Nothing sucks the fun out of indoor gardening quicker than pests and molds.  And, once you make the horrific discovery that your indoor garden or greenhouse has been infiltrated by spider mites or mildew spores, your tranquil Shangri-La is suddenly transformed into a hellish battlefield.  We’ve come across countless indoor gardeners who, after repeated infestations of some persistent pest or problem, have dismantled their entire set-up, scrubbed and bleached all surfaces, set everything up once more and WHAM, the enemy strikes again within days or weeks!  Arrrrrrrgh!  Mamma! Make it stop!  It’s enough to make a grown man cry!

So is it any wonder that our ears pricked up when we heard that there might be a new super weapon for us indoor gardeners to use in our fight against pests and molds?  What might this be?  UV-C?  Are we serious?

Ok, so UV-C’s germicidal properties have been known since the 1930s but it’s remained an unfamiliar technology to most indoor gardeners.  And now with new UV-C T5 lamps becoming commercially available, could UV-C technology be used safely within our indoor gardens as a room sterilization tool, part of an ongoing preventative routine, or even as a reactive treatment for bugs and molds on the plants themselves?

If you put a UV-C lamp anywhere near your plants and leave it on for a few hours, the leaves will soon appear to be damaged / scorched.  Again, I repeat, UV-C kills stuff!  So what place, you may well ask, does UV-C light have in an indoor garden?  We don’t want to kill our plants!  We want them to grow and thrive!  I understand your concerns, Urban Gardeners, but please give me time!  Despite being a relatively old technology, this particular application of UV-C is right on the cutting edge.  I’ve also got a very scary looking lawyer stood right behind me advising me to type these warnings in big, bold font.

The Basics

Okay, for those of you who aren’t so familiar with UV, let alone UV-C, here’s a quick run through the basics.  Ultraviolet light (UV) occurs naturally from the Sun.  UV has a wavelength that is just outside of our visible range.  We tend to refer to light that is visible to our naked eye as various “colors.”  The lowest wavelength color we can see is “violet”, hence the name for light with a wavelength just lower than this is “ultraviolet.”

Now it turns out that the term “UV” refers to a relatively broad spectrum of light – anything from 100 nanometers to 400.  So UV has been further divided into UV-A, UV-B, UV-C and UV-V.  The part we are interested here is UV-C.  It’s the section of UV between 185 and 280 nanometers – also known as “short wave ultraviolet radiation”. UV-C rays have the highest energy and are arguably the most dangerous part of UV light.  (Although some would counter that UV-B is more dangerous as it causes skin cancer.)  Solar radiation in the UV-C range is absorbed almost entirely by the atmosphere.  Artificial UV-C lamps have been shown to be super effective in the laboratory at destroying bacteria, mold, viruses and certain plant pests as well as other biological contaminants in air, liquids, or on solid surfaces.

UV in Nature

UV is Mother Nature’s blanket method of controlling pests and pathogenic microorganisms.  Crops grown in greenhouses (which filter out UV) and indoors under high pressure sodium lights (which emit virtually no UV) have tended to be more susceptible to pests and pathogenic fungi.  Higher humidity levels inside greenhouses and indoor gardens can also promote pathogenic fungi such as Phytophthora and Botrytis – serious pathogen families which can decimate crops.  And until now most growers have resorted to using expensive fungicides to combat these problems.  However, there is an increasing groundswell of public opinion against the use of these products, especially when used on crops intended for human consumption.

UV-C destroys a whole host of undesirables – from viruses, bacteria, mold, and mildew to plant pests like spider mites. UV-C rays are able to penetrate the outer membrane of microbes (e.g. algae, bacteria, mold or viruses) and stop them from reproducing.  The same is true of many plant pests (and their eggs) – the smaller the plant pest, the more susceptible they are to UV-C.  The specific wavelength of 253.7 nanometers is known to break the DNA of pathogens and smaller plant pests so that they are unable to reproduce.

But isn’t UV-C damaging to plant tissues too?  The short answer is yes!  But the same is true of Hydrogen Peroxide, Nitrogen, Phosphorus and Potassium when applied at levels that are too high!  The key question is:

Can precise doses of UV-C be used to treat plants directly in order to eradicate or control pathogens and plant pests without doing harm to the plants themselves?

If so, the net effect of UV-C treatment could be fewer pests and increased yields – because if your plants aren’t using energy fighting off pests and pathogens then they can put that energy back into growth and bloom.

Intellectual Property

In May 2007, two Dutch inventors, Arne Aiking and Frank Verheijen, were granted an International Patent on a method of treating live plants and mushrooms against pathogens and pests with UV-C light.  (International Patent Number: WO 2007/049962 A1.)  In the past UV-C had only been used to sterilize things like air and water.  Typically the germicidal effects of UV-C were achieved through the heuristic of “overkill”.  Use triple the amount of UV-C you think you need and you will definitely kill whatever it is you want to kill.  The water or air still remains perfectly intact after sterilization.  The difference, of course, with proposing to use UV-C to fight pathogens and pests on living plants is that you shouldn’t use any more than is necessary, otherwise there is a risk to health of the plant.

While the general method of using UV-C to kill pests and molds is now public knowledge through the World Intellectual Property Organization, the owners of this and associated intellectual property are keeping the details very close to their chests.  For instance, in the aforementioned patent, the application of UV-C is only broadly described:

“It has been found that amounts of UV-C light between 0.0025 and 0.15 J/cm2 during a period of 24 hours enables not to induce any, or at least not to induce plant tissue damage which has a negative effect on growth and yield of the plants while still having an anti-pathogenic effect, i.e. controlling pathogen growth.”

What does that mean in plain English?

Aiking and Verheijen's UV-C lamp would travel up and down water heating pipes like these in between the plants.

Well, it’s probably a good idea to look at the practicalities. Aiking and Verheijen’s invention is a mobile UV-C lamp that travels up and down the water heating pipes you commonly sees in commercial greenhouses.  This lamp periodically travels through the crop, dosing the plants either side with UV-C light.

How much UV-C light?  Well, again, specifics like these appear to be pretty closely guarded secrets right now. The emission or light intensity of a UV-C germicidal light bulb is usually expressed in a term called “microwatts per square centimeter” (μw/cm2) not J/cm2 (Joules per square centimeter.)  Aiking and Verheijen appear to be suggesting a range of between 2,500 and 150,000 microwatts of UV-C energy over a given 24 hour period.

But then the plot thickens when the patent describes the UV-C lamps to be used in the invention:

“UV-C lamp intensity of between 2 and 100 Watts with an effective exposure period of between one second and one minute and a proximity to the pathogen growth of between 2 cm and 200 cm.”

Hang on a second.  That’s quite a range of variables there!  Let’s just break those down:

If we take the lower end first:  We can safely estimate that a 2 Watt UV-C lamp will output approximately 1000 microwatts of energy over a square centimeter, in one second, from a distance of two centimeters away.  Remember, the inverse square law applies to all artificial lighting sources.  At 150 cm it’s less than 1 microwatt.  At 200 cm, it’s barely anything at all.

A 100 Watt UV-C lamp, on the other hand, will output approximately 14,000 microwatts of energy over the same area, in one second, from a distance of two centimeters away.  If we leave it there for one minute (the upper limit of the duration range specified in the patent) we have to multiply that figure by 60!  840,000 microwatts!

I guess, if we’re to make any modicum of sense of this huge range of numbers, we need some data on how much UV-C light is required to effectively kill various things.

For instance, tests have shown that powdery mildew is killed when given a dose of 1720 microwatts of UV-C per square centimeter. So, if I took the aforementioned 100 Watt UV-C lamp and positioned it two centimeters away from the mildew, I would need to switch on the UV-C lamp for just 1/10 of a second to kill it.  This is calculated by taking the effective dose rate (1720) and dividing it by the amount of microwatts reaching the target (14,000).  At ten centimeters away only about 3,600 microwatts of UV-C is delivered to the target, so about half a second’s exposure is needed.

Spider mites could possibly also be effectively treated with UV-C but with amounts that are hundreds of times more compared to something like powdery mildew.  Lower doses of UV-C may be able to control the increase of this bug, but it would be difficult to kill off large populations that have already established themselves.

As for as the effect of UV-C on plant tissue, another patent indicates that if the UV-C dose is under 200,000 microwatts, leaf damage was not observed.  This appears to be in the same region as the limits proposed by Aiking and Verheijen, however, at this point, these shouldn’t be treated as firm numbers.  Certainly the safest way to use UV-C on plants appears to be regular, smaller doses rather than a single, large hit.

POSSIBLE APPLICATIONS

Room Sterilization Between Crops

UV-C lamps could be used for sterilization of small growth chambers before plants are introduced.  Of course, the grower should still clean the chamber in the regular way using a mild bleach solution and a sponge, but then it would be quite feasible, as an added precaution, to expose the growth chamber to UV-C light to deactivate any residual pathogens, insects or eggs that might still be lurking in the growth chamber.

What about a UV-C Sterilization Chamber?

One possible application of UV-C could be to create a ‘cleaning chamber’.

A small ‘clone-box’ sized light-proof grow tent could be fitted with  UV-C lamps hanging from the top, and banks of UV-C lamps fixed on each side panel of the grow tent.  Plants could be placed inside the chamber when the UV-C lamps are switched off, the grow tent is zipped up and light-proofed, and the UV-C lamps are switched on for a precise amount of time to kill / control the pest or pathogen while remaining under the range where plant tissue is damaged.  Remember, when it comes to UV, plant tissue is hardier than your skin!

Certainly, small plants would present less of a challenge than larger ones, simply because there are fewer places to hide.  A key consideration when using UV-C for pest and pathogen control is crop density, which makes a dedicated UV-C chamber a more attractive possibility.  Of course, plants would have to be mobile (e.g. grown in pots) and the UV-C lamps would still need to be arrayed in such a way that there was a sufficient spread of UV-C energy hitting all parts of the plant.  Remember, the inverse-square law of indoor lighting intensity also applies to the germicidal properties of UV-C in that they decrease exponentially the further an object is from the artificial UV-C source.

A dedicated UV-C grow tent would also be very useful to anybody wishing to research the levels of UV-C exposure that healthy plants can tolerate without affecting growth and yield through comparison with a control plant that does not receive any UV-C.  It should also be noted that younger plant tissue can tolerate less UV-C without sustaining visible damage.

UV-C Within the Indoor Garden?

UV-C applied every day for short periods of time could keep some pests and pathogens under control, and help to sterilize the surrounding air.

For UV-C to be used within an indoor garden, banks of UV-C lights would need to be arrayed so that all plants were being hit fairly evenly from top to bottom with UV-C energy.  This would obviously involve placing UV-C lights in between plants at even intervals.  The UV-C lamps would then be switched on for a fixed amount of time each day (we’re talking seconds or minutes depending on the wattage of the UV-C lamps.)

An important note:  the target bug or pathogen must be hit directly with the UV-C rays in order to be affected.  If it is shaded / protected by a leaf, for example, the UV-C will not be effective.  UV-C will not penetrate through leaves so it would be really important to get the UV-C to the underside of the leaves too.  This could be achieved by lower lamp placement at the level of the soil or growth media.  If there is also air movement, the leaves of the plants will move and more leaf surface will be exposed to the UV-C.

If UV-C is going to be used in an indoor garden it would be advisable to incorporate some sort of “kill switch” that automatically turned off the UV-C lamps if the grower inadvertently entered the room while the UV-C lamps were operating.  Just to reiterate (again!) …. YOU DO NOT WANT TO BE ANYWHERE NEAR A UV-C LAMP FOR ANY AMOUNT OF TIME WHEN IT IS SWITCHED ON.

Research is Required

The Japanese corporation, Matsushita (parent company of Panasonic), has years of data logging detailing how to apply UV to control various species.  But they too are keeping a tight grip on that data.  There are many combination of factors for a private researcher to consider: (Strength / distance of UV dose amount, time duration of UV dose, location of fixture and angles, continuous dose or intermittent dose, time in between doses, before or after germination, before or after fruiting, type of use such as disinfection, pretreatment, etc.)  And of course, safety mechanisms are a big part too.  Matsushita have developed a complete method based on their extensive research and applied it in real use with success.

However, once again, this is all Matsushita’s intellectual property.

What we do know about UV-C is that it controls smaller pests and pathogens very effectively when used correctly within the right parameters.  Furthermore we can see no reason why experienced growers who understand and appreciate how to use UV-C safely would not be able to determine their particular “sweet spot” for indoor garden pest and pathogen control and thus reduce their reliance on chemical insecticides.

We realize that using UV-C on plants is a highly controversial topic – so give us your opinion!  Do you think this is a step too far in the battle against pests?  Let us know: post a comment.

WORDS: Everest Fernandez