Has your pond
or water feature gone to pot? Then read our complete guide to cleaning
Over the past few years
the popularity of garden landscaping and design has encouraged many
of us to install water features in our gardens. They look fabulous
initially, but over time they can turn into slimy, mosquito-infested
bogs. Green water algae (or pea soup algae as it's also called),
together with slime algae and blanketweed, are some of the many
frustrations of pond and water feature owners.
Algae are like the aquatic
version of garden weeds, if you like. And like other weeds, they
need light and nutrients to survive. If the surface of the water
is constantly exposed to sunlight, algae will thrive. And if there's
an abundance of nutrients in the water and not enough plants to
compete for those nutrients, algae will be thanking you for that
The simplest way
to discourage green water algae is to shade the surface of your
pond with floating plants and water lilies. The more plants
you have the better. As a rule of thumb, you should be aiming
for a leaf coverage of about 75%.
submerged plants will also help to keep down the algae. Oxygenators
are leafy, soft plants with small roots; they depend on their
leaves for the absorption of nutrients, which are also required
by algae. In the competition for nutrients, the oxygenators
generally win and gradually starve out the algae. One of the
best oxygenators to use is Hygrophylla polysperma .
Another very good one is Hygrophylla diformis , which
is actually used overseas in sewage water purification plants.
There are algaecides
on the market that will clear your water (such as Algae Fix).
These are OK to use with fish, so long as you use the correct
dosage. (Overdose, and your fish could die.)
For organic control,
Bioactive Algaway and Pond Zyme are a couple of good products
to use. Bioactive Algaway contains a natural plant agent that
helps break down the cell walls of algae. Pond Zyme consists
of either bottled or powdered bacteria which consume the organic
wastes and nutrients in the water, thereby starving out the
algae. Both of these, however, must be used on a regular basis
- that is, fortnightly. Whereas chemical algaecides will kill
algae in three days, these organic products will take six to
eight weeks before you start getting a result.
If you want to absolutely guarantee clear water, use a UV clarifier.
What happens is, water passes through a sleeve which contains
a UV bulb that kills off the algae.
The blanketweed algae
is a lot more persistent and difficult to control. It's a filamentous
algae that looks like green cottonwool and its spores are spread
via the wind. Blanketweed is a particular problem with concrete
water features and water features that have river boulders in them,
because it likes a high pH, and those things create a high pH.
To control blanketweed
you need to manage the pH, maintaining it around neutral (between
6.5 and 7.5). There are products available to do that, such
as Pond Balance.
There are also
algaecides that will kill it, but that's not going to solve
your problem long-term. It will give you short-term relief,
but if you haven't changed the water conditions, it will just
start regrowing again once the algaecide wears off.
Partial water changes
is something else pond/water feature keepers can do regularly
throughout the year, scooping up any blanketweed or dead leaves
and plant debris as you go. About 30% of your pond's water should
be removed every six to 10 weeks. Simply grab a bucket and drain,
then top up with the hose. Or here's a great idea why
not dunk the watering can into your pond every now and then
and use that water on your garden. Then, every now and then,
just top up your pond.
are ideal for water features because they're the best means of controlling
bugs and pests (such as mosquito larvae and water lily aphids).
Every now and then you just need to give your plants a blast with
the hose so the bugs drop into the water for the fish to eat. If
you notice mosquito larvae in your pond, stop feeding your fish
for a while. Hungry fish make the best mosquito predators.
By the way, mosquitoes
hate moving water, so if you keep the water moving (with a fountain
or waterfall), you won't have a problem. What they do like is shallow,
stagnant water. So the steeper the pond's sides, the less sheltered
area available for mosquito larvae to live. The egg-laying females
will seek out the sheltered spots even in the top of planter
pots that are half submerged in water, or the shallow water in between
rocks. Both these places provide an area for mosquito larvae to
escape from fish.
If you don't have moving
water, here's a clever trick to discourage mosquitoes add
a few tablespoons of vegetable oil to the water. Mosquito larvae
breathe through air tubes that link to the water's surface; the
oil prevents them from obtaining oxygen by forming a barrier at
the surface. But a word of caution, if you have fish, do not use
this method you shouldn't need to, anyway. The oil will inhibit
gas exchange (that's the giving off of CO2 and the taking in of
oxygen) at the surface and as a result suffocate the fish.
If you've got fish,
they will need some special attention coming into winter. Being
cold blooded, their metabolism rate is governed by the temperature
of the water. In the winter, they're slowing down. When the water
temperature reaches about 10 degrees around May it's
important to feed them a winter food. A winter fish food is lower
in protein and higher in digestibles, and is usually based on wheat
germ. The reason you feed them that through winter is that, with
the lower temperatures, their digestive system doesn't work as well
and you start to get partially digested food in the stomach. When
spring comes and the weather warms up, that starts to ferment and
they get health problems. When the water temperature gets down to
about 5 degrees, stop feeding altogether. In Auckland, that's unlikely
to happen, but in Invercargill, for example, definitely.
Fish can handle the cold
weather no problem. They'll just go to the bottom of the pond and
remain inactive until the weather warms up. If you're down south
and your pond freezes over, simply cut a hole in the ice to allow
for gas exchange. Never smash it with a hammer or the shockwaves
may kill your fish.
One word of advice
for those of you in the deep south who use a UV clarifier for the
control of green water algae, dismantle it and bring it inside for
the winter. If the water that's inside them freezes, it can rupture
the glass sleeve that's inside.
You may have noticed
that many ponds experience a sudden flush of algae in early spring.
The problem is, your plants, especially water lilies, will either
die down in winter or become dormant. That in itself isn't so bad,
because the algae will also die down or become dormant as light
conditions drop. But come spring, the algae will often get a head
start, growing a lot faster than the plants. That's where a winter
clean-up is essential.
If there's a lot of dead
leaves and fish droppings sitting at the bottom of your pond, rotting
and releasing nutrients into the water over winter, the algae is
going to feed on those nutrients come spring and get a good head
start. Over winter clean out any residual blanketweed, leaves or
plant debris to reduce the nutrients in your pond.
Keep an eye on the pH,
too. As mentioned, the ideal pH for pond water is between 6.5 and
There's one last thing
to consider if you own a water feature. Once a year you should undertake
a complete clean-out. June, July, August is the perfect time for
that because most plants will be dormant, and you'll be heading
into spring with a clean pond.
Take the whole lot out.
Drain it, clean it, remove all the sludge and debris from the bottom,
and divide your plants. Hardy water lilies can be placed back into
the clean water, but tropical water lilies should be lifted and
stored in moist sand over winter.
Good old elbow grease
is all you need. Never use any detergents. A really good scrub with
a stiff brush is enough for most people, but if you've got a really
bad case of sludge, you can waterblast it.
Winter's also a good
time to spray neighbouring fruit trees for aquatic pests. While
water lily aphids disappear from your pond during winter, they will
overwinter in nearby fruit trees, especially cherries and plums.
Spray with a winter oil to reduce these little nasties next season.
And if you've done all that and want to introduce some colour into
your pond over winter, consider the water hawthorne (Aponogeton
distachyos). This plant has oval leaves and vanilla-scented
flowers that bloom well into winter. Feed fortnightly with a water
lily fertiliser. Apart from its favourable winter-flowering appeal,
it will grow in deep water and in moving water, where water lilies
Thanks to Jansen's
Pet & Aquatic Centre.
All products mentioned
in above article can be purchased at
Jansen's Pet & Aquatic Centre.
with permission from NZOOM Home and Garden content,
from the previous
The views expressed here are not necessarily those of the RNZIH