Gerbera & Roses growing article January 2008

Western flower thrips (WFT), Frankliniella occidentalis; 

Some growers who use Integrated Pest Management (IPM) find it difficult to control Western flower thrips (WFT); nearly every effective chemical for WFT has an effect on the biological predators and will reduce the balance between the insects and the biological predators. 

At low temperatures (15^oC), WFT requires 13 days or more to complete a generation. However, at (30^oC), the generation time decreases to 4.3 days and the population increases 8.5 fold with each generation.  

Before the temperatures rise you need to build up the predators so they can handle the faster generation time, when you have to spray to get the thrips under control it’s very important to spray at least every 5 days when temperatures are high. 

Other measures that could be taken; 

1-  Remove weeds in the greenhouse and around the greenhouse, 99% the thrips will come from outside the greenhouse especially when there are weeds and high grasses around the greenhouse.

2-  Remove all the effected flowers and throw them away far from the greenhouse, sometimes I see the damaged flowers just outside the greenhouse where they could breed again and come back in the greenhouse.

3-  Hang up (blue) sticky traps and monitor the numbers of thrips on these sticky traps, when the number of thrips increases you know that you have to take action. 

If you can’t control the WFT in your flowers when you use IPM the best thing to do is to stop using IPM for a few weeks / months and use harsher chemicals to get the WFT under control. After the required waiting period for the used chemicals you could start again with the IPM.  

Zinc; 

When you take drain water samples, the sample could show a high level of zinc, most of the growers then reduce the amount of zinc given or take out the zinc completely (normal rate +/- 70 gram of Zinc Sulphate). When you take out the zinc completely there is a risk that the plants could show zinc deficiencies, my advice is to keep at least 75% of the standard recommendations in the fertilizer program. 

Zinc deficient younger leaves are splotchy and chlorotic in colour, but the definitive symptom is that one half of the leaf blade ceases to expand and develop, while the other half is normal in size and shape. This uneven development causes the leaf to bend into a "C" shape.   

Sometimes the flowers could be affected without showing deformed leaves; I remember one case by a gerbera grower where we saw uneven shaped and smaller flowers before we saw any affected leaves. 

Reasons why the zinc could be high in the drainwater; 

1-  Sometimes the zinc level in the drainwater could be high because the drainwater gets in contact with the galvanized bench system. The galvanized iron bench system could release the zinc in the drainwater, but this zinc is not easy available for the plants.

2-  The gutter which collects the rainwater from the roof could also releasing zinc especially when the gutters are not coated. 

Reasons why the zinc could be low in the irrigation water; 

1-  When you use bore or mains water there is nearly no zinc in the water, especially when you change from rain water to bore or mains water you should increase the zinc.

2-  With EDDHA iron (high PH iron) the zinc could be reduced, I experienced this when we started growing gerberas on rockwool in 1984 in the Netherlands. When the PH is high only use 50% EDDHA iron and 50% normal iron.

3-  With a high PH the plant has more trouble to uptake the zinc / trace elements. 

Irrigation; 

Sometimes a growing media could become to wet and worried growers start to reduce the amount of water given to the plants, the effect of this could be that the growing media could become to dry. When the growing media is wet then dry and then wet again the roots could be affected with deceases (especially Phytium). 

A few things to avoid over watering or keep the growing media to dry; 

1-  Make sure that the pot or growing bags have enough holes at the bottom to get rid of the excess water, often I see plants standing in a pool of water or growing bags with not enough drain holes.

2-  Measure the drain % and EC a few times a week, when the EC gets higher the amount of water given to the plant is most likely not high enough.

3-  When you take drain water samples the level of Sodium and Chloride indicates if you give enough water. Test in the Netherlands have show that a sodium level above 5 MMol could give your plants a slower growth and smaller flowers.

4-  Start with the irrigation 1 hour after sunrise and stop around 3 - 4 hours before sunset, because the time of sunrise and sunset change during the year this start and stop time needs to be adjusted at least every few weeks. 

Many growers give Agriphos (phosphoric acid) to improve root growth, you could give every 4 or 6 weeks 2 or 4 litre Agriphos per hectare (the amount depends on the volume of growing media and what type of crop).  Some growers have an extra tank by their irrigation unit to give the Agriphos to the plants, others wait until the B tank is nearly empty and then add the Agriphos. It’s important that the Agriphos is given in a one day period.

**Although we trust that this information could be of considerable help to the success of the crop, we cannot accept any liability for the results of your crop. All the information in this growing newsletter is without committing ourselves.

This article is written by Job Roskam of Roskam Young Plants Pty Ltd. and also published in the Australian flower industry magazine.