Use of Polythene
In the UK, Polythene sheeting is generally used in Poly tunnels to protect plants from severe frost or to force plants into growth earlier in the season.
Here in Spain, when planting lawn seed in the Winter, we recommend the use of polythene sheeting. This can significantly increase the temperature for warm and cool season seed, and aid germination. And particularly in the case of Warm Season grass seed can extend the sowing season well into Autumn.
When planting lawn seed, polythene sheeting is great for keeping moisture in the soil. It also increases the soil temperature during the day. However just because there is condensation on the inside of the polythene, do not assume that the soil is moist. Check daily to ensure that the seeds and soil has not dried out, and if so remove the polythene and water the seed. Keep the polythene in place until the grass seeds germinate. In fact keep the polythene in place for a week to 10 days after the first seeds germinate to encourage the stragglers to germinate. After removing the sheet you will still need to keep the soil damp for a week or so, see watering a newly germinated lawn.
This photo is a 1M x 1M patch of our wintergreen nursery grass, sown in March 2011, 4 weeks after germination. It has had its first cut and is ready for its first application of fertilizer. To the left is the next test patch of lawn, sown under geotextil.
As the temperatures start to rise above 20 degrees in the Spring, polythene should NOT be used as the daytime temperature underneath the sheet can get very high and ultimately kill the seed. Instead a permeable membrane (obtainable from most builders merchants) called GEOTEXTIL should be used. Another advantage of this material is that the seed can be watered without removing the membrane, as the water passes through. Note: If gray is an unpleasant colour, then similar green sheeting can be used, but is a little more expensive.
Tip: If you have problems with birds eating your grass seed, then polythene sheeting/geotextil is great as an absolute deterrent.
You can use polythene sheeting/geotextil on a 1M x 1M section to repair a lawn, long 1M strips or large sheets depending on your requirements.
For example, if you intend sowing an entire lawn, you can sow 1meter strips of seed, separated by 1Meter strips of unseeded soil. When the grass seed has germinated, you can go back and sow the unseeded strips.