Last week I wrote about the Spring Watch survey being conducted by the BBC and the Woodland Trust to monitor the arrival of spring across the country.

Like many gardeners I've made mental notes over the years of the flowering date of the first daffodils, the arrival of swallows, swifts and suchlike, and I thought I'd like to know a bit more about the science involved in recording the timing of natural events.

It's called phenology - the study of the response of the life cycles of plants and animals to seasonal and climatic changes in the environment.

All over the country (and indeed the world) there are people recording the timings of such things as plant budding, flowering and fruiting, bird migration, first frosts, insect activities and leaf fall in the autumn.

The earliest records date back to 1736, and show that some species - ash trees and coltsfoot for example - do not appear to be affected by warmer and earlier springs. Other species, such as oak, hazel and birch, are coming into leaf earlier than they used to, while some typical spring flowers are increasingly being seen coming into bloom in November and December.

One of the unlikely results of warmer seasons is that, because many trees and grasses are flowering earlier and over a longer period, there has been an increase in the length of the hay fever season.

And before all you non-hay fever sufferers have a good laugh at the expense of the rest of us, you might like to ponder the fact that in many parts of the country mowing the lawn has become a year-round activity.

While plants respond to climate change by changing their flowering dates, insects are able to respond by changing their natural ranges.

The comma butterfly is now regularly seen much further north than previously; we saw them at Brockhole Visitor Centre, near Windermere, in 2003 for the first time ever.

Many birds are nesting earlier than they used to, while others are overwintering in this country instead of migrating to warmer parts.

I've even found some phenology records in my own gardening column: on February 5, 1999, I wrote that Hacquetia epipactis was just coming into flower. It's an unusual plant with small, butter-yellow flowers surrounded by a ruff of bright green leaves.

As I write, in the last week of January 2005, the flowers are starting to go over!

Jobs for this week...

Sow broad beans in trays or pots in the greenhouse or cold frame to give them a head start on outdoor sown crops.

Cut away the old foliage of plants of Epimedium and the Lenten lily, Helleborus orientalis, so that you can see the flowers properly before the fresh new leaves are produced.

Mark with short sticks or make a note of any areas where you would like to plant extra spring flowering bulbs. By the time the autumn planting season arrives you may have forgotten where your existing clumps are planted.