When to Aerate the Lawn
This seasonal schedule should give you all you need:
March to November
Spiking with solid tines (2" to 6") using your lawn aerator sandals, rolling lawn aerator, garden fork, powered or towed aerators can be done at any time from March to November. Repeat as often as you like as it's very beneficial. Avoid very wet soil that smears or when frost is imminent, otherwise get stuck in.
Shallow spiking or pricking is purely for the drier months when the spikes won't penetrate much usually prior to seeding, fertilising or watering. This will let the lightest of showers get into the soil.
Spring and Autumn
You can add slitting or chisel tining (flat spike rather than a round spike) to the spiking if you have the appropriate powered equipment or have a ride on mower with the attachment. If you have a slitter then spring or autumn is great. Late autumn, October to November is also good as the ‘pruning’ effect of these tines works well on root development throughout the cooler months. Watch out when it gets colder or warmer as the slits can open up quite badly with frost or during dry weather!
Coring with a hollow tine using a hollow tine aerator is the more invasive or the treatments and as with other invasive procedures should only be done when good recovery can be achieved. I suggest end September through to mid-November as the target months because the weather will be getting wetter rather than drier. The aerator will improve drainage and increase soil temperature; ideal at this time of year. If you aerate the lawn in the spring on the other hand, the additional drainage and drying effect could cause problems for your lawn if the weather becomes prematurely hot and dry. Once again, watch out when temperatures drop as any freezing soon after hollow tining will cause the lawn to heave up!