There are different types of lawn scarifier. For smaller lawns a hand held version such a spring bok rake might well fit the bill, although this can be backbreaking work over large areas - better suited for such gardens are a powered version. If you have a severe thatch problem then even over a small area, a powered scarifier will remove more material than a hand rake.
Deep scarification is normally performed in the early to mid autumn as part of an annual lawn care program or in spring to aid moss control. This allows time for the lawn to recover whilst still in the growing season and is less likely to suffer from droughts or frosts. Most motorized scarifiers have a depth setting to help you achieve the desired results. In truth light lawn scarification can be carried out at any time during periods of strong growth. Great care should be taken to make sure that it is not excessive or coupled with a drought. Ensuring that there is enough growth to allow recovery following deep scarification will prevent entering the winter months with a thin sward, and therefore inviting lawn issues such as moss invasion and disease. If you have a moss problem, scarifying and raking should be utilised in the spring following a moss killer treatment - again dont scarify too early in the year when temperatures are still cold, it is better to wait until temperatures to rise in order to get the best recovery growth. Effective scarification will also increase the effectiveness of other lawn care processes such as overseeding and top dressing, which can be applied immediately after scarifying.
This depends on a lot of factors - the type of grass, the condition of your lawn, and the time of year. As mentioned above, spring and autumn are the main months when scarifying should be utilized as these are the periods of best growth and therefore healthiest recovery. If you fertilize your lawn and/or dont collect your grass clippings then you should consider scarifying more regularly as thatch will build up quicker under these conditions. Start by scarifying once and monitor your lawn so you understand how long it takes your particular lawn to recover, and if issues such as moss coverage have been improved. In most cases annually may be enough, in more extreme situations scarifying or using a lawn raker every few months may be required. If you have your scarifier set a to a lower setting then more frequent passes may be possible, whereas a deeper setting will have a more profound effect and require longer to recover from.
A small layer of thatch (1-2 cm) is actually beneficial, resulting in a hard wearing and resilient lawn. Any more than this and your lawn becomes exposed to several issues. During periods of wet weather, the thatch will hold moisture impeding drainage and like wise during dry weather prevent the moisture available reaching the roots. In addition it can lead to shallow rooting and therefore a lawn vulnerable to attack from fungal diseases.
Thatch accumulates by several means. Over watering results in a lack of air within the soil, inhibiting the bacteria required to break the thatch down. Over fertilizing creates a situation in which the natural process of thatch breakdown are unable to keep pace with the rates of fretilized growth. Not collecting grass clippings adds more material to the thatch layer - large amount of grass clippings can smother the grass and take a long time to breakdown.
Dont over fertilize your lawn. Dont over water during periods of drought. Water just enough to keep the soil moist, not saturated. Regular raking, brushing and scarifying will prevent build up and lateral growth. Good and regular aeration will help with drainage and provide the best environment for bacteria and microbes to breakdown remaining thatch.
Very affordable scarifiers are readily available on the market to suit all budgets. It is true that for a tool which is only used once or twice a year, you may consider it a luxury purchase but if you are someone who strives to have a healthy and vibrant lawn it really is a tool you shouldnt be without. If you dont want to own one yourself you will probably be able to hire one on a daily rate basis or why not share one with neighbours, friends or family?
Whilst scarifying alone will improve the health of your lawn over time, the best results involve scarifying as part of a wider lawn care program. Especially, aeration (or hollow tining), top-dressing, overseeding and feeding.
This is really down to you, your lawn size, its condition and your budget. Other factors to consider are weight, storage spage required for the various models and whether you would like a manual rake or petrol or electricity driven model. If you have a very large lawn then the length of power cable may become an issue. In gerneral a rake brushes through the surface whereas a scarifier gets down into the grass have a bigger impact.
Lawns can look great from a distance but take a closer look and issues can be developing all the time. And as with so many things prevention is better than the cure, severe moss and fugal problems can take a good deal of time and effort to correct.