Pot or ground?

In a pot, and if you're flushing the medium as you water, FP 9-3-6 at production rate is a very good choice. A good starting point would be to fertilize every 3rd or 4th time you water. It's best to tie the frequency with which you fertilize to the number of times you've watered and it's rained any notable amount than to go by the calendar; this, because watering/rain is what primarily dilutes the fertilizer salt concentration in the soil solution. Of course, the purpose of flushing as you water is twofold. It prevents accumulations of dissolved solids that will eventually limit water and nutrient uptake, and it prevents the ratio of each nutrient to all others from becoming badly skewed, which causes antagonistic deficiencies (too much of nutrient A can limit uptake of nutrient B ...... and C and D ....... ). For example, an overabundance of phosphorous can limit uptake of calcium, potassium, copper, zinc, and especially iron, and cause excess (synergistic) uptake of magnesium.

In the ground, no one can give meaningful advice w/o knowledge of what nutrients are already in the ground and available for uptake, and taking that into account when issuing recommendations.

Al

Mandevillas are beautiful plants, and I can understand wanting them to cover a large trellis. When it comes to fertilizing, the best soil for plants and grass would be one that is rich in organic matter and nutrients.

For container plants like your Mandevilla, it's important to use a high-quality potting mix that includes ingredients like peat moss and perlite to promote healthy root growth.

As for fertilizers, I've found that using a balanced liquid fertilizer with equal parts nitrogen, phosphorus, and potassium works well for Mandevillas.

During the flowering season, I would recommend fertilizing every two weeks or so to keep the plant healthy and promote blooms. Just be careful not to over-fertilize, as this can cause burn and other issues.

In my own gardening experience, I've found that using organic fertilizers like compost tea can also be effective for promoting healthy plant growth and flowering.

These natural fertilizers provide a steady supply of nutrients and are less likely to cause burn or other issues.

I hope this helps, and best of luck getting your Mandevilla to cover that trellis!

@tapla (mid-Michigan, USDA z5b-6a) My original post did say "container Mandevilla" so most of your post is on target.

* Could I just use Osmocote Plus to establish a base level of nutrition and then complement that with a once-a-month synthetic fertilizer at 9-3-6?

* I added photos to the original post. Based on how the leaves currently look, would you say this is a lack of nutrition that is likely to be addressed by proper fertilizer?

* I read online that Mandevilla likes a heavier phosphorous ratio, to encourage flowering. I guess this might suggest it would respond to a rose-type fertilizer? What is your thinking about ways to promote heavy flowering?

@tapla (mid-Michigan, USDA z5b-6a) This plant is in its second year in a very large pot, and I think it is nowhere close to root bound.

The soil I mixed for this is maybe even too well draining. It has a problem holding onto water, so I am going to place it on drip irrigation soon. Unlikely that the roots are too wet here....

A potential problem with this soil - which was made when I had a little less experience with bark as a substrate - was that some garden waste compost was used in the mix instead of just composted bark. For reasons I do not understand well, garden waste compost makes a fantastic mulch for in-ground plantings, but plants simply do not like it as a component in container soil. This is about more than particle sizes, and this particular garden waste compost is very fast drying.

I of course use carefully sized bark in gritty mix and anything remotely like 5-1-1 although I have had a problem with such bark when used for plants like Rhododendron or Azalea. The Rhododendrons do extremely well for 18 months, and then they start to head downhill FAST until I change out the soil. I wonder if the bark starts to rob nitrogen from the soil as it decomposes?

To contrast, I found a local supplier of composted bark that seems much more stable over time, probably because it is already being composted.

@GardeningZilla How are you making your compost tea?

When it comes to fertilizing, the best soil for plants and grass would be one that is rich in organic matter and nutrients. Please expand on this thought. The most productive container medium I've ever Very unlikely. Nitrogen immobilization is mostly a problem associated with fresh bark, and that can almost always be overcome or deal with if the grower is aware of the potential for it to occur. As the cellulose in bark is consumed N immobilization continually decreases until all that's left are priimarily natural polymers - lignin and suberin. Lignin is what makes wood woody/strong and suberin is nature's water protection for plants. When you think of suberin, think of the water resistance of a wine cork, made from the bark of the cork oak, it holds a favorable ratio of water:air and is able to hold nutrients for a reasonable length of time after fertilizing.
As for fertilizers, I've found that using a balanced liquid fertilizer with equal parts nitrogen, phosphorus, and potassium works well for Mandevillas. The average plant uses about 16% as much P as N and about 60% as much K as N. It's not an accident that fertilizers with a 3:1:2 NPK RATIO (examples: 24-8-16, 12-4-8, 9-3-6 are all 3:1:2 ratio fertilizers supply nutrients at nearly that exact ratio. It would be very difficult to justify using a 1:1:1 ratio product in a container when there are virtually no plants that use nutrients in a 1:1:1 ratio


I've found that using organic fertilizers like compost tea can also be effective for promoting healthy plant growth and flowering. Linda Chalker-Scott, PhD, has a much different take on that. Dr Scott has carved out an international reputation for her work dispelling gardening and horticultural myths.

Compost tea:
PDF Part I

PDF Part II

PDF Part III

PDF Part IV

These natural fertilizers provide a steady supply of nutrients and are less likely to cause burn or other issues. Nutrients locked tightly in the hydrocarbon chains that make up organic soil particles are not available for plant uptake. As such, they require the assistance of soil biota to cleave the chains and reduce nutrients to a form in which can be assimilated by the plant. Unfortunately, conditions under conventional container culture are generally more hostile than welcoming to soil life. The boom/bust population cycles this causes makes delivery of nutrients far more erratic and unreliable than the precise control offered by soluble synthetic fertilizers. Because nutrients often become quickly available when media conditions favor large populations of biota, plasmolysis (fertilizer burn) and ammonium toxicity are more likely to occur when using organic forms of nitrogen (including urea) than when using fertilizers that provide more than half their nitrogen in nitrate form.

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A potential problem with this soil - which was made when I had a little less experience with bark as a substrate - was that some garden waste compost was used in the mix instead of just composted bark. For reasons I do not understand well, garden waste compost makes a fantastic mulch for in-ground plantings, but plants simply do not like it as a component in container soil. If you do a net search using the word "eutrification", you'll quickly see what's going on.

The Rhododendrons do extremely well for 18 months, and then they start to head downhill FAST until I change out the soil. I wonder if the bark starts to rob nitrogen from the soil as it decomposes? Very unlikely. Nitrogen immobilization is mostly a problem associated with fresh bark, and that can almost always be overcome or deal with if the grower is aware of the potential for it to occur. As the cellulose in bark is consumed N immobilization continually decreases until all that's left are primarily natural polymers - lignin and suberin. Lignin is what makes wood woody/strong and suberin is nature's water protection for plants. When you think of suberin, think of the water resistance of a wine cork, made from the bark of the cork oak, Quercus suber.

Al

@tapla (mid-Michigan, USDA z5b-6a) Eutrophication is normally about the runoff of fertilizers in water, which results in algae blooms in lakes where the fertilizer accumulates. Is your point on garden waste compost that it may contain too many fertilizers and therefore would create mineral imbalances in the soil? So I guess there is not much I can do to correct that other than do not repeat the mistake of using garden waste compost in a soil mix again? Maybe some of the imbalances have created shortages of other nutrients, and I can compensate for that somewhat with additional fertilizer, but it is not an ideal situation.

Do you have any theory on why a forest canopy plant like Azalea or Rhododendron would do extremely well in a sized bark soil (1/4 bark as used in the gritty mix) but would suddenly die off after about a year? This is a pattern that has repeated for me over and over again, many different plantings/containers and many different plants. Always acid lovers Azalea/Rhododendron in a near-pure-bark soil mix.

By the way, I love Dr Scott's blogs, but if you have ever been on her FB forum, it is a pretty sadistic and cruel place. Her admins run it like a college classroom where no one is allowed to contradict any idea the teacher has. I tried to defend some of your ideas around Turface, and I was scolded and told it had no place in any soil mix. I quoted some peer-reviewed studies because her group was supposed to be a "science-based" group. That's hilarious, because in fact they don't allow you to use studies at all, and when you do you are punished. Then they threatened me for violating their commercial product policy by using the word "Turface". So I edited all of my posts to use the generic term "calcined clay" at which point I was banned for being insubordinate. Really?

After a year, I asked to be added back into the group and posted another question, but one of the admins there then deliberately hid all of my replies in the thread to Dr Scott, using a feature that lets me see the reply but does not let anyone else see it. When I confronted her about that, she denied it but after enough screenshots, she said "Well, it doesn't look like it was important anyway". So let's not add "honesty" to sadism on the list of their essential qualities.

It looks to me like your mandevilla might be in gritty mix. In my experience in SoCal on a hot stucco deck, mandevilla did very poorly in gritty mix. Even last year, your mandevilla had lost a lot of lower leaves and was not thriving. Mandevilla can survive very dry conditions, but will have lots of bare areas, especially the lower vines. It should be moist pretty much all the time when the temperature is optimal for growth.

Also, I have found that mandevilla does not do well if its root zone is too warm, and the gritty mix gets too warm in my climate/microclimate in midsummer. I no longer use Turface for anything because it gets too hot and holds the heat.

Mandevilla needs plenty of fertilizer. The intense rain we got in CA last year leached all the nitrogen out of the soil/growing media.

A thriving mandevilla has dense, lush green leaves everywhere and prolific blooms. For my best mandevillas, I have done the following:

- plant in a barky potting soil. Last year I used Kellogg Patio Plus with great results. It drained freely.

- use some CRF. I have used Osmocote Plus, and more recently, Dynamite (which is 13-13-13). Have you ever noticed how beautiful the mandevillas are in the garden centers, and even Lowes/HD if they haven't been there too long? They are growing in a barky, peaty mix with little green balls of CRF.

- fertigate with FP 9-3-6. Mandevilla thrives with FP 9-3-6, cranking out loads of blooms. I use a Chameleon sprayer and fertilize at 1 tsp/gal for most waterings --- every day in midsummer, and sometimes twice a day when it is above 86, which was a lot last summer. I sometimes use 2 tsp/gal, especially if temps are 76-80. Once a week or so, I use plain water.

- Last year, I had to battle spider mites (the dots on your yellowed leaves might indicate spider mites) and aphids. I misted off as much as possible during the day, and then sprayed Safer Insecticidal soap in a hand-pump sprayer under all the leaves after sunset (so that it will stay wet longer, and not affect the bees). I used the soap treatment repeatedly until they were under control.

If you don't want to change your mandevilla's growing medium right now (I would do it here in SoCal, but we have another 6-8 weeks of growing period), I would get a Chameleon sprayer and fertigate with FP 9-3-6. I would water that gritty mix probably twice a day. I would remove all the yellowed/brown leaves and look for signs of spider mites, and mist them by day and spray with Safer soap by night. If that pot is sitting on concrete or stucco, and there's Turface in it, I would insulate beneath the pot. One way is to set the pot on 3 or 4 wine corks (and in a saucer, for some evaporative cooling), or a slab of wood, or an overturned plastic or foam pot (NOT clay or ceramic).

@andersons21 I was able to recover the Mandevilla by using a healthy amount of Osmocote CRF and maybe once every two weeks some diluted FP 9-3-6 liquid fertilizer. I have it on an automatic drip system.

Based on your comments about bark soils, I guess it would do very well in the 5-1-1 mix. I do not have it in gritty mix, but the soil used garden waste compost instead of composted bark, and I think that garden waste compost is better as a mulch than as a soil component.