Best Time of the Year for Indoor Hydroponic Plants
The best time of the year for indoor hydroponic plants, especially when the environmental conditions (air temperature, water temperature and light) are controlled, is not really a matter of concern. You can have success with any suitable plant species all year round as long as you know what you are doing with the environmental conditions control.
That is the whole idea of indoor hydroponics. To mimic a perfect environment for your plants so that you can grow whatever you like any time of the year.
Of course, if one of the air, water or light is not correctly controlled then the conditions are not ideal and you should seriously start thinking about how to compensate that with nature’s power.
For example, if you do not have artificial lights, then an indoor hydroponic system will be very hard to succeed. As a counter measure, you would have to find a well-lit space outdoors, most likely a glasshouse in your back yard. Even if you live in a place where sunshine is a given, this might not be perfect in winter. Unless you are close to the equator where sunlight may be without great changes all year round.
Or if you chose to go indoors then a patio or a large window could work if it gets enough hours of sunlight. All that is not ideal, and only work as mediocre counter-measures to artificial lights because yields will likely be lower.
If you have no means to control the air or water temperature, then again, you should wait for the growing season to arrive or sometimes earlier if you have a glasshouse. There will be cases where you place your plants close to a window to get enough sunlight but during winter you will get cold drafts that may shock your plants. Again, these are parameters you need to consider when you make your decision to invest or not in a controlled hydroponic environment which eliminate the issue of when is the best time of the year for indoor hydroponic plants!
Water temperature is also important. Roots are quite sensitive in temperature. Bear in mind that the different plant species will have different optimum water temperatures. For example, lettuce and other small leafy plants have optimum water temperature at 68F (20C) (Sakamoto and Suzuki 2015), potatoes like it at 77F (25C) (Chil et al. 2001), cucumbers a bit higher at 82F (28C) (Daskalaki and Burrage 1998) while others like onions have a much higher upper limit at 84F (29C) (Rahmat et al. 2019).
If you want to keep it simple, you need to aim for water temperature of 66-70 degrees F (19-21 degrees C). Below the lower temperature limit roots slow down in growth and functionality. Ideally keep the water temperature at 68-70 degrees F (20-21 degrees C).
So having a controlled environment including controlling the temperature of the water solution takes away on more of the parameters of concern when asking what is the best time of the year for indoor hydroponic plants.
If you want to go into more detail into the limiting factors in growing hydroponically and what you can do to prevent or rectify problems, have a look at my Post in 12 Hydroponic Problems and Solutions to Avoid Disaster.
So long story short, best time of the year is ANY time. If you are diligent in the process, indoor hydroponics is ideal for growing any plant suitable for hydroponics, any time of the year. Also have a look at the 6 Factors to Success When Selecting Hydroponic Plants to find out more about factors to consider when selecting your plants.