K's Garden

Building a large garden on a budget

When can I put my plants out? Wednesday 15 April 2009

Filed under: Uncategorized — kgarden @ 12:00 am

I have heard a number of people ask when their last frost will be or when they can put their plants out or – either for hardening off, or to transfer them from a windowsill to a cold greenhouse – Tomatoes get pretty big if they sit on the windowsill until 1st June!

So here are some steps to review the temperature in your area using wunderground.com – hopefully not too nerdy!

wunderground.com has weather data recorded by personal weather stations. You can find one near you and see what the temperature has been in previous years, and form a view on when the majority of the nights are warmer than, say, 5C and then choose that as your point to put your plants out – on average there will be relatively few nights afterwards when you will have to cover them, or bring them in. Keep an eye on the forecast though!

First you need to get to the Wunderground Personal Weather Station Map page – I suggest you Right Click this link and choose “Open in New Window” so that you can toggle backwards and forwards between these instructions and the wunderground.com site (or print out this page)

You should see a map that looks like this:

Wunderground.com Personal Weather Station Map

Wunderground.com Personal Weather Station Map

  • 1) Use the controls to pan the image, or click-and-drag
  • 2) Use the + / – controls to zoom in

Pan and zoom in on your local area (more stations will appear as you zoom in)

Map legends

Map legends

The “flags” show current temperature (Number degrees C) and the wind direction (number of bars on the the wind direction tail indicate strength).

Major weather stations

Major weather stations

You may have some major weather stations near you – they have a weather symbol next to them. There are a couple of air force bases near me – they are likely to have data going back further, historically, and will probably have more accurate measuring equipment and one would hope better procedures!

Anyway … click on a suitable flag near your location.

Weather Station Details

Weather Station Details

Details of the weather station should appear in a “speech balloon” with:

  • 3) The Station’s name
  • 4) The Station’s code with a link to the detailed data (click on this)

Click on the code link (4)

Daily Summary Data

Daily Summary Data

You should see detailed data for today

  • 5) The data shown is the “Daily” data (for the date above, which should be today’s date)
  • 6) We need to choose a Custom data range – click on this.

Click on the “Custom” tab link to the right

Choose Custom Date Range

Choose Custom Date Range

I have chosen a date range from March – 1st June in 2008 to review.

  • 7 ) The Start Date
  • 8 ) The End Date

Its worth noting how far back the select list for the Start Year lets you choose – generally this is the range of historical data available, for this station – if it is too recent you might want to select a different station, maybe a bit further away, that has a history going back for more years

Press GO

Find a Different Station

Find a Different Station

If you are viewing a Personal Weather Station (rather than an airbase or similar) there will be a “View Interaction WunderMap” link (9) in the right pane which will return you to the map to allow you to choose a different station.

Scroll down a bit to see the Temperature Graph for the date range you have selected:

Temperature Graph

Temperature Graph

The Temperature Graph is a bit titchy (maybe there is a way to get just the temperature, and with a larger image?) but it should give you an idea of historical temperatures and help work out when the majority of nights are above a given temperature – such that you cold put plants out, and bring them in only on odd occasions. Don’t forget to check the weather forecast though!

Looking at this graph 20th April looks to be the date that temperature stayed above about 5C – which would be warm enough in my cold greenhouse for my Tomatoes. There was a cold snap around 18-21 May though. But so long as I kept an eye on the forecast I could take care of the occasional cold snap.

I could look at previous years to see how they compared:

2007 Temperatures

2007 Temperatures

For 2007 20th April looks to be about right too – and that year there were no later cold snaps.


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