond plants. Aha, I hear you cry, nothing original about that, lots of people plant ponds with native plants. Well, up to a point. About ten years ago I started working in nature conservation, and had to learn the various native water plants for my job. The vast majority of these – including many species highly suitable for growing in ponds – are simply not available in cultivation. Most of the native ‘pond plants’ that you can get are not really pond plants at all – they’re marginals that like growing with their feet wet, things like kingcup (Caltha palustris) or ragged robin (Lychnis flos-cuculi). Why are so many native species ignored – are people not aware of them, are they difficult to grow or are we just so set in our ways that we don’t try anything different?

We constructed the pond in 2007. It’s a standard pond liner sort of affair, about 70cm deep in the middle and about 4m x 3m across. The shape is rather uneven partly because I like it that way and partly because the ground is very stony (of which more in a minute). The hole was lined with old carpet, followed by the liner, followed by quite a large quantity of low-nutrient substrate – mainly some old builders sand that was knocking about, some slate waste, and a couple of bags of commercially available aquatic compost to provide the small amounts of nutrients that are needed. With hindsight maybe the slate waste was not such a great idea as it has sharp edges and could puncture the liner. So far I’ve been lucky though. Slate waste does have the advantage that it’s cheap and very readily available in these parts.

Constructing the pond proved a greater challenge than I expected. I had known that the garden soil was stony and shallow from my experience planting shrubs and trees (lets just say that a crowbar is an invaluable garden implement). After digging for a time I hit a large stone, which we only managed to move with the assistance of my Marine brother-in-law.

The big boulder - you can also see the surrounding heathland pretty well.

The following weekend I started digging again and almost immediately hit an even larger boulder, which could only be removed with the assistance of the local farmer and his tractor. We opened up the sheep-proof fence that surrounds the garden, fixed a chain around the boulder and towed it onto the heath.

Since the ground is sloping, I used the spoil and assorted rocks dug from the hole to build up a sort of dam / earth bank that looks quite similar to one or two of the smaller dams on the reservoirs in these parts. If I’m feeling pretentious, I can pretend it’s a homage to a glacial moraine. The result looked pretty bare for a while but now it’s planted up (and to a certain extent been allowed to revert) it looks better. I don’t think it will ever look really natural though. Here’s how it looked in February 2008:

The pond in February 2008, complete with rock dam

After filling the pond, it went through the usual new pond phases. First the water turned brown as all the sediment from the sand and slate waste became suspended in the water column. This steadily changed to a green soup as microscopic algae grew in the pond and used up the nutrients in the water column. However, this then cleared fairly rapidly as populations of waterfleas and other animals built up to eat the plants. The pond was almost immediately colonised by pond skaters and whirligigs, and soon afterwards by the local palmate newt population and to my consternation, by a great diving beetle. These enormous beetles are ferocious predators capable of eating virtually anything else in a pond smaller than them, and surpassed only in appetite by their larvae, which will also eat animals larger than their own size. If you are wondering how they do this, they inject digestive juices into their prey and then suck up the resulting soup through their jaws. There is a gruesome account in King Solomon’s Ring by Konrad Lorenz of two approximately even-sized diving beetle larvae grabbing and killing each other simultaneously in a deathly embrace.

After some soul-searching (and fishing the diving beetle out once) I decided to let nature take its course. She laid eggs and there were a few larvae around that summer, but they didn’t seem to establish and I haven’t seen an adult or a larva for quite some time. Diving beetles were not the only predators to arrive at the pond relatively rapidly, but that’s for another time.