How to treat yellowing grevillea?

ashjell

Hi team,

I have a question regarding grevilleas with yellowing leaves. They started off on the top (new foliage) with the typical yellow with green veins so I applied 3 treatments of foliar iron chelates. No change. Now after lots of rain the yellowing has progressed and now other plants are showing symptoms. 

They are all in pots waiting for my house to be build so I can plant them. They are in native well draining soil, fed with osmocote native in autumn during potting up. I also gave liquid bush tucker as advised by a nursery.

Did ph tests today on the worst affected and results vary between 4 1/2 and 5 1/2. Healthy looking grevillea test was ph 6. So I'm guessing it's too low and this is causing the chlorosis and not iron deficiency. 

Could there be any other causes?

Any help would be great.

Photo shows grev moonlight front & bonnie prince Charlie behind. 

  

Is my best bet to raise the soil ph with dolomite? I've never done this before.

JacobZ

Hi @ashjell,

Thanks for the detailed background. It sounds like you’ve done everything right so far. 

From your description and pH readings, the most likely cause of the yellowing is excess soil acidity, not iron deficiency. Other potential issues could include root binding, compacted mix, or root stress, but given the timing and pH, low soil pH is the most likely culprit and should be our focus.

Grevilleas prefer a pH of around 5.5 to 6.5, so the readings of 4.5–5.5 are too low. At this level, key nutrients like phosphorus, calcium, and potassium become less available, and excessive iron or manganese uptake can actually cause toxicity symptoms like chlorosis.

The recent heavy rain may have leached nutrients and further lowered the pH in your pots, while also stressing the roots and limiting oxygen. 

Your iron foliar sprays likely had no effect because the issue isn’t a lack of iron but the plant’s inability to use nutrients properly in overly acidic conditions. To correct this, apply a small amount of garden lime - around a teaspoon per 20cm pot - and retest pH every few weeks. Avoid overcorrecting too quickly. Make sure the potting mix is still draining freely and not staying too wet or breaking down. 

Avoid further foliar feeding for now and consider flushing pots with clean water if you’ve been applying fertiliser regularly, to avoid salt buildup.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob