The vertical orientation of green walls causes a risk of uneven water distribution within the growing medium, and thereby stress on the plant roots. Therefore it was studied how the root and top growth of different species were affected by the water holding characteristics of the growing media.

Five species of hardy perennials (Campanula poscharskyana ‘Stella’, Fragaria vesca ‘Småland’, Geranium sanguineum ‘Max Frei’, Sesleria heufleriana and Veronica officinalis ‘Allgrün’) were grown in 3 types of growing media (coir and 2 of rockwool) in vertical boxes under greenhouse conditions. Root distribution was registered over 52 days and the activity of individual root systems was studied via 15N uptake and plant parameters were measured. The water holding characteristics of the growing media was determined on a sandbox.

From day 21 and throughout the experiment, the plants growing in the coir medium showed stronger root growth compared to the two rockwool media. From day 28 onwards, there was also a difference between the two rockwool media, with a higher root frequency in the less dense medium. This pattern was consistent for all species, even if they showed different root growth dynamics. Fragaria, Geranium and Veronica showed steadily increasing root growth throughout the experiment, whereas Campanula started slow, but showed a strong root growth towards the end of the experiment. Sesleria showed little root growth throughout the experiment. Dry weight and root activity measured as 15N uptake was higher for plants grown in coir than rockwool. The coir medium showed a more gradual water release with increasing tension than either of the rockwool media, corresponding to the water content measured locally in the boxes.

The results confirmed that the growing media affect root and aboveground plant growth. This is consistent with the differences in water retention, as the more even vertical water distribution in the coir medium resulted in stronger growth compared to the rockwool media. The five species showed different root growth dynamics and different abilities to grow in the different media, and 15N uptake showed that low root growth in the rockwool media also resulted in low root uptake activity.