Greenhouse
热门新闻
A shade net is one of the simplest pieces of equipment in a nursery, greenhouse or crop field, and one of the easiest to install badly. The percentage printed on the roll describes how much light the fabric blocks, not how much shade the crop underneath actually experiences. That gap between the label and the field condition is where most problems begin.
Shade rates are normally quoted as the percentage of photosynthetically active radiation that the woven fabric blocks, and the complementary figure is the percentage transmitted. The practical figure most growers care about is the reduction in light reaching the crop canopy, and that depends on several variables at once.
| Nominal shade rate | Typical light transmitted | Best suited crop stage | Seasonal use |
|---|---|---|---|
| 30 to 40 percent | 60 to 70 percent | Established warm season vegetables | Mid season heat reduction |
| 50 percent | About 50 percent | General nursery and vegetable production | Spring to early summer |
| 60 to 70 percent | 30 to 40 percent | Young transplants and leafy greens | Summer peak |
| 75 to 90 percent | 10 to 25 percent | Root cuttings and hardening off | Short periods near the end of propagation |
The most common material is high density polyethylene, knitted into a mesh with a defined hole size. Knitted construction stretches in one direction, which makes tensioning easier but also means the fabric can distort if it is pulled unevenly. Woven polyethylene tape is stiffer and holds its dimensions better, but it resists stretching and can tear if tensioned too tightly.
Check the weight and the expected service life together. A light fabric of roughly 100 to 150 grams per square metre is usually enough for single season nursery use and will break down noticeably under constant ultraviolet exposure. Heavier fabric in the 200 to 300 gram range resists tearing and ultraviolet degradation better and can often be reused for several seasons, provided it is stored dry when not in use.
The mesh size deserves attention for another reason. Very fine mesh blocks air movement as well as light, so a dense net laid directly on a crop canopy can trap humidity and encourage fungal problems. Coarser mesh gives better airflow and tolerates wind loading. Round monofilament strands also shed water and dust more easily than flat tape, which is a practical advantage in dusty conditions.
The net never causes the failure; the support does. A typical field installation uses poles set along both sides of the crop row, with a wire or rope running the length at the top. A house frame with purlins gives a flatter, more even sheet. In either case the aim is a surface that will not collect rainwater, because a sagging pocket of water adds considerable weight to a fabric that was designed to carry almost none.
Set the poles far enough apart that the net does not stretch more than a few percent between anchors. A common failure mode is a net pulled tight in the middle of the run and slack at the ends, which produces a diagonal twist that folds the fabric and cuts the crop underneath. Allow for a slight sag, secure the leading edge with a rope rather than a single cord, and tie the fabric to the support at regular intervals rather than only at the corners.
Rolling the net out along the row in the same direction as the prevailing wind reduces flutter, and working from the upwind side keeps the fabric from folding over itself. Fasten the leading edge first, then the trailing edge, then work along the length in a steady sequence. Leave the middle sections slightly slack and adjust them afterwards rather than pulling hard against the anchors.
Hardware matters more than most people expect. Use galvanised or stainless clips and hooks rather than plain wire ties, which can abrade the polyethylene and cut into it under tension. If the run is long, a continuous lock line or rope edge along the full length distributes the load far better than a series of individual ties. Overlap adjacent panels by roughly 100 to 150 millimetres so that light does not leak through the seam, and overlap in the direction that lets rain run off rather than collect.
Inside a greenhouse the installation problem changes completely. The structure already exists, so the work becomes a question of attachment points, ridge height and how the net interacts with existing covers.
Fix the net to the gutters or to a continuous rail rather than directly to individual glazing bars, so that the load is shared and a single failing clip does not concentrate force on one bar. Leave a small gap between the net and the polyethylene cover of the greenhouse so that warm air can still move between them; if the two layers touch, the greenhouse loses a significant part of its ventilation area.
A common mistake is to lay the net directly over the roof glazing, where it presses onto the plastic and abrades it over time. Where the roof must be shaded, a separate hoop set above the glazing is usually more durable than a net laid on top of it.
Most complaints about shade nets come down to one of four causes: the wrong shade rate, uneven coverage, wind damage, or excessive heat underneath. Each has a recognisable signature.
Uneven shading across the row. If one end of the row is noticeably darker than the other, the fabric is not level or the tension is uneven. Check the height of the support poles and the anchor points, and look for fabric folds. A shallow sag near the ground or along the seam will also produce a bright strip.
Ponding of rainwater. Standing water in a low point adds weight and can stretch the mesh, creating a permanent sag. Look for the low point along the support line, then either raise the support or add an intermediate anchor to lift the fabric.
Fraying and torn edges. Fraying is nearly always abrasion against a sharp edge, a wire, or a pole. Cover exposed metal ends, smooth the cut edge with heat, and inspect the perimeter at the start of each season.
Rippling in strong wind. A rippling net has little slack and catches the wind like a sail. Increase the sag slightly, widen the spacing between supports, or move to a heavier fabric. Rippling also strips UV stabilisers from the fibres and shortens the life of the material.
Heat still building under the net. If the temperature under the net is not dropping, the net is too open for the light level, the canopy is too dense, or the net is too close to the crop. Confirm with a light meter held at canopy level rather than judging by eye. Reducing the watering volume and improving air movement often matters as much as adding shade.
Shade nets are engineered to carry almost no load, yet they are routinely asked to hold snow, hail or a heavy summer storm. The combination that causes the most damage is a saturated fabric in strong wind. Wet polyethylene becomes dramatically heavier, and if the net is also holding rainwater in a sag, the load on the anchors multiplies.
The table below summarises the common failure modes and the corresponding countermeasures.
| Failure mode | Typical trigger | Warning sign | Countermeasure |
|---|---|---|---|
| Tearing at anchors | Wind flutter, sharp hardware | Fraying at tie points | Galvanised clips, protected edges |
| Sagging under rain | Insufficient intermediate supports | Ponding after rain | Add anchors or raise the line |
| Full panel failure | Wet fabric plus wind load | Sudden drop after storm | Tension, drainage, heavier fabric |
| Uneven coverage | Level support line, folded fabric | Bright band down the row | Re-level and re-tension |
| Premature degradation | Continuous ultraviolet exposure | Brittle, flaking fibres | Rotate, replace, store dry |
Where a structure cannot be engineered for wind load, a common approach is to divide a large area into smaller panels so that a given panel can fail without taking its neighbours with it. Another is to fit a continuous tensioned lock line that allows the fabric to slide and equalise load along the run.
Treating the net as seasonal equipment keeps the cost down and the performance predictable. At the end of each season, remove the fabric, shake off accumulated debris, and wash it with a hose. Dry it completely before folding. Storing damp fabric folded into a stack produces mildew that permanently weakens the fibres, and the first sign of the problem appears as a bad smell the following spring.
Inspect the fabric during removal. Check the edges for abrasion, look along the seams for UV damage, and count the clips so that none are left in the field where they can be driven into a tyre. Repair small tears with a patch of similar material and a length of monofilament line stitched through the mesh, rather than with adhesive tape, which fails in summer heat and blocks airflow over the hole.
If the fabric has been on a structure for two or three seasons and the yarn feels brittle or slides freely under finger pressure, replace it. Ultraviolet degradation is not reversible, and a compromised net that fails in a storm costs more than a new roll.
After the net is up, measure rather than assume. A simple light meter held at canopy level, with and without the net in place at the same hour, gives a direct reading of the reduction actually achieved. Compare it with the quoted rate. If the measured reduction is well below the figure on the label, the cause is usually low tension, side spill from an elevated net, or a gap in the coverage.
Temperature and humidity are equally worth logging for a season. A simple maximum and minimum thermometer placed at canopy height inside and outside the shaded area, with readings taken at the same time each morning and afternoon, quickly shows whether the installation is doing what was intended. Where the measurement shows too little difference, the usual remedy is a denser shade rate, a lower profile, or a combination of the two. Most installations that disappoint turn out to have the wrong fabric rather than the wrong frame.
Installing a shade net properly means treating the fabric, the support structure and the crop as one system. Choose the shade rate from the light level the crop actually needs rather than from a habit or a habitually reused percentage, match the fabric weight to the exposure and to the expected service life, and build a support line that can carry the fabric in wind without stretching it.
相关文档
