Hydroponic at Home Post-Webinar Q&A

Extension specialist Jonathan Ebba answers real questions from gardeners about home hydroponics


In a recent webinar from UNH Extension and UMaine Cooperative Extension, Jonathan Ebba delved into home hydroponics and answered viewer questions.

We didn’t get to all of the questions viewers had for us, so we are sharing relevant questions and answers here as a written Q&A. Enjoy!

What temperature should your home hydroponics system run at?

Optimal growth of leafy greens wil be at 68 °F, but lower temps produce satisfactory results.  If it's comfortable for you to live in, it should work! Seedlings often prefer it a bit warmer… if you have a separate nursery area, you can target closer to 74  °F for many plants, but the warmer you go, they more stretch you will get if your lights are inadequate!

Is the rock wool at big box stores an inexpensive, appropriate source of material? Is there a reason not to use insulation as a rooting medium?

Insulation rockwool is denser and seems to have less pore space for root growth.  I also don't know if it is treated with substances  to increase its suitability as a construction material but which may be undesireable in a food production system.

How much time do you need to spend a week to obtain a weekly supply of 6 lettuce heads a week?

That's a great question!  After setting up the system, consider (roughly) each week:

  • Prepping cubes and sowing seed: 15 minutes
  • Transplanting: 15 minutes
  • Harvest: 15 minutes
  • Cleaning: 30 minutes
  • Checking, filling and mixing nutrient solution: 30 - 45minutes

How do you clean? What cleaning agents? In the dishwasher?

Often simply scrubbing in the sink with hot water, soap and elbow grease is sufficient.  We often use quaternary ammonium cleaners (like GreenShield or Physan) in commercial production.

If hydroponic plants go to seed, will the seeds be as viable as they would in geoponics?

If the growth factors are correct for seed development they will be.  An interesting concept that we didn't address in the webinar is pollination!  We don't need pollination for leafy greens, but we do for seed and fruit production.  If you are growing for seeds or fruit, you need to devise a pollination  method depending on what plant you are growing.

Is a 5000K lamp (Daylight) a better choice than a 3000K lamp? How lumens per square foot is appropriate?

"Lumens are for humans"... Kelvins and lumens are really functions of human vision more than photosynthesis.  Check out this fact sheet describing light in plants.

That said, 3000K tends to have more red wavelengths which are photosynthetically active.
The amount of light available to a plant per lux (lumen per square meter) varies greatly by the light source. You can find conversion tables online for different light sources and use the above fact sheet to roughly calculate a sense of how much light is available to the plant from different sources.

An example:  for growing lettuce with cool white fluorescents running 20 hours per day, you may need about 19,000 Lux (lumens per square meter), but remember this is measured at the crop level, not directly below the lamp!

If you're really into photosynthetic light, you can purchase a light meter which measures Photosynthetic Photon Flux Density (in micromoles per square meter per second)!

What is wrong with low-priced, small LEDs for hydroponics?

Although many of these lamps have great wavelengths, they simply don't produce enough photons to grow the full sun plants we use as vegetables.

Can one make ‘pots’ out of screening or porous materials? 

You want to make sure that the holes are large enough to accommodate roots as they grow (interestingly, this is a problem with some commercially available net pots!), but people certainly have experimented with a wide variety of containers for the systems which require a container.  Typically, raft systems need the net pots... in NFT systems the root cube is often set directly on the gutter bottom , and of course medium-based systems use a wide variety of containers.

If your well water has arsenic/uranium/radon, do you need to grow in filtered water?

Arsenic has been positively shown to accumulate in soil-grown vegetables, and I assume it could in hydroponic vegetables as well.  I am not clear on the risk from radon or uranium, but my concern (other than accumulation in the plant) is that aerating the water in your house will release radon gas from the water into the air.

Work with a reputable water purity company to investigate your options.

Do systems have problems with black mold? What does one do to solve that problem?

"Sooty" mold on plants is from a harmless fungus growing on exudate from insects such as aphids, scale, whitefly or mealybugs.  Any of these can be a pest on hydroponics; treat the pest and the sooty mold will go away.

"Black mold" as a potential household toxin does not grow on hydroponic plants or systems, HOWEVER, these systems can produce indoor humidity that needs to be dealt with. (This is a function of the plants more than the system).  This increased humidity may result in the growth of potentially harmful household molds and mildews.

Make sure to have good airflow and regular air exchanges.  If growing a large hydroponic garden in an enclosed area, you may need to consider mechanical dehumidification.

I've seen the tents that have reflective interiors, can this reduce the amount of light required from the lamps?

Reflective interiors are nice because they diffuse the light being bounced back to the leaves resulting in a more even illumination with less shading.  What percentage of the light they "save" is too influenced by system design, light type, stage of growth, etc. to quantify, but a reflective tent can certainly reduce the required run time of your lamps. 

Got questions? The Ask UNH Extension Infoline offers practical help finding answers for your home, yard, and garden questions. Call toll free at 1-877-398-4769, Monday to Friday, 9 a.m. to 2 p.m., or e-mail us at answers@unh.edu.



Jonathan Ebba
Landscape & Greenhouse Field Specialist
Asst Field Specialist
Office: Coop Extension, Taylor Hall, Durham, NH 03824

Nate Bernitz
Community & Economic Development Field Specialist
Asst Field Specialist
Phone: (603) 678-4576
Office: Cooperative Extension, Nesmith Hall Room 204, Durham, NH 03824