Why Plants Are Happier Together

Plants modify the environment around them through transpiration, shading and their effects on airflow. Temperature, humidity and carbon dioxide concentration near leaves can therefore differ from measurements taken elsewhere in a room or greenhouse.

This local environment is the microclimate. Grouping plants can change it, sometimes buffering hot or dry conditions, but also creating humidity, light or air-exchange problems when a canopy becomes too dense. The title refers to these physical effects, not an emotional preference for company.

Transpiration changes heat and moisture exchange

Plants take up water through roots and release much of it as water vapour, mainly through stomata. Evaporation within the leaf consumes energy and can cool the leaf, while the released vapour adds moisture to the surrounding air.

The effect depends on leaf area, water supply, stomatal behaviour, radiation and ventilation. Even an isolated leaf has a local boundary layer, while many transpiring leaves together can modify conditions across a canopy. The resulting air may be cooler and more humid than outside the canopy, but this is not guaranteed in every environment.

Neighbouring leaves also provide shade, reducing radiation received by some tissues and changing their temperature. These processes help explain why a distant temperature or humidity sensor may not describe conditions at the affected leaves.

The canopy influences its own environment

The canopy comprises the aboveground leaves and stems. Within it, vegetation obstructs air movement while transpiration adds moisture. Canopy depth, leaf arrangement, spacing and ventilation determine how strongly these effects develop.

During hot, dry weather, shading and evaporative cooling may reduce exposure to extremes. That potential benefit has limits: dense vegetation can also retain heat or moisture where exchange with surrounding air is insufficient. Grouping plants is therefore a change in growing conditions, not an automatic improvement.

Air VPD and leaf-to-air conditions are not identical

Vapour pressure deficit, or VPD, describes a difference between saturation vapour pressure and actual water-vapour pressure. Growers often calculate air VPD from air temperature and relative humidity. Higher temperature at the same relative humidity, or lower relative humidity at the same temperature, increases that calculated deficit.

The evaporative gradient between a leaf and the air also depends on leaf temperature. A room-level calculation using air temperature does not necessarily describe the conditions at each leaf, particularly where radiation, transpiration and airflow create temperature differences.

Canopy cooling and humidification can modify this gradient, while stomatal regulation and water supply influence actual transpiration. Environmental demand should therefore be considered alongside root function, as discussed for Cannabis in Overwatered or Underwatered? Understanding Cannabis Droop.

Density can create competing effects

Close spacing can reduce air exchange, sustain high humidity and increase the duration of leaf wetness when wetting or condensation occurs. Such conditions can favour particular fungal or bacterial diseases, depending on the host and pathogen.

Dense canopies can also restrict carbon dioxide replenishment and light penetration. Where plants share a limited root environment, competition for water, nutrients and rooting volume can add further constraints.

The aim is a canopy suited to the crop and climate rather than maximum density. In Cannabis, the microclimate within dense flowers illustrates why room humidity alone may not capture disease risk. These crop-specific risks do not mean every grouped planting requires leaf removal.

Air circulation affects the leaf boundary layer

A relatively still boundary layer forms near leaf surfaces. Its resistance to exchange depends on factors including leaf size and shape and the speed and direction of airflow. With limited movement, heat, water vapour and carbon dioxide can exchange more slowly between the leaf and surrounding air.

Circulation can improve exchange, but its effect on transpiration depends on stomatal behaviour and the surrounding temperature and humidity. Strong airflow can increase evaporative demand or create damaging local exposure; very weak circulation can leave persistent pockets of heat or moisture.

Adjust circulation to the canopy rather than treating maximum fan speed as the objective. If changes to leaf area are being considered, the trade-offs discussed in Defoliation: Does Removing Cannabis Leaves Actually Increase Yield? provide a crop-specific example. Increased airflow after removal does not by itself establish a yield or disease-prevention benefit.

Measure representative canopy conditions

Compare measurements near representative leaves with room or greenhouse readings. Consider different canopy heights and positions, especially where light, airflow or symptom distribution varies. One sensor within a canopy does not describe every leaf.

Sensor placement and exposure matter: direct radiation, wetting or an unrepresentative air stream can distort interpretation. Air temperature and humidity measurements describe the sampled air, not leaf temperature or internal tissue conditions directly.

Repeated measurements under comparable conditions are more informative than a single reading. Together with observations of growth, water use and canopy density, they help identify whether grouping is buffering environmental extremes or creating a new limitation.

Manage the local environment rather than plant proximity alone

Plants influence one another through shade, water-vapour release and airflow. These effects can make a group experience conditions different from the surrounding open space, without implying that closer spacing is always better.

Useful management combines suitable spacing, water supply, light distribution and circulation with measurements taken where the crop is growing. The relevant outcome is a functioning canopy, not simply more plants in the same area.

References

1. Westreenen, A. et al. (2020). Substantial differences occur between canopy and ambient climate: Quantification of interactions in a greenhouse-canopy system. PLOS ONE, 15, e0233210. DOI: 10.1371/journal.pone.0233210.
2. Wright, A. et al. (2024). Plant traits, microclimate temperature and humidity: A research agenda for advancing nature-based solutions to a warming and drying climate. Journal of Ecology, 112, 2462–2470. DOI: 10.1111/1365-2745.14313.
3. Raupach, M.R. (1989). Turbulent transfer in plant canopies. In: Plant Canopies: Their Growth, Form and Function. Cambridge University Press.