Eric Hopper

Takeaway: Finding the ideal operating temperature is one-way growers can help ensure growth rates and bountiful harvests. But as Goldilocks found out, determining ideal temperatures can sometimes take some experimentation.

The atmospheric conditions in an indoor garden have a tremendous impact on the quality and quantity of the finished crop. Growers of all skill levels are continuously trying to master the subtle environmental factors that can make a significant difference in growth and yield over the course of a plant’s life cycle.

When discussing the atmospheric conditions of an indoor garden, people are often referring to humidity and temperature. Although humidity is an important aspect, this article will focus on temperature, which plays a vital role in the way a plant is able to process light and intake water and nutrients.

Temperature affects a plant’s ability to photosynthesize, and it also plays a key role in seed germination and fruit and flower development. If temperatures fall outside the desired range, the garden becomes less efficient. Over time, these inefficiencies can lead to smaller, lower-quality yields.

Maintaining a Consistent Temperature Inside Your Grow Room

One of the most important things a grower can do is maintain a uniform and consistent temperature. A uniform temperature means the growroom’s temperature is the same in all areas, from one end to the other.

Most growers use high-intensity lighting equipment in their growrooms, which produces excess heat from one focal point. If this is not addressed, the temperature around the light source will be much warmer than in the other areas.

Maintaining a uniform temperature can normally be achieved by creating adequate air flow within the room. Oscillating fans will continuously move the air around the room so the heated air underneath the light fixtures will mix with the cooler air in the room.

Consistency is also important when it comes to growroom temperatures. Plants love consistency and respond well to it. The better growers can maintain a consistent temperature range, the better their yields will be.

To achieve a high level of consistency, a grower will usually invest in a climate controller with ventilation fans or an air conditioner.

For ultimate control, air conditioners are the way to go. Mini-split air conditioners allow growers to maximize control over temperature consistency. Those who choose to invest in a properly sized mini-split air-conditioning system will not regret it—nothing is better than being able to simply push a button and have absolute control.

The Ideal Temperature Range for Indoor Gardens

To maximize efficiency, try to determine the garden’s ideal temperature range, which is the temperature at which plant growth is maximized. The ideal temperature is crop-dependent because plants differ greatly in their responses to temperature.

For most fast-growing annuals, a good initial range at which to operate a garden is 70-80°F (21.1-26.6°C). Most varieties will prefer the upper end of this range. This temperature range can be used for both the vegetative stage and the fruiting/flowering stage.

Growers with climate-controlled rooms can use the temperature range as a starting point when trying to figure out the room’s best operating temperature.

When experimenting, consider the way the air temperature will affect the root mass of the plant. In most indoor gardens, plants are grown in hydroponic systems or planting containers, and these plants are more affected by changes in the air temperature than plants that are planted directly into the ground.

For this reason, growers must always be aware that higher air temperatures can lead to higher temperatures around the plant’s root mass.

Growers with climate-controlled rooms can set up an experiment to find the ideal temperature by slowly increasing the temperature and monitoring vegetative growth. By using this method, a grower can measure plant growth as it correlates to temperature and then determine the optimal temperature for the vegetative growth of that plant variety.

Remember that different varieties of the same plant will have different optimal temperatures. For that reason, growers with multiple plant varieties will have to shoot for a happy medium to keep all plant varieties as content as possible.

For most plants, the ideal temperature range for vegetative growth will also be the ideal temperature range for the fruiting stage, although some varieties prefer slightly cooler temperatures for flowering.

Many plant varieties I have grown performed best at 78°F (25.5°C), although I have had a few varieties that liked it a bit warmer (80-84°F). For novice growers or those without an air-conditioning system, a thermostat controller and fan can still get the garden close to the 70-80°F (21.1-26.6°C) range.

"In a semi-enclosed environment, a significant drop in temperature can lead to condensation and moisture issues in the growroom and on the plants themselves."

Unfortunately, temperature control is only as good as the equipment, but don’t get frustrated if the ventilation system isn’t perfect to begin with. Instead, do your best to maintain consistent and uniform temperatures with the equipment that is on hand and work towards maximizing climate control in the future.

Temperature Differential

The temperature differential, or the temperature change between the lights-on (daytime) and lights-off (nighttime) periods, is an important aspect of controlling temperature that often gets overlooked. Once growers determine the temperature of the lights-on stage in the garden, they can start to determine what temperature is needed for the lights-off period.

The best lights-off operating temperature is generally about 10-15°F (5-7°C) cooler than the lights-on operating temperature. This temperature drop is significant enough to allow a plant to go through its normal process of rest and transpiration, but is not so great that problems are caused.

Keeping the lights-off temperature within 10-15°F of the lights-on operating temperature serves as a safeguard against pathogens, especially molds and fungi. In a semi-enclosed environment, a significant drop in temperature can lead to condensation and moisture issues in the growroom and on the plants themselves.

Over time, this will result in a perfect environment for molds, fungi, and other plant pathogens. For most indoor gardens, the ideal temperature for the lights-off period is 55-70°F (12.8-21.1°C). Adding an electric heater to warm up the garden at night goes a long way towards preventing problems and maintaining healthy growth rates. (Read More: Tips for Avoiding Mold Growth in the Grow Room)

CO2 Enrichment and Temperature Inside a Sealed Grow Room

Growth rates can be increased by enriching the garden’s environment with carbon dioxide, but to gain maximum benefits, growers need to increase the room’s operating temperature.

The increase in temperature enables plants to properly process the additional CO2 in the atmosphere and increase the rate of photosynthesis, which increases plant growth.

When operating a garden with CO2 levels of 1,200-1,500 parts per million (ppm), it is a good idea to increase the temperature range to 80-90°F (26.6-32.2°C). From there, slowly increase the temperature and monitor plant growth to find the ideal operating temperature.

Best Temperatures for Clones and Seeds

Just like fully established plants, clones and seedlings thrive on consistency. In fact, most seed varieties will only germinate once the temperature is consistently over the seed’s threshold temperature for sprouting.

For many plants, this is a temperature range of 60-70°F (15.5-21.1°C). In indoor gardens, a good temperature range for the seedling and cloning stage is usually around 72-82°F (22.2-27.8°C). Seedling heat mats or small electric heaters are great ways to maintain a consistent temperature warm enough to promote germination and early root development.

Again, each plant variety is different. When starting from seeds, check the seed packet for instructions. Some cold-weather plants can be germinated in relatively cold conditions, whereas some summer plant varieties may require much warmer temperatures for germination.

Experiment to Find the Ideal Temperature

Every indoor garden is different and there are many variables that can affect the way a crop performs. Although growers can use the general operating temperature range of 70-80°F ((21.1-26.6°C) as a starting point, finding the ideal temperature for an indoor garden is something that can only be done through experimentation.

As with most horticultural experiments, finding the ideal operating temperature will take time. It is hard to determine how a small change in the garden will affect the overall results until the garden’s harvest can be measured.

For that reason, it may take multiple garden cycles before a grower is able to collect enough data to make a solid conclusion about the ideal operating temperature.

Although the process can be a bit tedious, committed indoor gardeners should have no problem documenting temperatures and how they relate to the growth of the garden. After all, growers are always in search of ways that will increase their return on investment.

Finding the ideal operating temperature for an indoor garden is just one-way growers can help ensure their gardens have consistent growth rates and repetitive, bountiful harvests.

Written by Eric Hopper

Eric Hopper’s past experiences within the indoor gardening industry include being a hydroponic retail store manager and owner. Currently, he works as a writer, consultant and product tester for various indoor horticulture companies. His inquisitive nature keeps him busy seeking new technologies and methods that could help maximize a garden’s performance.

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