SCIENCE TO THE GROWER: Do Plants Need Water? Science Has the Answers!

By Richard Evans

I sighed when the Editors asked me to write about drought-related research. (Our long California drought already was getting me down. I recently was standing in an airport restroom, wondering if the guy in line behind me would think I'm a loser for not flushing the urinal, or if he'd think I'm a loser for flushing it. So I asked him. He told me I'm a loser.) I told the Editors my philosophy: When life gives you drought, resort to dry humor. I offered to relate some jokes about camels and deserts. The Editors ignored that and reminded me of the deadline for my article about drought-related research. I felt like I was lost with a witch in a tunnel of love.¹

O.K., fine, I’ll do it. Maybe you'll laugh when I tell you that scientists have studied the possibility that drought could improve ornamental crop production. It's true!

This research has followed two main paths. The scientists on one path recognize that greenhouse and nursery plants will be affected by water deficits; they ask whether deficit irrigation — always irritating with some fraction of the total amount of water the crop could use — could replace use of chemical growth regulators. Scientists on the other path may agree with their colleagues, but they wonder whether there are stages of crop development during which the plants are relatively insensitive to drought. Their objective is to save water, not control growth, so they propose using deficit irrigation only at times when the crop is drought-insensitive — a method they call regulated deficit irrigation.

Research on deficit irrigation of ornamental crops has yielded mixed results. The growth of one-gallon rosemary plants that were irrigated at soil moisture sensor setpoints near container capacity was about twice that of plants grown in drier media, and roughly similar results were obtained with columbine (Aquilegia canadensis) and pinks (Dianthus gratianopolitanus).² When one-gallon Bougainvillea plants irrigated to replace 100%, 50%, or 25% of normal daily water use, those in the latter two treatments were smaller and had fewer leaves.³ However, water stress also increased the number of flowers and cut the total irrigation volume in half. The authors of the study concluded that deficit irrigation could be used to save water during a drought without losing the ornamental value of Bougainvillea. Six-inch geraniums (Pelargonium×hortorum) subjected to deficit irrigation (60% or 40% of normal irrigation) were smaller, with fewer leaves, but they also had fewer flowers.⁴ The height, weight, and leaf number of four-inch marigolds (Tagetes erecta) decreased in proportion to the amount of moisture maintained in the growing medium.⁵ Unfortunately, height was less affected than plant weight or leaf area, so deficit irrigation did not increase the compactness of the plants. Together, these results cast some doubt on the utility of deficit irrigation for business that produce a variety of crops.

Regulated deficit irrigation was introduced in the 1970s as a method to reduce the need for summer pruning of peach trees. The earliest ornamental research subject, as far as I know, was rhododendron.⁶ One-gallon plants were exposed to 150%, 75%, or 25% of normal daily water use, and the deficit treatments were maintained for 8 weeks at four different times of year. Later experiments with Choisya ternata, Cornus alba, Cotinus coggygria, Forsythia ×intermedia, Hydrangea macrophylla, Lavandula angustifolia, and Lonicera periclymenum produced roughly similar results.^7,8 Severe deficit irrigation generally produced more compact plants, and damage was limited to minor tip burn of leaves in a few cases. However, the studies were conducted in England, which is not exactly Drought Central, and the authors note that careful management would be necessary to prevent leaf damage under high light and high temperature conditions. Potted geraniums subjected to mild regulated deficit irrigation in Spain were smaller than control plants, but flower production was normal unless the deficit irrigation occurred during the flowering phase.⁹ 

These experiments tell us that deficit irrigation can be used to reduce nursery water use by about 20% to 40% without causing severe damage, but flowers and vegetation may be affected differently in different species. Furthermore, the timing and intensity of the deficit irrigation affect outcomes. Since regulated deficit irrigation involves closer control of water stress, it is more likely that it can be employed successfully, as long as species requirements are attended to. However, the success of these methods depends on both accurate measurement of plant irrigation requirements and accurate delivery of water to the plants. Nurseries that meet those requirements already save a tremendous amount of water. And that’s no joke.(Editor's note: see Summer 2014 Issue article: "Maintaining Irrigation Efficiency in Greenhouses and Nurseries"

1. I borrowed this from a New York Times article by Dr. Seuss that appeared on 17 November 1957.
2. Zhen, S., Burnett, S. E., Day, M. E. & Van Iersel, M. W. Effects of substrate water content on morphology and physiology of rosemary, canadian columbine, and cheddar pink. HortScience 49, 486-492 (2014).
3. Cirillo, C., Rouphael, Y., Caputo, R., Raimondi, G. & De Pascale, S. The influence of deficit irrigation on growth, ornamental quality, and water use efficiency of three potted Bougainvillea genotypes grown in two shapes. HortScience 49, 1284-1291 (2014).
4. Sánchez-Blanco, M. J., Alvarez, S., Navarro, A. & Bañón, S. Changes in leaf water relations, gas exchange, growth and flowering quality in potted geranium plants irrigated with different water regimes. Journal of Plant Physiology 166, 467-476 (2009).
5. Van Iersel, M. W. & Nemali, K. S. Drought stress can produce small but not compact marigolds. HortScience 39, 1298-1301 (2004).
6. Cameron, R. W. F., Harrison-Murray, R. S. & Scott, M. A. The use of controlled water stress to manipulate growth of container-grown Rhododendron cv. Hoppy. Journal of Horticultural Science and Biotechnology 74, 161-169 (1999).
7. Cameron, R. W. F. et al. Regulation of plant growth in container-grown ornamentals through use of controlled irrigation. Acta Horticulturae 630, 305-312 (2004).
8. Cameron, R. W. F., Harrison-Murray, R. S., Atkinson, C. J. & Judd, H. L. Regulated deficit irrigation: a means to control growth in woody ornamentals. Journal of Horticultural Science and Biotechnology 81, 435-443 (2006).
9. Álvarez, S., Bañón, S. & Sánchez-Blanco, M. J. Regulated deficit irrigation in different phenological stages of potted geranium plants: water consumption, water relations and ornamental quality. Acta Physiologia Plantarum 35, 1257-1267 (2013).