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I picked a bunch of basil last week so we could put up some pesto.

I brought the leaves inside, washed them, and set them on the counter in the kitchen while gathering other ingredients for pesto (grating cheese, etc). Many of the leaves started to turn black -- especially at the tips and in the veins. It didn't seem to affect the quality of the end product.

I'm planning to dry the next harvest batch, and I'd like to make sure it doesn't turn black during drying.

  • What makes basil leaves turn black?
  • What should I do post-harvest to prevent this?
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have you experienced a sudden drop in temperature where you are? –  Mike Perry Jul 28 '11 at 16:43
    
@Mike: This is only post-harvest (not leaves still growing on plants in the beds), if that matters. This was last Friday, temps were in the neighborhood of 90°F with 173% humidity, so we had the A/C running in the house. Does a temp change from 90 to ~78/80 cause blackening? Or washing in cold water? (And yes, outdoor temps change constantly -- 90s last week, then 50s overnight this past weekend.) –  bstpierre Jul 28 '11 at 17:03
    
I was thinking daytime temperatures, nighttime temperatures do not normally have a negative effect on plants, unless there is a freeze (or near freeze). In fact most plants need that cooler nighttime temperature, it helps them remain healthy eg Allows them to recover from the midday heat of the summer sun. –  Mike Perry Jul 28 '11 at 17:09
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also take a look at this answer, Why does my fresh basil turn black?. –  Mike Perry Jul 28 '11 at 17:10
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I would use a pair of scissors..... –  lamwaiman1988 Aug 8 '11 at 1:45

3 Answers 3

up vote 4 down vote accepted

From an article linked in a comment by Mike Perry:

Your basil is turning black because it is oxidiizing [sic] ... the exposed new surfaces will go black becasue [sic] they are exposed to the air around them

and

Be very gentle with it, it bruises so easy.


My experience [not] following this advice leads me to believe bruising was the cause:

  • I picked a couple leaves yesterday for a salad, handled them gently, no blackness.
  • I picked a couple of leaves for salad today, but I stacked some cucumbers on top of them in the picking basket and they had some black spots where they had been bruised when I went to slice them onto the salad.
  • In my original question, I had packed a bunch of leaves into the picking basket (which was overflowing, so they were packed). It must have been this packing that caused some bruising and led to the blackening that prompted my original question.
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I have had that happen when I put the basil into a sink of water to wash instead of washing the cuttings under running water. The newest growth turned black and even older growth did at the "crotches". I maintain indoor plants for a living, this is exactly the same reaction as over-watering does to a plant. My understanding in that event is that it is caused by root rot. In the case of my basil, I think the high growth areas soaked up so much water from the sink and had no way to drain it again, as there is an incomplete circulation system once cut. The effect was the same as bruising, or early rot. I hesitated to use them in pesto, but they dried just fine. No recurrence since I have stopped washing them in a sink of water, just running water.

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From your comments above, the heat drop seems like a very likely reason. Basil plants like the heat outside and a sudden drop in temperature from outdoors to indoors is sufficient to turn it dark. I don't know the chemical reaction, but it could very well be oxidation as in the link from Mike's comment

This page also mentions that leaves turning dark could be an indication of recent rooting and suggests that you repot it with with some good potting mix.

B[l]ack usually indicates that its under stress due to recent rooting. If you cut off leaves and chop them, they also turn very dark. A soilless mixture may be a bit too 'clean' for it too, as compared to a potting soil. Suggest that it gets repotted again, and try to remove some of the soilless mix, and replace it with regular potting mix.

Although in your case, they turned black only when you brought it inside, watch out for leaves starting to turn black on the plant, in which case you can try following the advice above.

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They're growing in a garden bed, and they've been in the ground for a few weeks, so I don't think root shock is the problem. The plants outside still look fine (they're almost ready to cut again). I'm leaning towards temp change and/or rough handling as the cause. –  bstpierre Jul 29 '11 at 3:20

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