ALGAE—especially tenacious filamentous string algaes or blanket weed—can quickly turn a water garden into a battleground. I have been enjoying, and managing, two in-ground, rubber-lined garden pools for more than 20 years, and you know what? It’s not that hard, despite the sometimes-tenacious, gooey green stuff. And most important: There is no other feature of the garden that brings more joy—or sustains more wildlife, from birds to dragonflies, salamanders to frogs–than a pool or pond. My essential spring water-garden care tips:
Reduce debris (organic matter such as leaves on the bottom of the pool), which adds nutrients to the water as it decays and can thereby “feed” algae growth. I always do this just as soon as the ice on the pool allows—typically in March—using a net, then turn the plumbing back on. (Details on filtration below.) I make another pass with the net and my hands in April once the perennial water plants start to sprout so I can see which parts are alive and dead (removing the latter, but being careful not to disturb masses of frog and salamander eggs). That’s a pile of dead plants and leaves at poolside scooped out last weekend, in the top photo, and a spotted salamander below in my gloved hand). I discard the smelly, mucky debris around shrubs; they seem to enjoy the treat.
Excess nutrients can also enter ponds in the form of fertilizer runoff. Are you feeding flowerpots beside a water garden, or is the lawn adjacent (and being fertilized)?
Think like the English, and add barley straw to your pond—whether actual straw stuffed into a floating sachet-like net bag, or one of the newer extract products (pellets, powders–both seen below–and even liquids). Barley straw helps make the water inhospitable for algae. The bagged straw sachets last about six months, the other products a shorter time.
Non-toxic additives: The last two years I have been experimenting with other non-toxic and/or biological additives (liquids, powders, sometimes formulated in combination with barley extract as mentioned; photo below). Depending on the formulas, they may help settle particles out of the water onto the bottom, and/or add enzymes and “helpful” bacteria, and I am favorably impressed, I think. According to label directions, these are added periodically through the season.
Changing the water, by the way, doesn’t work, and can actually backfire. Work to improve what you have, consistently keeping a vigilant eye out and adjusting things throughout the season.
Aerating the water (with a small spillway or waterfall, for instance) can help fight some species of algae, but not all, but is preferable to stagnant water for various other reasons—and who doesn’t love the sound of moving water?
Filter, the biological way. I have pumps and biological filters for both my pools (one in the water in my smaller pool; one external unit for the larger). Inside biological filters, a community of helpful organisms including bacteria builds up on the filter “medium” (often made of foam or what looks like packing peanuts) and helps digest the unwanted pond wastes. Highly recommended. My units have each lasted many years, though every so often foam inserts may need to be replaced.
UV “clarifiers” can help, too. Some pond filters incorporate the use of ultraviolet lightbulbs. When the water passes by these “clarifiers,” or “sterilizers,” as they are called, algae is prevented. Independent clarifying devices—not housed within the filter box—are also available at pond-supply vendors. I seem to manage without these, but in a large, sunny pool I think I might rely on one.
Critical: Shade the water (at least one-third of the surface should be shaded, some experts say). Ponds in full sun will be most challenging to manage, and algae will romp. Shading can be accomplished by (surprise!) siting your pond in part shade, or planting shrubbery or grasses or other shade-casting things nearby. Most important and easier:
Floating water plants will shade the pond surface (and some also help with overall water health besides blocking light that would encourage algae). They are essential. I love tiny Azolla, for instance, but you might prefer big, bold water lilies, or towering pots of large-leaved elephant ears. Note: Never put water-garden plants into natural waterways, lakes, ponds, etc.
One more shading tactic: non-toxic black water dye (sold as a powder or liquid; the powder seems far more concentrated), which I also like because it helps hide the submersed plumbing in my small pools. The dye lasts a few weeks or a month, then needs to be reapplied.
If floating foamy-looking or stringy algae does occur, reduce it promptly. You can use a net, but I do it with my hand, moving in a swirling motion to gather it into clumps. Squeamish? Use a bamboo cane in the same motion.
Net the surface in fall, or not? Experts advise placing a net over the pond in fall to prevent debris from re-entering the pool, but my frogs, birds and other pond visitors and residents would strongly object (and even potentially be injured trying to navigate such netting).
Speaking of the other end of the season: Follow my fall pond-care steps, which do not include water changes and scrubbing the liner. They do include keeping a hole open in the ice that might form on the surface, to prevent buildup of gasses under the surface (and also welcome wildlife 365 days a year).