Majority of the fruits develop colour only after they reaches their full size. Strawberry being a non-climacteric fruit, it is best to harvest at defined maturity level. For a particular cultivar, an expected colour can be observed. If you the know the colour, you can use "RHS colour chart" of identify the same maturity level at different pickings

I agree with Kapil in the use of RHS colour chart. I would suggest taking temperature data and correlating the stage with heat unit accumulation using the growing degree day concept once this is defined the time for reaching harvest stage can be predicted accurately using this data. 

Use RHS colour chart or use minolta colour reader. Beside using heat unit, may be you can calculate day after flower bloom (DAFB) as a guide to complement with your predetermined maturity index and of course the size of particular cultivar. Good luck..

Harvesting

Strawberries are usually hand-harvested, graded, and packed in the field. Harvest when temperatures are cool, either early in the morning or evening. Pick berries that are ripe but not over-mature (pink or red but not mushy)2/3 red, leaving only a short stem and the calyx, and place directly into pint baskets or flats. Berries that will be used for dipping should be harvested with long stems and placed into flats so that the stems do not touch other fruit. Avoid bruising or nicking fruit. Transport immediately to the shed or shade in the field if this is not possible.

Cool fruit immediately after harvest by forcing cool air through the flats. This is preferable to room cooling, as forced air can cool berries to 34 degrees F within an hour, whereas room cooling may take 9 hours.

Thank you very much all, for the answers, I have some follow up questions.

1. Mr. Kapil and Paul, about the RHS colour chart, it should be purchased right? Are there any soft versions which I can download?

2.Mr. Jupikely, I have tried the Days after blooming method, but it is not much practical to apply for a large green house. 

3. Mr. Paul, I have read about the growing degree day concept, but I'm not much familiar, how it can be applied to this scenario. If you have any document or suggested reading, can you please attach it here?

4. Mr. Jupikely, I'm currently using minolta colour reader. So, if it is to be used for selecting similar maturities, you mean that I should consider similar L*a*b* values?

5. Mr. Sidiq, I'm currently following the methods you suggested. Thanks. But 2/3 red thing is difficult to use for the cultivar I'm using. It is called Sachinoka and is it is picked 2/3 red, the top part is completely white. It kind of reddens whole fruit wise, which makes it much more difficult.

I dont think RHS colour chart is available in soft versions. You need to buy it.

For DAFB, first you need to identify how much days it takes from full bloom to the maturity you are harvesting. Repeat it for different fruits having different full bloom date and its average can be considered as DAFB.

For growing degree days, along with DAFB you need to calculate max. and minimum temperature from full bloom to harvesting date. With correlating with temperature above a base temperature, you can get data on how much temperature is needed from flowering to fruiting. It is one of the method proposed for date palm.

Reviewing Growing Degree Day Accumulations at Clayton

— Written By Barclay Poling Fig. 1.  It was rather shocking to see this many open blossoms on Chandler yesterday on a farm north of Greensboro, NC.  With the usual interval of 30 days from open bloom to harvest, this crop of Chandler plugs that had row covers this winter could be ready for harvest in mid-April.

Dear Agents, Agronomists and Growers,

I know we’ve all been observing the rather spectacular advance of this year’s strawberry crop since last Monday, March 7.

These are the daily Growing Degree Day accumulations at Clayton since March 1st:

Fig. 1.  Daily Growing Degree Day Units since March 1st.  With many recent days approaching 20 GDDs per day, and yesterday exceeding 22 GDD units, it is no wonder we are seeing such rapid changes in the strawberry crop since 3/1/16.  At this time of year it is more common to see GDD numbers in he single digits (not upper teens and low 20s).

Believe it or not, we also saw this kind of heat accumulation occur back mid-to-late December:

Fig. 2.  Note the remarkable GDD units experienced back in mid-to-late Dec at Clayton Central Crops.  That warm spell triggered blooms at Christmas in Camarosa as well as some other early varieties.  THOSE BLOOMS DIED, OF COURSE, BUT LEFT GROWERS WITH ANOTHER PROBLEM…BOTRYTIS INOCULUM GALORE (FIG. 3).

FIG. 3.  BOTRYTIS INOCULUM (3/14/16) ALL OVER THE DEAD BLOOMS AND PEDICELS OF A STRAWBERRY PLANT THAT WENT INTO AN EARLY BLOOM BACK IN DEC 2016.  THESE WERE SUBSEQUENTLY WINTER KILLED, AND NOW SERVE AS A VERY NICE SOURCE OF INOCULUM.

In case you are wondering, here is the

STANDARD FORMULA FOR CALCULATING GROWING DEGREE DAYS:

TAKE THE DAYS HIGH AND THE DAYS LOW TEMPERATURE ADD THEM TOGETHER AND DIVIDE BY 2. SO IF THE HIGH FOR THE DAY IS 70F AND THE LOW WAS 50F THAT GIVES US A TOTAL OF 120 DIVIDED BY TWO EQUALS 60.

OUR BASE TEMPERATURE FOR THE STRAWBERRIES IS 50F. YOU NOW SUBTRACT 50 FROM 60 AND GET 10 GDD.

USING THIS FORMULA, YOU CAN DETERMINE THAT YESTERDAY (MARCH 16) WE ACCUMULATED 22 GDD UNITS AT CLAYTON:

EXAMPLE:

MARCH 16TH:  MAX =85.1, MIN=59

SO, 85.1 + 59 = 144.1

DIVIDE BY 2 = 72.05

SUBTRACT BASE 50 FROM AVG:  72.05 – 50 = 22.05 GDD UNITS

Date/Time of ob

Max Temperature

Min Temperature

at 2m (F)

at 2m (F)

03/10/2016

78.8P

57.9P

03/11/2016

81P

64P

03/12/2016

76.5P

56.7P

03/13/2016

76.6P

59.4P

03/14/2016

77.2P

61.7P

03/15/2016

76.8P

56.3P

03/16/2016

85.1P

59P

AS OF TODAY (ST PATRICKS) WE HAVE NOW ACCUMULATED A TOTAL OF 213 GDD UNITS; AS OF LAST FRIDAY (3/11) WE HAD 143 UNITS. WHEN YOU REACH ABOUT 450 UNITS, YOU CAN EXPECT TO BE DOING SOME PICKING!

FIG. 4.  GDD UNITS THIS YEAR AT CLAYTON:  7.9 IN JAN; 60.7 IN FEB; AND 144.9 FROM 3/1-3/16.  TOTAL = 213.5.

POTENTIALLY, CLAYTON WILL REACH 398 GDDs on April 14th (few ripe) and by April 19th it will be at 456 GDDs (light pick) and over 500 by April 22 (full picking).

FIG. 5.  Cumulative GDDs at Clayton Central Crops using temperature forecast information for this zip code (27520) from AccuWeather.  This will definitely change between now and even next week!  But, as of today, it does look as if we will be reaching approximatly 400 GDD by mid April, and this is when I would anticipate some light picking.  By the following week (around April 21), there should be a relatively full picking.

WRITTEN BY

Dr. Barclay PolingFormer Professor and Extension Specialist, Strawberries and MuscadinesHorticultural Science - NC State UniversityUPDATED ON MAR 17, 2016

Thank you very much all.

Dear Sakalya, a few things to consider after I read through all the answers given to you. All the suggestions are good to define harvest date, but none of them will be good enough for postharvest treatments if your experimental unit is too reduced. So, I suggest, use as harvest index either method suggested (I use mostly at least 3/4 colored fruit), but make sure you have enough fruit for each replicate (the more replicates/treatment - for strawberries five is a good number - the better) and experimental units of at least 500 gram fruit. Success with your trials. Best regards, Rjbender

Dear Renar, thank you very much. I was thinking of all the suggestions and reading on the matter. I came across the three forth red harvesting index for strawberry, but that is bit difficult for the variety that I am using, as it starts reddening the whole fruit. It turns pinkish, orangish and then red. It is very difficult to identify through the naked eye. I'm of course using a large sample, of about 10 fruits to reduce errors. Another thing is that, fruit size also differ widely and brix values seems to vary according to the fruit size. I'm thinking of categorizing them in to 3 weight groups. In that case I can measure, atleast about three fruits per category. Any suggestion?

Dear Sakalya, I still think your number of fruit is too small. For color measurements we use allways at least 5 fruit/experimental unit and it does not matter if we subcategorize or not. For other analyses (like sugars, acids, vit C and etc...) again only three fruit is too reduced - if you are limited by the total amount of fruit available, eventually you could use only two sizes of fruit (this would reduce in one third the total amount you need to run your experiment). 

Dear Renar, Thank you for the suggestions. Using only two sizes of fruits seems OK for the present circumstances.