Hydrogels and Water Availability

It seems recently that everyone has a hydrogel to sell to help your plants stay healthy and well watered. But what are hydrogels and do they really help?

Hydrogels are crystals that suck up water, sometimes 600 or more times their weight in water. These crystals look like tiny pieces of quartz before they’re saturated with water and remarkably like clear shredded jell-o when water is added. These gels are sold as additives for soils and container media for the purpose of reducing the soils need for watering. There have been some experiments on these products over the years, most of these experiments were performed a long time age with gels that are no longer on the market. In general these experiments showed that hydrogels resulted in reduced growth and didn’t supply the water to the plant that they should have. But what about the newer hydrogels? Could they supply extra water to a plant and reduce the need for watering? We tested 5 different hydrogels on geranium (grown in large flower pots) and 3 different hydrogels on ninebark (grown in 1 gallon containers) to see how long we could keep the plants healthy without watering.

Plants were grown with the hydrogels incorporated into the media. Growth was roughly similar among the control and the different hydrogels tested with the exception of hydrosorb which stunted the growth of the ninebark. Hydrogels were added at a rate similar to the recommended rate. Plants were irrigated whenever the media in the container was less than 80% saturated with water. After most of the plants reached a size that was considered saleable watering was stopped and the plants were allowed to dry out. The amount of water contained the plants was determined by using a “pressure bomb” which is an instrument that measures how much water is in a leaf by compressing the leaf with air pressure until sap comes out of the petiole (base of the leaf). Geraniums were noticeably wilted when water potential was measured as a 9 or greater (this is typical of succulent plants). Ninebark were noticeably wilted when water potential was 30 or greater (this is typical of woody plants).

The results are shown in the graphs included with this report. None of the hydrogels kept the plants supplied with water for any longer than the controls which had nothing added to the media.

It is worth mentioning that in the ninebark study the hydrosorb does appear to keep the plants at a healthy water potential for longer than the other hydrogels and the control. However, this is almost certainly because of the smaller size of the plants.

Hydrogels do have a place in the nursery industry. They will help to keep the roots of bare-root plants moist while they are waiting to be transplanted. Besides this though they seem to be overrated. We are conducting an experiment to see whether extremely high porosity mixes will benefit from hydrogels, but these results are pending.