Why do my Tomato plants turn purple?

A tray of our Funky Fresh Kitchen Tomato pots, College Challenger I think.  Getting a little "hard" i know.

We walk a fine line at this time of year, we lost the first two batches of Basil before even sending them out. The demand is so crazy that we have to try to have them available but it doesn't take much to put them off.

I've also had inquiries this week about Tomatoes losing colour. Perhaps more accurately taking on the hue of a three day old bruise, purple and yellow. This colouring is a symptom of the plants being hungry. Especially for nitrogen and phosphorus.  Tomatoes are a great crop, we have significant demand and they are relatively easy to grow except for the fuss of staking them. There are two issues that we have to watch for: stretching and black spots (haven't seen any yet this year!). To protect the plants from the winter and early spring elements we hold the plants in a poly house. This ensures the plants can grow without stress but encourages them to grow very tall and skinny and if the foliage is too soft the black spot bacteria can infect the plants easily. We try to control this culturally by deliberately under watering them and by supplying only just enough fertilizer to keep them green but not enough to grow like crazy. Another trick is to use what is often called a 'native plant' fertilizer that is very low in phosphorus, this encourages strong cells and inhibits 'stretchy' growth.

When the plants get too hungry they start to change colour: yellow foliage indicates plants need more nitrogen and the purpling is a sign that a little phosphorus is needed. In fact in the nursery this colouring is quite a good sign because we know we can generally turn these plants around very quickly with a little extra liquid feed. But here's the down side, when we are managing plants so closely they can start to look very hungry very soon after leaving our loving care. Now that the weather is warming we can take a calculated risk and move plants outside sooner, this hardens them up naturally and allows us to start applying a little extra fertilizer. In the garden centre a weekly application of complete, liquid fertilizer should help maintain colour and shelf life but I understand that this is not always possible.

For gardeners, a little yellow and purpling is not at all a bad thing.  Once plants hit freshly prepared soil or quality potting mix they jump away, I'd suggest much more reliably than plants that go into the garden too soft. Just make sure it's purple shading, not small dark spots.

Hope this helps, please don't hesitate to call if I have only raised more questions.