Bonsai In A "Bag"

Derek Bentley's Growing System

by Jerry Meislik


There are many growers of indoor bonsai out there and many more that would like to grow bonsai indoors. Most indoor growers select their bonsai materials, usually plants that tolerate low light and low humidity and then grow them in the brightest light in their home that is often a rather dim windowsill. Others use supplemental lighting with fluorescent lights allowing them to grow low to medium light requiring trees. Others use high intensity lights, most often metal halides, to provide light levels that allow relatively high light requiring plants to be grown. Of course lights are just one of the environmental manipulations that can be made for growing plants. Water, soil, fertilization, humidity and CO2 levels can also be manipulated.

Derek Bentley living near Seattle Washington has taken a very interesting approach to growing his bonsai in a plastic “bag” or tent, also called a 'HydroHut'. In this way he can grow bonsai indoors in any room in his house while being able to provide the best growing conditions for his trees with control of almost all the environmental factors that influence plant growth.

Derek says here is what I am doing:

"I have been growing my tropical bonsai which are mainly Ficus and Schefflera in a HydroHut. The HydroHut is basically a waterproof plastic tent that comes with the modifications to manipulate all the factors needed to grow bonsai.  The Hydrohut comes in many sizes but the one that I am using is 4 by 8 feet and about 7 feet high. The floor is waterproof so that spills are not a problem even in a home environment. My system is in my garage.

Picture of the HydroHut in Derek's garage.


Hydroponic growing

Plants in the HydroHut are growing with a hydroponic growing system that automatically pumps water into the soil providing low levels of fertilizer two or three times daily for about 15 minutes. Water is then drained from the pots. The “soil” is baked clay pellets to allow good root growth by providing proper moisture, fertilizer and air to be provided. Ultimately plants will be moved to a more “normal” bonsai mix in conventional bonsai pots to finish and display the trees. The current system is used to grow the trees as quickly as possible to bring them into the size and condition for them to transition to normal life in a bonsai pot.

Hydroponic controller tank that pumps water and fertilizer to the grow buckets.

Plants aranged on the floor of the Hut. Black pipes carry hydroponic solutions to the growing pots.


Holding tank for the hydroponic solution located outside the tent.


I have been experimenting with the nutrient solution as my primary variable.  I am changing the solution about every 2-3 weeks, although some recommend every 1-2 weeks.  I need to do more testing before I have some conclusive results.

I did see some amazing growth about a month ago, but there were a few changes done simultaneously, and I am not sure which one, or combination of them caused the accelerated growth.  The changes I speak of were a warming of the outside temperature as spring arrived, It was the first time I used a salt leaching solution as part of my nutrient solution change, and third I used some organic components in my nutrient solution along with half my normal amount of GH Flora Nova fertilizer.


I have a 1000Watt metal halide (MH) light on 12 hours a day; which is less time than I would like but I am trying to conserve electricity, and carbon dioxide (CO2). The CO2 is only released when the control box light sensor is activated.  Wben the light is not on there is no extra CO2 released into the system. There is a CO2 meter that will trip a solenoid when CO2 drops below 1500 ppm. to keep the CO2 level at optimum growing range.

Metal halide light with ventilation ducts entering and exiting the lamp housing.

Temperature and CO2

The sealed unit allows very precise control of the interior environment. Temperature is regulated with a fans set to ventilate the tent when temperatures exceed optimum levels. With a metal halide light generating lots of heat the vent system pull excess heat out of the light housing and simultaneously moves cooler outside air in.

A tank containing carbon dioxide (CO2) sits outside the tent and an interior sensor raises the CO2 to levels that stimulate maximum plant growth. Fans within the tent also mix the CO2 so that it does lie at the lowest levels of the tent since CO2 is heavier than air.

CO2 tank outside the tent.



I started out using a humidifier, but ended up bringing two units back to the store, because they weren't designed to be run 24/7.  I finally found a unit that lasted 3 months (a Honeywell Ultrasonic cool mist unit).  After being in the high humidity for 3 months the motor bearings for the air accelerating fan gave out.  I ditched the humidifier, and now I just have the bottom of the HydroHut flooded with a mixture of water and H2O2 (Hydrogen Peroxide).  This which creates enough evaporation to keep the humidity at about 65-70% during the day, but when the temperature falls at night the humidity reaches higher levels. The H2O2 prevents green algae buildup. (Note the plants are never sitting in water as their root level in the pots is above the floor.)

Warmer air can hold more moisture, so a humidistat reading 70% humidity when it is 85 degrees is different than 70% humidity at 65 degrees. I do mist the plants in the morning to give them an extra boot of humidity which brings the hut up to 92% for a few hours. When I was using a humidifier I saw evidence of "rain" in the tent, as water droplets were present on all the plants leaves in the morning. I can only assume that the humidity was reaching 100% at night when the air cooled, and this released extra water in the form of rain.


Temperatures and Ventilation

There is a duct that brings in outside cool air and passes it over the hot metal halide light and then vents the heated air outside the HydroHut. The light ventilation fan turns on at 86 Degrees F, but it rarely reaches that temperature.  Once the hot summer weather hits I'm sure I will be taking advantage of that Vortex fan to cool my light, which in turn will reduce heat build up in the HydroHut.  The HydroHut is almost a sealed system, so minimal CO2 will be sucked out.

I have a floor standing oscillating fan which runs all day, and the timer turns it on and off at night a few times.  I also have two fan strips in the ceiling of the HydroHut to blow warm air down and stir up the CO2 from the floor.  The two strip fans are controlled by a timer as well.


Initial Costs


Setup:  HydroHut Deluxe (4'W x 8'L x 7'H)    ~$500

        Ebb & Grow Hydroponics System          ~$500

        CAP CO2 Reg. &Meter & Control         ~$300

        MH Light System                                      ~$350

        Vortex Fan                                                 ~$100

        Misc Supplies, fans, EC/PH meters         ~$300


Total                                                                      $2050

Ficus microcarpa growth in 4.5 months!


Future and Comments

Derek comments:

I am planning on removing the plants from the system at the end of the summer, and potting them up in real bonsai soil.  The system does become dirty, especially if organic components are used in the nutrient solution.  I will have to slowly adapt the humidity back down to normal indoor levels so as to preserve as many of the aerial roots as possible.  The aerial roots are the most amazing things to watch.  I have literally seen roots grow 1cm in 24 hours.  It's amazing, and now I need to figure out how to produce that result again.  Some varieties of Ficus microcarpa seem to grow aerial roots much more easily than others.  The two plants I have that grow the most aerial roots are the ones with larger leaves.

Ficus microcarpa showing massive growth of aerials inside the HydroHut.


Ficus microcarpa showing extensive growth of roots in the baked clay pellets.


For the future I would like to investigate what types of nutrient solutions, organic, or chemical, and how many times per day to flood and drain.  Right now I am flooding and draining twice a day for 30 min each time (before I was doing 5 times a day at 15 minutes, and have tried other combinations).  My nutrient is GH Flora Nova at 1800ppm.  I have also tried Earth Juice Organic nutrients, and am considering Eco Enterprises Eco Grow"M" next.

Jerry's comments:

Derek's growing system is a wonderful way to have total control of growing bonsai in a home situation. His results clearly demonstrate fabulous growth with his Ficus and Schefflera bonsai. Although costs for the initial system are high, those on a limited budget can utilize some of the concepts in a modified home system.

Also note that Ficus aerials will die back if they are not rooted into the ground and the trees are subjected to normal typically low home humidity levels. Allow the aerials to establish themselves in the ground before returning the bonsai to the home atmosphere.

All photos by Derek Bentley.

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