Scarifier or power rake...which is which?
Most scarifier machines came with two cartridges. Most people pick one at random or just use the same one every time.
They do completely different jobs.
The scarifier cartridge (below) is the one with fixed blades. It cuts deep into the lawn and rips out thatch - that spongy layer of dead grass, old roots and debris sitting between the surface and the soil.
If your lawn feels soft underfoot, or you can see moss has taken hold, this is the one you need. It is aggressive by design.
It will make your lawn look awful. So when your partner questions your sanity, just respond with "trust the process".
See below Scarifier cassette. 
The power rake cartridge (below) is the one with spring tines. Think of it as a mechanical version of a hand rake. It works the surface - lifting light debris, loose moss and dead material without cutting into the soil.
If your lawn just needs freshening up, this is the gentler option.
See below Power Rake cassette.
Which one should you use right now?
Spring = power rake. You want to tidy the lawn up and clear winter debris without being too aggressive just before temperatures rise. A light going-over to give the grass a clean start.
Autumn = scarifier. That is when you go deeper. Thatch removal, heavy moss clearance, the full reset. The lawn has rain and mild temperatures to recover before winter.
If you use the scarifier in spring, you risk opening up the lawn too aggressively right before dry weather. If you only ever use the power rake, you never actually deal with the thatch underneath.
Right tool. Right time.
As promised, lawn tips in under a minute.
Liam
Founder, Great British Lawn Co.
