algae in terracotta pots

andersons21March 2, 2010

Hi everyone, I just stumbled upon this forum recently, and I'm excited about everything I've learned. Thanks to all who contribute such great information!

I have 7 empty 20" Italian terracotta pots on my deck. They have been empty for a couple years, but before that, roses struggled to survive in them. I'm hoping that this time, using the gritty mix, the right fertilizer/nutrients, and vinegar to compensate for minerals in the water will eliminate a lot of the problems I had before.

Just recently, with the rains we got, a bunch of green algae (I assume) appeared in the inside of the empty pots. I want to plant roses in the pots soon, like this month. What, if anything, should I do about the algae? I was thinking of trying to kill it and otherwise disinfect the pots with a bleach solution, but I have not figured out a way to keep liquid in the pots for very long.


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tapla (mid-MI z5b-6a)

No need to do anything about the algae (it could be moss, too). It's not growing in standing water? - there are drain holes in these containers - yes?


    Bookmark   March 2, 2010 at 2:19PM
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There's no standing water in the terracotta pots. Just a lot of green stuff on the inside walls. There is a large drain hole, and the pots are elevated on feet. Water drains out fast. I tried plugging the holes with foil, but a filled pot still drained out completely in a couple minutes.

    Bookmark   March 3, 2010 at 12:49AM
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greenman28 NorCal 7b/8a

I'll second that you needn't worry.
A few of my terracotta pots are developing a fine green sheen, particularly on their northern sides... ;)


    Bookmark   March 3, 2010 at 9:33AM
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calistoga_al ca 15 usda 9

Many gardeners encourage the growth on their pots, hoping to achieve a look of a very old container. Al

    Bookmark   March 3, 2010 at 9:45AM
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Mine do that all summer, and actually looks beautiful to me...Some of them got so thick with it, the pots got slimey...Especially the ones hidden behind the tall grass..My pots almost faded into the nice green grass.:-)


    Bookmark   March 3, 2010 at 11:54AM
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I don't care for that slimy feel, but the "make the pot invisible" look described by Mike sounds like an aesthetic achievement to me.

    Bookmark   March 3, 2010 at 2:52PM
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I love the look of old pots with moss on the outside, but this unfortunately does not look like that. It looks like the nasty slime that grows in a shower, but greener. Every time I look at it, I feel the urge to clean it out, but on the other hand, I have a bad back. The ugly stuff won't be seen when the pots are planted, and if it won't hurt the plants, for the sake of my poor back I should refrain from cleaning it out.

    Bookmark   March 8, 2010 at 10:28PM
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This article was in the Sunday paper, thought it was pretty interesting:

Like many objects of value, terra-cotta pots take on character as they age. The clay darkens, assuming a whitish cast from fertilizers and the minerals in water. When kept in the shade and watered frequently, the pots gradually acquire a verdant sheen of algae or moss. But you don't have to wait for that look. These six easy techniques help pots undergo a transformation within weeks. Start now and you'll enjoy their vintage charm this summer and for many seasons to come.


Accelerate the appearance of white deposits by filling the pot with a highly concentrated fertilizer solution for a few weeks. Pots aged this way are safe for plants because the salts won't wash from the pot to the soil.

Tools and materials: wine cork, candle and water-soluble fertilizer


Plug pot's drainage hole with a wine cork. (A standard cork will fit a 10-inch pot perfectly. For smaller pots, whittle the cork; for larger ones, slice additional corks to fit, and wedge in place.)

Light candle. Let wax drip over cork on outside of pot to seal. Let cool.

Fill pot with water. (Hard water accelerates aging.)

Add five times more fertilizer than package directions recommend. The longer the pots sit, the more dramatic the effect. Remove water, wax and cork.


One of the most natural-looking patinas can be achieved by simply slathering plain yogurt on a new pot. Yogurt applied to dry pots yields more dramatic results. For a subtler look, first soak pots in water for 15 minutes.

Tools and materials: plain yogurt and a 2-inch foam brush


Stir yogurt.

Use brush to coat surface of pot with yogurt, covering it completely.

Set aside in a shaded place until pot achieves the desired look, at least one month.


Combining buttermilk and moss to encourage moss growth is a common tactic. The moss serves to hold the runny buttermilk in place and vary the texture, as well as to promote growth.

Tools and materials: moss (or sheet moss), buttermilk and 2-inch foam brush


If you've gathered your own moss, remove as much soil as possible. Tear moss into small pieces, removing materials such as bark and pine needles.

Pour buttermilk into a bowl, add moss, and combine.

Use brush to paint the mixture over pot.

Set aside in a shaded place until pot achieves the desired look.

If necessary, use a metal-bristle brush to remove any heavy clumps of moss.


It's easy to make a pot appear as if it had been unearthed in an archaeological dig. Just apply soil found in your backyard. Moist soils with high clay content are ideal, since they adhere to terra-cotta best.

Tools and materials: clay soil and flexible wire brush


Rub soil over surface of pot, moistening the soil with a little water if it doesn't stick.

Place pot in a shaded area for at least one month while soil bonds.

Brush pot to create a varied, textured surface.


This method provides instant gratification. The lime solution quickly tones down the harsh orange of many new pots.

Tools and materials: hydrated lime (available at hardware stores), natural-bristle paintbrush, spray bottle and 150-grit sandpaper


Dissolve 1 cup hydrated lime in 2 cups water, stirring until no clumps remain. (This amount will age several small pots or 2 large ones. You can make varying quantities of the solution, but always use 1 part lime to 2 parts water.)

Using random strokes, brush pot with lime solution, applying thickly in some areas, and thinly in others to simulate the subtle streaks of old pots.

Fill spray bottle with water, set it on the "stream" setting, and coat pot in various spots while lime is still wet. This thins the coating for a natural look. Let dry.

Sand pot in random directions, wiping dust frequently, until you have achieved the desired look.


Sometimes, the simplest methods bring the most satisfying results. Soak a pot in a tub of water until algae grows on its surface. Algae grows best in the sun, so be sure that vessels sit in bright locations and that water is replenished as it evaporates.


Although each technique will yield unique results, a few common truths apply to the various methods.

Ingredients: It's fine to use dairy products that aren't fresh or have expired. Low-fat products will work, but higher-fat versions tend to be thicker and therefore less likely to drip off.

Application: To achieve an authentic appearance, vary the thickness of the materials and the direction of application. Look to true aged pots for inspiration.

Storage sites: Shaded locations are ideal for most pots while they "age." Do not stack the pots. Spray them occasionally with water, or place them where rain can reach them. Pots coated with food products may smell strongly for a few days after the ingredients have been applied; keep them away from living areas.

Waiting: The longer a pot sits, the more pronounced the effect will be. It's up to you to decide when you think it's ready. Most pots will continue to "age" even as they are being used. Be creative. Try combining methods for different effects.

Here is a link that might be useful: OC Register

    Bookmark   March 9, 2010 at 7:51PM
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tapla (mid-MI z5b-6a)
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