Dendrobium Speciosum with burn marks on leaves.

[DennisKennedy]

I suspect my problem may be overfeeding but I cannot rule out too much sun.

My Dendrobium Speciosum has developed burn marks on the edges of its leaves with just a couple of marks in the centre of the leaves. I note too some tip burning on my Dendrobium Kingiannum.

Is using the recommended strength of orchid food weekly a mistake. I recently read that I should treat half the recommended strength as the usable standard and increase or decrease slightly from there.

As a complete novice, I would welcome your comments.

[Ray]

"Recommended-" and "half strength" are meaningless. What is the formula, and how much are you using?

My guess is that you are using too much, but without numbers, there's no way to know for sure.

[DennisKennedy]

Ray

I have been interchanging two fertilisers both commercial brands. The first is Aquasol (N 23% P 4% K 18%) at 4g per 5 litres of water. The second is Yates Orchid Food (N 21.5% P 8.3% K 13%) at 10g per 4 litres of water. These were the strengths recommended on the packs.

I have been feeding once per week since flowering finished with two fresh waterings in between. At this time of year 3 waterings per week are sufficient though this will increase considerably when our summer arrives.

Dennis

Since writing the above I have availed myself of the fertilizer PPM calculators you make available on your site. Along with the associated info you provide they are fantastic for someone with my lack of experience. Greatly appreciated.

[Undergrounder]

Dennis i'm not sure since i don't grow specs to show-quality standards, they all just grow naturally in the yard, but i'm guessing the burn is from over fertilising. Also if they're not used to full sun, then a 30+ day with full sun and dry wind will burn them. If they're used to sun though, only the very hottest 35+ days will hurt them.

I have a string of speciosums in a rock, all of which i never ever watered (natural only) and never ever fertilised for about 6 years. They get full sun all day and also get eaten by scale and den beetles occasionally. As you can see from the pics, they're doing fine. They were totally neglected during the drought and some of them even self-seeded and spread along the rock. They're about the hardiest orchids there are.

If the fertilisers you have are intended for normal garden use, then using their reccommended strength weekly is too much. The oft-used phrase is "WEAKLY, weekly". I would use HALF the reccommended strength weekly and make sure they're not getting over-watered, they need to dry out totally between waterings.

[Ray]

I think you are overfeeding, and the two formulas are not significantly different that you need to use them differently.

If we assume the bulk density of the fertilizers to be about 1g/ml, then you're likely feeding the first formula at a rate of 180 ppm N and the second one at 525 ppm N. The first isn't so bad if you feed only every other time, but the second one is WAAAAAY to strong of a concentration to every use on your plants!

My approach would be to use a moderate dosing at almost every feeding. I have a varied collection, including a speciosum raised from a tiny seedling, and I feed at 125 ppm N at every watering.

[DennisKennedy]

Undergrounder

I love your speciosums. Where abouts in Oz are you?
I am in Melton, 35k West of Melbourne. Our hot dry northerlies and low rainfall probably prevent us from growing them here without some sort of shelter. I use a simple 50% shade house.

Thanks everyone for your help

Dennis

[Undergrounder]

Originally Posted by DennisKennedy

Undergrounder

I love your speciosums. Where abouts in Oz are you?
I am in Melton, 35k West of Melbourne. Our hot dry northerlies and low rainfall probably prevent us from growing them here without some sort of shelter. I use a simple 50% shade house.

Thanks everyone for your help

Dennis

I recently added another level or specs to a lower crevice in that rock, and after a few of the hotter days we got last month they've yellowed significantly and a few of the smaller kings lost leaves. I just stumbled upon an old pic i took just after i planted them. I'll take a photo of what they look like tomorrow, its interesting to see the difference.

It just goes to show both how important it is to establish plants gradually to new conditions, but also how resiliant specs can be once they're properly established. I'm hoping i can get them through summer without too much damage, but after that they'll be fine. I remember another grower telling me once that if you bring your specs in and out of the sun every season they'll burn every year but if you leave them out they'll burn initially but will adapt.

I'm in Sydney... I wish i had a shadehouse, it would make it a lot easier. Unfortunately at my place its either straight sun or deep shade. I do have shadecloth nailed to the railing of my balcony so that serves as a shield for my other orchids in summer.

[Undergrounder]

Added comparison:

[DennisKennedy]

Undergrounder

I see what you mean about the yellowing. I'm afraid I would be panicking by now and probably doing more harm than good by trying to molly coddle them.

Dennis

[Undergrounder]

Yeah i'm sure i'm burning them but it's tough love for the poor specs in this house. The ones that have lived up the top for years stand as an example to the newbies below on how they need to suck it in and quit complaining. I got them from shadecloth growers so they're a bit spoilt.