Ask the Professor – Prev. Questions 1

Dear Professor Hydro-

My students and I am building a hydroponic system for our classroom. It is going to be a perlite/vermiculite mix. My question is this, what pH level should the nutrient solution be for growing tomatoes? How about Lettuce? Is there a happy medium that will work for all plants?

Liz  B.


We are always glad to here from a teacher. Most vegetable plants seem to like a pH of around 6.0 (slightly acid) to 7.0 (neutral). This of course depends on the type of plant you are growing. Perlite and Vermiculite mix is pH neutral so mix your nutrient solution to 6.5 pH and you should have no problems with tomatoes or lettuce. NOTE: Most popular hydroponic growing media is pH neutral, except Rockwool, which is alkaline. You would normally mix your nutrient solution to 5.0 to 5.5 pH if you are using rockwool. This balances out the higher pH of the Rockwool, so the root zone is balanced out to be around pH of 6.5.



How much can you “fudge” on the fertilizer strength? I have been giving my plants about 1 1/2 times the recommended strength on the package. I grow in Rockwool, does that change how much nutrient I can use?

My plants were doing great, but lately I have noticed that some of the leaves are getting brown at the tips and some are beginning to curl under. I have checked them for bugs and found nothing. I think I might have a fungus and my wife thinks that it is caused by over fertilization.

Jeff P.


I think you might have a smart wife. Sounds like over fertilization, maybe 1 1/2 times recommended strength is too high. As Mom likes to say “just because a little is good, it doesn’t mean that a whole lot is great, in fact it could be deadly”.

Look for salt deposits on your Rockwool, it’s a sure sign of over fertilization. Flush the Rockwool with slightly warm water to remove the salt build up. Drain the reservoir and run the system with plain pH adjusted water for a few days. Refill the reservoir with nutrient solution mixed to recommended strength.

Some plants can take higher fertilizer strengths than others, but you should do your homework before you go messing with the recommended nutrient levels.

It doesn’t matter matter what the growing medium is, too much fertilizer is too much fertilizer.



Can I use mirror in my grow room to increase the light? Is there anything better?

Fred S.


Reflecting light back towards your plants is always a good idea, however, using mirror is not the best method of doing this. Mirrors are not actually all that reflective, I have heard that mirrors are only about 80% to 85% reflective, also mirrors do not defuse light which can cause “hotspots”. Flat white paint is actually more reflective than mirror and it defuses the light very well.

There are several products on the market that are specifically designed to go in a grow room. The most popular is probably Mylar which is about 95% to 98% reflective. Mylar comes on rolls of 4 feet x 25, 50 or 100 foot  in 1 or 2 mil thickness.



I have been taken cuttings a few times over the last couple of months and have had very good success, the clones have rooted up in about a week and a half or so. But the last batch I did took much longer, they still rooted, but they took over three weeks. What gives? I did everything the same as the other times. Does taking repeated cuttings from a “mother plant” make them root slower? Any ideas?



Taking repeated cuttings from a mother would have no effect on the speed of rooting. I believe that perhaps your problem may be temperature related. This is a common problem. The cuttings like to be kept between 72°F and 80°F., if the temperature increases or decreases the cutting will root slower.



What are the disadvantages of hydroponics?



As much as I hate to admit it, there are a few disadvantages to growing with hydroponics.

The most obvious disadvantage of hydroponic gardening is the initial cost. Some hydroponic systems are very expensive. However once the initial investment is made the cost of running and maintaining a good system can actually be lower than dirt gardening (especially if you take labor into account). Which means that in the long run it may actually be cheaper to garden hydroponically. There are many ways to grow hydroponically that are practically free, and building a system yourself can bring the price down to less than that of a good shovel, rake and hoe.

Some hobby hydroponic systems are very susceptible to power outages. All hydroponic systems would be affected by very long term outages, (commercial growers usually have back-up generators). Hobby gardeners might have to hand water their systems in the event of a long term power outage. I personally have a 12 volt bilge pump (from a boat) and car battery to use as an emergency back-up (luckily I’ve never had to use it).

There are many people that think organically grown produce tastes and smells better than hydroponically grown. This is debatable and is obviously a matter of opinion. Although it is difficult (but not impossible) to have a totally “organic” hydroponic system, a “hybrid” system works very well. There are many great 100% organic nutrients that can be added to the normal nutrient solution in the hydroponic system. This seems to give the gardener the best of both worlds. The tremendous plant growth and high nutrient value of the crop supplied by the hydroponic nutrients, and the increased flavor and aroma generated by the organic nutrients.



I have root weevils munching on my rhododendrons, privet and ivy. I just saw the first chew marks on the new growth. Surely there is something that I can use other than the toxic chemical Pestkil. Please help before my leaves have been completely eaten away. Thank you!



I agree with your choice of going the non-toxic route, I always advise people to use organic pest controls when ever possible. I do not have any experience with root weevils, however below you will find a link to a great site that has “least toxic” methods to control many types of pests. You should be able to get the information that you need. Please feel free to contact us if you have any questions. Click below (don’t forget to use your “BACK” button to return to Hydro-U. CLICK HERE



I need a chiller for my outdoor hydroponic system,can you help?



There is a company that specializes in chillers for hydroponic systems. The company is named Frigid Unit and they are located in Toledo Ohio. Their phone number is 419-474-6971.

There is another low-tech method that I have seen used, take two plastic bottles (two liter soda bottles work well) and fill each about 3/4 full with water and put them in your freezer. Every day (or as needed) simply put one of the frozen bottles into the reservoir of your hydroponic unit and put the warm bottle in the freezer to re-freeze.It’s kind of a pain to remember to swap out the bottles every day, but it is a much cheaper way to cool your nutrient solution. 


Old wise Professor-

Please tell me the best technique and medium to use when rooting seeds, and how to transplant rooted plants to an NFT system using no grow medium only nutrient solution.



Wise Old Professor? I’m offended! Just who do you think you’re calling WISE?:)

Planting seeds is best done in either rockwool cubes or Oasis cubes. These small cubes are extremely popular for propagation from seed. Many people use “peat pellets” to start seeds But I think that they hold too much water and don’t always produce good results. Peat pellets are also kind of messy if you use them in a NFT system. Once the seedlings are ready for transplant into the NFT system they need to be supported so that the roots hang into the nutrient solution. this is usually done by putting the cube containing the seedling into a net cup. Net cups are plastic cups that have many holes in them that allow the roots to grow through the cups and hang down into the solution. These cups come in a variety of sizes but 2″ and 3″ are the most common. NFT systems are usually made out of some sort of tubing (4″ and 6″ PVC pipe is very common), holes just slightly smaller than the net cups are drilled into the top of the tube and the cups are placed into the holes. the tube holds the net cup in place while the net cup holds the starter cube with the rooted seedling. The net cups are inexpensive and can be found at Simply Hydroponics or any good hydroponic shop. You can also make your own if you wish by drilling or cutting holes into a small plastic cup that will fit into your system.



Despite your excellent answer to an earlier question entitled ” How do I root Plants for a NFT system?”, I’m still a little confused. When the seedling is transferred to the net basket, the roots aren’t growing thru the basket yet, so how do I ensure root contact with the nutrient film? The drawings I’ve seen always show a mature root system with the root tips hanging in the solution, but the bottom of the basket is well above the bottom of the channel.



That is an excellent question that I should have covered in my previous answer. There are basically two ways to get the nutrient solution to the roots until the roots grow long enough to reach the bottom of the NFT channel. You can raise the level of the nutrient solution in the NFT system so that it comes in contact with the starter cubes, or you can make an intermediate water culture system. We use a water culture system at our facility which is basically a large plastic tray with a piece of Styrofoam floating on top. The Styrofoam has holes the proper size to hold the cups so that the bottom of the cup is in the nutrient solution. Simply aerate the nutrient solution and let the seedlings float around until the roots grow enough to be placed into the NFT system. Depending on the plants, the roots should be long enough in just a few days. If you decide to raise the water level in your NFT system, you need to return the water level back to normal as soon as the roots are long enough. (Be careful that you have enough room in your reservoir for the excess nutrient solution when you lower the level).



What an informative web site…..But, maybe I am a little dense…but, What kind of vegetables can I grow like this? You said the only stupid question was the one not asked…..

Kate S.


Thank you for your question, it is NOT a stupid one, it’s a good basic question that wasn’t covered on this website until now. The answer is a good basic one also: You can grow anything hydroponically. The beauty of hydroponics is the ability to control the aspects of the plants environment. The amount of water the plants likes, the amount and type of nutrients, pH levels and, with an indoor growing environment, the ambient temperature as well as the intensity and duration of the “sunlight” can all be easily controlled. In other words, if you know what a particular plant likes you can easily duplicate the ideal growing environment.

For most hobby hydro-gardeners the most popular vegetables to grow are probably Tomatoes, Peppers, cucumbers and lettuce. However I have seen virtually any vegetable you can think of grown with some form of hydroponic system or another.


Dear Professor Hydro,

I am new to hydroponic gardening, and need to know how many hours of direct sunlight do most vegetables need?  Due to space limitations, I have only a screened-in patio to work with. I would also like to know if it is a good idea to alternate a nutrient solution such as DynaGrow with an organic fish emulsion fertilizer.  I would like to keep the plants as “organic” as possible for feeding and pest management.

Thank you for your help!



Most vegetables need as much direct sunlight as possible. However you need at least a good 1/2 day of direct sunlight to get good healthy growth. As far as alternating the nutrients with organic and chemical fertilizers. This can be done but could potentially be a headache. What Most people do is add organic fertilizers along with the hydroponic fertilizers. This seems to give a “best of both worlds” result. The organic nutrients can be added directly into the reservoir or can be follier applied (sprayed directly on the plant).