Monday, September 13, 2010

Bacteria and Mold

I received two emails recently regarding mold/bacteria on walls. Sam wrote:

"Hi Matthew,

My name is Sam, i'm from auckland NZ and we built a large green wall based off your video on you tube after reaching a lot of brick walls with landscape companies, so we decided to do it ourselves.

Our wall is progressing really well, the density is great, some plant types have not done so well, but almost all have survived, and thats about 1500 plants! The one issue we are having is smell. After 2 months, the felt we used has water in it of course, and has started to smell damp and musty. On invetsigataion with our hydroponics consultant he told us is was being caused by anaerobic bacteria in the felt. He advised us to use "waterclear" that is available that goes in the water system and kills the bacteria, and he has assured me that it is not only safe for the plants but it improves the oxygen available to them. So we have tried this and while not much has happened since we applied it, it is early days.

I was wondering if you had come across this or have any advice?


I replied to Sam via email and asked him how often he was watering. If his wall is going through distinct wet/dry cycles I'd think the bacteria wouldn't really be an issue. Also it seems odd to me that anaerobic bacteria would be an issue with plants in synthetic felt as it is a fairly open material. Was this wall made with synthetic felt for it's moisture retention layer? Sam if you are out there please let me know more. Also I'd love to post pictures of your wall so please send them in.

Jared wrote in about mold growing on his wall:


First off I just wanted to say I love your blog. Without it I don't know if I would have gotten my greenwall up and running nearly as well. I'm having a problem, though, that I was hoping you might know a solution for. My wall has only been up for about 4 weeks, but a little over a week ago I noticed small amounts of mold growing on the felt material I used. I've been using a vinegar/water mixture to kill it, and it works without hurting my plants, but it only kills what has already grown, and doesn't prevent further growth once the vinegar is diluted. Do you have a safe solution that might work? I was considering moving my wall so it receives direct sunlight. I heard mold has a much harder time growing with enough sun.

My wall doesn't compare to yours, but I've attached a picture in case you wanted to see. It's a self-standing wall which is why I'm able to move it when I need to. It's not completely done yet. I'm a poor grad student so I'm adding plants little by little when I can spare the money. I'll be adding another 15 today actually. In the picture I'm sending only about half is covered so far. If your interested I can send you another picture once it is complete.

I hope to hear from you soon, and thanks again for your blog. It is amazing!
-Jared Brown"

I've never had a problem with mold on my wall. I'd be curious to know about your wet/dry cycles. How often are you watering; amount and length of time. What material did you use for your felt? Let me know in the comments please.

If you have any helpful suggestions for Sam or Jared please comment. I'm no expert when it comes to mold/bacteria so any and all help would be appreciated.




  1. Hey Matt,

    I used 100% polyester felt from a local clothing store. I was watering 4 times a day for 10 minutes. Once I saw the problem I changed it to 3 times a day for 10 minutes. I've considered that because I live on the Texas coast where the humidity is always high the felt stays wet longer then a place with lower humidity thereby improving the conditions for mold growth.

    During your watering cycle does your felt become completely saturated? That is where mine is at right now. The felt will drip quite a bit about the last 4 minutes of my watering cycle.


  2. Hi Jared,

    I think you are on the right track regarding the humidity in your area. For a wall your size you may also be able to shorten the cycle so you aren't super saturating the felt. The wall should get damp but it shouldn't be soaked. Water will always drip out of the bottom regardless how much or little you water (within reason). Is the wall drying between cycles?

    Please continue to send photo updates of your progress and it would be great if you could post your approach and results to solving this problem in the comments.

    Good Luck,


  3. Hi Matt, congratulations on your blog!

    Regarding this topic, I was wondering if perhaps the mold was already there to begin with? Maybe in the roots of plants or bits of soil that went into the felt... and it only spread from there?
    Of course mold loves moisture, but I'm thinking that maybe moisture only helped spread the mold. Perhaps it would help to lightly spray plants with vinegar/water mixture when planting if you're not sure that the plants you bought are sterile.
    Only a thought...

    Would also love to see more pictures of Sam and Jared's walls and problem areas. Good luck to all, have fun growing green!


  4. In general, the more light your plants have access to, the faster it will grow and yield. The LED Grow Lights an electric lamp designed to promote plant growth by emitting an electromagnetic spectrum appropriate for photosynthesis. The emitted light spectrum is similar to that from the sun, allowing indoor growth with outdoor conditions.

  5. Alright, so it has been a few weeks since I posted my mold problem, and I'm happy to announce the problem has been resolved. Decreasing the watering cycles from 4 to 3 a day and from 10 min to 8 min each seems to have stopped any more mold growth. I was just watering my wall way too much for the amount of space (22.5 sq ft) mine takes up it seems. I haven't noticed any adverse affects on my plants from the change in water cycles. In fact they are all still doing really well. My wall has been up 6 weeks now and still going strong. Thanks for the advice everyone.


  6. Hello Matt,
    I an interior design student doing my final research now; my topic : D.I.Y green walls.
    I live on an island called Bahrain and the humidity can rise up to 90%!!! I live right by the sea ( literally a few meters away), this makes the weather around the house even more humid.
    My question for you is:
    Do you think it is still possible to install a green wall? What advice can you give me regarding the humidity?
    Thank you very much
    Your reply would be very much appreciated

  7. Hi Alya,

    Yes I think it's still possible to install a greenwall. You will need to pick the right plants that will be suited to the temperature, light, humidity/water, and nutrients you will provide. If your wall will be indoors you need to base your plant choice on your interior room conditions. There are many plants well suited to high humidity. As for mold and humidity you will need to balance your watering cycle so that your wall isn't wet all the time. The humidity will make your wall slower to dry between cycles. You may need to shorten your cycles and spread them out like Jared did (see above). Feel free to follow up with any other questions. Please document your wall building process and send me photos so I can post your wall on the blog.

    Good Luck,



  9. Hi

    Just a note on mold. I am a biologist and have been building and designing terrariums for.. a long time- which takes into account a lot of the same principles that apply to green walls.

    While mold is not that nice, it is a natural process that means that the ecosystem that you have built is establishing well. Molds are some of the first organisms that establish in an ecosystem and need to "play out" or "blossom" before seceding to other organisms in the succession of microorganisms. The molds' ecological niche is then taken over by beneficial bacteria and other fungi. So the best way is to ensure a healthy bacterial flora in the water reservoir. When you refill the water, do not empty it all, but leave at least 1/3. Otherwise the bacterial community needs to re-estalish which takes time, and that gives opportunity for new mold growth. Starting the reservoir with water from a well-established aquarium is a good source of a balanced bacterial 'primer' community.

    Most often the molds "flush up" for about 2 weeks, then subside. However, you can discourage it by saturating the water with a lot of oxygen. An aquarium oxygen pump (a "bubbler") could be placed right by the pump intake. But letting the felt dry up slightly between waterings probably works better.

    A more aggressive approach would be to add Sodium bensoate to the water (a milder fungicide). This is not recommended if you are growing edible plants though, as Sodium bensoate + Vitamin C --> Benzene.

    I try to go by the principle of finding a healthy balance, and not to try to fight the natural succession in an ecosystem. Like an aquarium or a terrarium, a green wall needs time to establish.

    Hope this helped in some way.

  10. Molds form and multiply in damp and moist areas; and green walls as beautiful as yours are a good place for them to grow in. Of course, you need to water them, so it’s impossible to keep the moisture away. It’s interesting to see that you solved the problem by merely decreasing your plants’ water cycle. Anyway, I think it’s still best to consult a specialist if you are confronted with the same problem.

    Allen Hoffman

  11. Jared how did you build that freestanding unit, I'm looking to build one and would love to understand the details. If you have drawings or a step by step that would be awesome!