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How to Get Rid of Green Algae on Seed Starting Mix

The Spruce / Ulyana Verbytska

Starting plants from seed is an excellent and budget-friendly way to create an array of plants in your garden. With indoor seed starting, all you need to do is gather light, seeds, soil, water, and food, just like when you grow plants outside. If you’re starting to grow your seeds indoors, it’s common to find a sudden plethora of green algae growing on the surface of your seed starting mix. This happens frequently when using peat-based mixes, which most gardeners use for seed starting. The good news is that it’s not likely to harm seedlings, and it’s fairly simple to prevent.

Removing Green Algae on Seed Starting Mix

To clear up any green algae growing on your seed starting mix, lightly cultivate the surface of the soil with a small tool, such as a chopstick or pencil. This simply breaks up the layer of algae. The algae itself won’t hurt your seedlings, but it could cause problems if you allow it to keep growing. For example, algae growth can get very thick over time and start to hold too much moisture near the seedling’s stem. This can cause issues like rotting.  

Green Algae Growth

Green algae is a large and informal group of algae that contains chlorophyll. These eukaryotic organisms live primarily in freshwater and come about in many forms including unicellular flagellates.

When the site is muggy, algae growth is bound to grow on your seed starting mix. You’ll know for sure when you see a bloom of pink, green, or brown sticky material moving across the surface of the soil. While it won’t kill the seed immediately, it can cause issues with your soil’s nutrients and water.  

Causes and Prevention

Over 7,000 species of green algae are found in freshwater, saltwater, and damp spaces. The cause of algae growth comes from soil that stays too moist and has a lack of air circulation.  

One way to prevent the growth of algae is to switch up your watering technique. For instance, if you’ve been watering from the top, you can try watering from the bottom instead. Simply fill the tray that your seedlings are in with about an inch of water. The water that isn’t absorbed, after about an hour, should be poured out so you don’t end up rotting your seedlings.

To help with air circulation, keep an oscillating fan on low near your seed starting racks. This will keep your plants from staying too damp and will encourage stronger growth.

Choosing a Seed Mix

Despite the algae growth commonality, peat moss mixes are an ideal base for many soilless mixes due to its hold on water and air. This light and fibrous mix can hold 15 to 20 times its weight in water alone. You can also consider a mix like composted pine bark or coir, which are renewable organic materials, unlike peat moss.

If you notice that the surface of your seed starting mix is turning green, you probably have green algae growth. Learn the cause and how to remove it.

Seeds of doubt over iron boost for algae

January 28th, 2016, by Alex Kirby

Microscopic examination of single cell algae that absorb carbon dioxide from the atmosphere.
Image: CSIRO via Wikimedia Commons

New research suggests that fertilising oceans with iron to increase the growth of algae that absorb carbon dioxide is not the hoped-for answer to reducing global warming.

LONDON, 28 January, 2016 – One keenly-argued possible way of moderating the build-up of carbon dioxide in the atmosphere may not work, scientists have concluded.

They say there is evidence that seeding the oceans with iron so that the algae that live there will multiply and devour more CO2 − thus preventing it reaching the atmosphere and intensifying the human contribution to global warming – is not as promising a solution as its supporters hope.

The extra iron can certainly stimulate the algae to grow more vigorously, but at a cost. More algae in one part of the oceans may mean there will be fewer in other areas, the researchers say.

Temperature rise

They report in Nature journal that the depths of the central Pacific Ocean contain ancient sediments that cast doubt on iron’s ability to slow the Earth’s steady temperature rise.

In parts of the oceans that lack the iron that plants need, algae are scarce. Experiments have shown that dumping iron into these areas can encourage algal growth, so large-scale fertilisation could theoretically reduce atmospheric CO2.

The seafloor sediments the team studied show that, during past ice ages, more iron-rich dust blew from cold and barren landmasses into the oceans, apparently producing more algae in these areas and, presumably, a creating natural cooling effect.

But the researchers say increased algal growth in one area can inhibit growth elsewhere, because ocean waters are always on the move, and algae also need other nutrients, such as nitrates and phosphates.

If you give them heavy doses of iron, the researchers say, the algae in one region may consume all those other nutrients, leaving the water with little to offer by the time it circulates elsewhere, so that adding iron achieves nothing.

“The basic message is, if you add to one place,
you may subtract from another”

The study’s lead author, Kassandra Costa, a doctoral student at Columbia University’s Lamont-Doherty Earth Observatory (LDEO), says: “There’s only a limited amount of total nutrients in the oceans. So if there’s greater use in one area, it seems you’d have lesser concentrations in other areas. The basic message is, if you add to one place, you may subtract from another.”

Much of the equatorial Pacific’s near-surface water comes from the Southern Ocean, where powerful winds circle Antarctica, helping to dredge large amounts of nitrates, phosphates and other nutrients from the depths where they tend to settle.

The nutrients are so abundant that the resident algae cannot use them all, and artificial fertilisation experiments have shown that adding iron there does cause more algae to grow.

Much of this nutrient-rich water eventually sinks and, in a century or two, reaches the mid-Pacific, where it meets opposing currents from the north and rises, making the nutrients available to near-surface algae. But most of these nutrients pass on by; the mid-Pacific is too far from iron-rich dust sources on land for algae to make much use of them.

Sediment analysed

In 2012, LDEO scientists took cores from the seabed in the region. Costa and her colleagues analysed sediments from the cores dating back to the last ice age, 17,000 to 26,000 years ago. They found two or three times more dust reaching the area compared with today, because of reduced plant cover in the cold, dry climate.

Marine plant growth might have been expected to have increased accordingly, but it didn’t. The sediments showed that productivity stayed the same, or even declined.

The team concluded that algae in the southerly latitudes, which were also dusted at the same time, snapped up the iron, along with most of the other nutrients, leaving the Pacific algae high and dry.

One of the study’s co-authors, Jerry McManus, LDEO professor of geochemistry, says: “This shows how different parts of the system are connected. If you push hard in one place, the system pushes back somewhere else.”

The study itself does not say so, but McManus adds that it suggests “we should be very careful about thinking we can use artificial fertilisation to combat climate change”. – Climate News Network

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About Alex Kirby

Alex Kirby is a former BBC journalist and environment correspondent. He now works with universities, charities and international agencies to improve their media skills, and with journalists in the developing world keen to specialise in environmental reporting.

Seeds of doubt over iron boost for algae January 28th, 2016, by Alex Kirby Microscopic examination of single cell algae that absorb carbon dioxide from the atmosphere. Image: CSIRO via