Hydroseeding

The problem

A bare slope left after excavation or fill starts losing its fine material with the first rains: first the small pits opened by raindrops, then rills as they join, and finally gullies as they deepen. The fine material lost fills the ditch, the culvert and the stream bed below, undercuts the edge of linings and makes maintenance costs permanent. On a slope without a vegetation cover this cycle does not stop by itself; with every event the thin layer in which seed can take hold becomes a little thinner.

The early signs are simple: parallel rills on the slope surface, fine sand and silt collecting at the toe and in the ditch, exposed roots and large stones, toe collapse on fill slopes, muddy runoff after rain. Aspect and gradient also carry information; on steep south-facing faces seed dries out in the summer heat. When these signs appear, the problem is not aesthetic but the loss of surface material.

How we decide

The data we measure and compile: slope angle and height, grain size distribution and organic content of the surface material, pH and salinity, stone content, aspect and wind, rainfall regime and seasonal window, lines where surface water concentrates, irrigation options, native plant species in the surroundings.

The deciding factor is the time window: germination and root establishment take weeks, and during that time what holds the surface is not the seed but the mulch and the mat. That is why the choice of mix and mat matters as much as the plant species.

System family

The mix consists of water, seed, mulch (cellulose or straw based), tackifier and, where needed, fertilizer and soil conditioner. Proportions and application rate are set for the site and written into the project file in kg/ha. When ornamental species unsuited to the local climate are used, a slope that looks green in the first year is bare in the second; species selection is the most often skipped item in this work.

Installation

The sequence starts with surface preparation: the surface is raked, the compacted crust broken, coarse stones removed and, if needed, fine soil spread. If a mat is used, it is anchored in a trench opened at the crest, laid downslope and fixed with pins; the direction of overlap is set by the direction of flow. Then the mix is prepared and sprayed; hose distance and gun angle are set for an even distribution. On steep sections experienced specialists work on rope access.

The seasonal window directly determines success: spring and autumn are preferred, midsummer and frost periods are not suitable. Checkpoints: acceptance of surface preparation, mix dosage and tank record, evenness of coverage, mat trench depth and pin spacing, first irrigation, germination count.

Acceptance and maintenance

Acceptance is by the vegetation that has taken hold, not by the quantity applied. Checked at handover: germination density per unit area, coverage ratio, distribution of bare areas, mat anchorage and overlap check, pin spacing, mix and dosage records. If coverage is below the target, bare areas are reseeded. The acceptance criterion and the time of measurement must be written at the start of the work.

During the maintenance period: first-season irrigation, weed control, filling in bare areas, surface repair and flow diversion where rills form, re-fixing mat ends that have lifted. Vegetation is not considered complete before it has been followed for two seasons.

Frequently asked

• Does hydroseeding work on every slope? No. If there is no fine material for roots to hold, or the slope is very steep, a mat or surface preparation is needed first.
• When should it be applied? In the spring and autumn windows. Germination drops in frost and in summer drought; for the limits: working in winter conditions (Turkish).
• Can a slope with rockfall be vegetated? Yes, but in order: the protection system is installed first and vegetation is planned on top of it. See slope stabilization.