Ive had several citrus tree customers email me lately asking what they can do to prevent fruit drop on their citrus trees. I know that the May fruit drop is a natural occurence to keep the trees from "over-taxing" themselves with a large crop. Here is one example of a "non" May fruit drop that Id like an answer to: A friend owns a keraji mandarin that he bought from me some 8 years ago. The tree was a seedling and just started flowering last year. It was covered with flowers and then shed every one of them off, not producing a single fruit. This spring, it was covered in flowers again, the bumble bees had a heyday! Again, this year, every one of the flowers have fell and there is no fruit? Does anyone have an answer or a cure for this?

Maybe Millet or Dr. Manners will chime in more knowledgably than I am able, but I suspect it may be a matter of nutrient availability at the right time. I think I messed up this year in my fertilizer application timing and had the very same problem of very poor/no fruit set. This really only bothers me with two trees, the others are young enough still that I'm more interested in growth than fruit. It still it annoys me that I blew my window like that though. This is for in-ground trees in deep sandy soil where nutrient leaching is a constant problem.

.....Alan.

First of all, we can expect up to 90 percent of all blooms to naturally drop from any citrus variety without ever producing fruit. This is normal abscission. This postbloom drop occurs within the first couple weeks of bloom. Other common causes of flower drop include insufficient pollination, poor NITROGEN levels, WATER stress, HEAT stress (two very common causes),and insect stress. I always spray my trees with a Potassium Nitrate foliar spray approximately two months before bloom to insure adequate nitrogen and potassium levels. It really helps with fruit set, and also with fruit size. Any inground citrus tree should be fertilized with a granlar soil fertilization a month or two before the normal blooming period, which helps in the setting of fruit. I do not have any idea if Keraji Mandarin is a self incompatable or is a poorly compatable variety or not. I believe Keraji might be a hybrid, does anyone know it parentage. If it is a mandarin X grapefruit hybrid many of these are self-incompatible. I have used it as a rootstock in the past but never actually had nor grown a Karaji Mandarin to maturity. - Millet

Potassium nitrate can be obtained at eBay as Rocket fuel if you have a hard time locating them from your local stores. Potassium nitrate also happens to be a bloom inducer in some plants like mangoes.

Another method is the bloom or fruit set inducer as sold from the stores, as they contain GA, but you should know what you're doing. GA (Gibberellic acid or GA3) when applied improperly before or at the very start of bloom, can revert your blooms back into vegetative growth losing all your blooms and fruits, but when applied during the peak bloom can make set tons of seedless citrus fruits. In fact GA3 is listed as one of the methods to make seedless fruits. An unpollinated fruitlet can "fool" the plant that it has been pollinated and will therefore set fruit. The side effect oftentimes, you will end up with thousands of marble sized fruits if you don't thin out.

I'd go with the potassium nitrate method suggested by Millet.

Gibberellic Acid does work, when 10 ppm concentration sprays are applied at full bloom. GA will set commercial size crops. However, it is usually only used on sexually self-incompatible varieties to produce crops of seedless, Robinson, Nova, Osceola, Sunburst, Clementine, Minneola and Orlando. Fruits "pollinated" by GA are reduced in size and the change to the orange peel color is delayed. Exceeding the concentration of Gibberellic Acid, or the spray dosage can result in severe leaf drop. - Millet

Even when we do not make this forum a priority to peer in daily
we do look in here every now and then. There are some issues
that have come up that have much greater implications that first
meet the eye.

At first we wanted to respond to this issues in this thread and
later decided not to as the premise query would entail a series
questions to be asked just so we can get a better footing of what
really is going on with the Keraji Mandarin and possibly other
plants as well. We may have some follow up questions later.

If the 8 year old Mandarin is a seedling and is blooming now
and not setting fruit, did this plant arise from a cutting grown
parent, a grafted parent or was the parent also a seedling?
Then the same question for the grandparent, the parent of
the parent. We are wanting to know if there is a pattern here
in that could it be that the Keraji Mandarin on its own roots
or germinated from seed is partly sterile, incapable of being
selfed but requires cross pollination in order to produce seed?
A Bumble Bee cannot pollinate a flower as easily or as
efficiently as a Honey Bee can and that can also be a factor
here.

May drop is the issue that kept bothering us the most as here
we are much more used to June drop, whereby the tree will
slough off the immature fruit and other fruit to better sustain
itself for the rest of the crop. May drop tells us something else
is going on to cause this as in a Southeastern state we really
should not see much May drop unless the tree has been overly
stressed in some way or we have several immature fruit that
have fallen off the tree due to incomplete fertilization, more
commonly seen in Fruit & Nut trees but we can see this
happen in Citrus.

Now, we need a better picture of what went on with this tree to
determine a direction to take as to how to answer the original
question of why is it that the tree reportedly bloomed well
enough and when it came time for the flowers to be pollinated
that none of them were and then how long can we expect this
to go on for us?

The issue of preventing June drop is a dubious one all in
its self as on one hand we want the tree to slough off fruit
giving us uniform size and roughly the same maturity from
the production end of things and on the other end, home
gardeners may not want any of the fruit to be sloughed
off so that the fruit can be picked at random times, not
all at once. Foliar sprays work best for both to aid in
sloughing off the fruit faster and also to aid in individual
tree fruit set for both the production and home gardener
ends.

Snickles

A long time ago we were having trouble with a Bower mandarine setting and holding fruit. If my memory is correct Dr. Calvin Lyons, extension horticulturist suggested cutting through the bark with a knife, completely girdling the tree. It did set some fruit after this was done but died in the severe freeze of December 1983.

The question the Snickles and I brought up still needs to be answered. That question is: Is Keraji Mandarin a sterile variety, that is incapable of self fertilization? Does Keraji require another variety for pollination in order to set fruit? I have Keraji as a rootstock on a couple trees, but otherwise I do not know much about the variey. - Millet

There are times we are asked a question in regards to one
particular plant in which we do not get told all that we need
to know to give a definitive answer. We are grasping at
straws when we have to rely on a second persons account
of what went wrong with a plant that he had sold and the
customer is having a snafu getting fruit to set. Sometimes
we have to go back to the grower or the propagator and ask
them as we have no where else to go with some of these
novel plants. Citrus we have been around for years can
fool us but when dealing with a Keraji Mandarin in which
there really isn't much known about it then we have to
defer back to the people that have been growing it or have
been around it to learn more of what may have gone wrong.
We can split hairs later once we know more about it.

When we first saw this thread we felt that the 8-year old
tree, originating as a seedling, may not have been old enough
to produce fruit as some Citrus may require a few more years
to develop before they can blossom and later have fruit set.
Then we wondered were there any other Citrus around when
this tree was flowering, then was there bee activity, could the
tree have been stressed as it was in bloom, could the blooms
have been a result of stress to the tree. What kind of soil is
this tree growing in and how much sunlight is it getting?
We can only go on so much but the issue of May drop is
what led us back to make the post we made yesterday.

It is not out of bounds to use girdling to aid in blooming
and resultant fruit set but there is a price to pay for doing
it. People can give advice to others to go ahead and do
it but will the people telling others to girdle trees do that
on their own trees or do they even have their own trees
or are they playing the game with someone else's trees?
Personally, to trade an open wound that can serve to be
an infection site just to aid blooming for Citrus is like
rolling the dice. We may get our wanted fruit but we
may also lose the tree in time because of it. It is not
an option that we can recommend people doing. As
an experiment sure, that is okay or if we have other
trees to be our backups but if it is a tree that may take
us a while to get another one, then we feel the reward
of fruit does not outweigh the risk we face by causing
injury to the tree. A very good point was made that
when we girdle the tree to get fruit we in turn weaken
it. A freeze may have killed the tree regardless of the
girdling but even still why take the risk?

Snickles

For the Potassium Nitrate spray recommendation by Millet, is there a recipe for the dosage? I looked into the Potassium Nitrate sold on Ebay. I'm assuming you just mix with water. How much should it be diluted?

Thanks,
Phillip