US Growing Zones, Planting Dates

I had a question about growing zones, which really opens a can of worms. There’s a lot more to timing for growing than simple fixed dates based on averages.

The question from newsletter reader Pauline:

I live in Deptford, south-east London. I am unsure what zone I am in. Some sites say 8A others say 9. Please can you tell me?

And when do I sow courgettes?

Sowing Dates

I’ll start with the easy part of the question, when to sow courgettes. If you’re sowing undercover, start them off in pots in April. You can direct sow in early May. There’s a chart on the site that shows sowing times for most vegetables. It’s a PDF so you can download it to your device and print it out.

The chart is linked from this page: Monthly Garden Advice which also has a little map showing the different areas of the country and the effect of location on growing times.

The garden planning software will also tell you when to sow and send individual reminders based on your location.

Growing or Hardiness Zones

UK Hardiness Zones Map based on USDA ratings from 2006

This brings me on to growing zones. These are an American system that was developed by the United States Department of Agriculture. More properly called hardiness zones, the USDA published its first official Hardiness Zone Map in 1960. It has been updated several times as more climate data became available, and the most recent updates reflect decades of weather data, not just single cold snaps.

It breaks the USA into 13 zones, subdivided into a & b zones. The ones most applicable to the UK are listed below:

USDA Zones, Minimum Temperature Average (Degrees Celsius)

• 7a: -20.6 to -17.8ºC
• 7b: -17.8 to -15.0ºC
• 8a: -15.0 to -12.2ºC
• 8b: -12.2 to -9.4ºC
• 9a: -9.4 to -6.7ºC
• 9b: -6.7 to -3.9ºC
• 10a: -3.9 to -1.1ºC
• 10b: -1.1 to 1.7ºC

These growing zones are based on long-term climate averages in the USA, not one unusually harsh or mild winter.

What growing zones do not tell you (important!)

Hardiness zones only measure cold tolerance. They do not account for:

• Summer heat
• Length of growing season
• Rainfall or humidity
• Soil type
• Wind or microclimates
• Extreme weather events

That’s why two places in the same zone can have very different gardening experiences. Think of growing zones as one piece of the puzzle, and not the most important by far.

They’re useful for:

• Choosing trees, shrubs, and perennials
• Knowing what can survive winter outdoors

Keys to Success

But success depends on knowing your specific garden conditions, if your plot has protection from wind or frost and, of course, plant care and timing

For gardeners and especially vegetable growing, the most important date is the average last frost date, which guides your sowing. On average. Remember, in the UK we have more weather than climate. When my daughter was born in late April, it snowed and was freezing. Three years later, her birthday party was outside and the sun was blistering hot. Happy days!

Rainfall varies across the country, wet west and dry east. Once again on average, certainly not this year!

The growing season is longer now than it used to be as the climate warms. There’s a wealth of data on the Met Office, although finding it can be challenging. A task for a rainy day.

London

Specific to Deptford, being in inner south-east London, it would benefit from the urban heat island effect, making it typically 1-2°C warmer than outer areas, especially during winter nights.

If you’re looking at planting shrubs and perennials, the RHS provide hardiness ratings Useful to know if you’re purchasing new plants.