One of the most frequent questions that is e-mailed to me is when and how much to fertilize.  When I started bonsai I consulted many sources on various subjects and got many varied answers.  Experience has been a good teacher. One of the things that it taught me is that there are various answers to the fertilizing question, all subtitled under the heading of ‘It depends’.  I will coherently try to outline what I learned over the years.

First the basics.  Q - Why do we fertilize?  A – To make the plant grow.  Simple, but in bonsai, this is something we want to control.  This makes timing  very important.  In landscape nursery growing, we want a strong push of growth in Spring. Not so in bonsai.  Strong early growth results in long internodes (the space between the leaves).  This ruins the compactness we strive for.  In fact, one of the reasons we repot and root prune in early Spring is to minimize that early push of growth.  So, the rule for fertilizing deciduous trees in Spring is don’t.

Is there a time when we do want fast growth in deciduous trees?  Yes, but only in the development stage.  The first set of leaves matures around the end of May.  This is the time we prune and defoliate.  I will usually water with a regular strength soluble fertilizer such as Miracles Grow or Peters 20-20-20 at this time then prune and defoliate in 1-2 weeks. Note I do not defoliate most of my trees.  I fertilize young developing trees in nursery pots full strength fertilizer every 2 weeks thereafter until the end of August.  Trees in bonsai pots usually get about half strength once a month throughout the same period.  Deciduous trees, especially maples, are more sensitive to fertilizer than conifers.  If you start to see long internodes, prune back and withhold fertilizer.  Do not fertilize after August. Doing so will results in non-hardy growth that will not harden off before winter.  Autumn is the time when the tree switches gears, so to speak.  It stores food in the roots in the form of starch for winter and it lignifies (hardens) the top growth.  Watering should be cut back as well.

When we discuss conifers we need to separate trees into two groups.  Those that get buds or shoots all at once and those that grow continuously.  Those all at once are Pines, that develop candles, and Spruces, Firs and Hemlocks that get shoots.  These trees produce buds 2 or 3 times a year.

Junipers and Cedars bud continuously so I’ll consider them first.  What I say for Junipers goes with cedars as well.  Fertilize junipers every 2-3 weeks with half strength liquid fertilizer or use fertilizer cakes.  I’ll talk about fertilizer cakes later.  Watch to see if the shoots are growing too fast and cut back fertilizer strength accordingly.  If using fertilizer cakes, remove one third of them.  Junipers should not remain static like plastic plants.  Let the shoots grow an inch or 2 before you pinch back with your fingernails (don’t use scissors).  Again, stop fertilizing at the end of August.

First I’ll talk about pines.  With Pines we want very strong growth in Spring.  This produces very long candles.  “Ack! Ugly! Why do I want that?!”, you say.  Two reasons.  Trunk and branch strength and back budding.  The long candles cause increased sap flow.  This causes the branches and trunk to thicken.  It also promotes new buds where you plucked needles last Fall.  In July you pinch off all those big ugly candles.  New buds (lot of them) come in at the cut.  You will keep 2 or 3 of them.  These are the buds you want to control.  What controls their size is not so much fertilizer but how late you pinch off the first candle.  So what is the schedule?  Full strength fertilizer every other week staring in April or lots of fertilizer cakes.  In the middle of June stop all fertilizer or remove most of the fertilizer cakes.  Take the rest off in early August.  That is it for pines.  Don’t forget to stay on schedule for wiring, pruning  and needle pulling.

With Spruces, firs and Hemlocks you will prune or shorten the first set of buds in Spring.  Stop fertilizing before it is time to prune the second set of buds.  Fertilize with half strength liquid fertilizer once a month after that until August.  Another trick with all conifers is to give a thorough watering of 1 tablespoon of Epson salts to a gallon of water in Spring.  The magnesium helps ‘green up’ the foliage.

Tropical trees vary by species.  They are also not my specialty.  A couple things to remember.  First, although they do not have a true dormant period, growth slows in the Winter.  Hold back on fertilizer at this time.  Second, during the growing season, watch the internode spaces.  If they get too long cut back on fertilizer.  Use half strength liquid fertilizer every week or two.  Avoid organics like fish meal.  It promotes fungus and bacteria which although may not hurt the tree, is unsightly at best and seems more suited to a petri dish. (at least that’s what I’ve found.)  In general, with indoor plants, much depends on the temperature and the amount of light the plant is receiving.

Fertilizer Cakes
Rape seed cakes are the ‘traditional’ bonsai fertilizer.  That is because it was the best thing available in early Japan.  Rape seeds are like millet.  You can buy them via mail order.  There are better organic top dressing fertilizers such as BioGold  also available from bonsai suppliers (not me).  Many bonsai artists make their own cakes.  There are as many recipes as there are bonsai artists.  The advantage of fertilizer cakes are that they last a long time.  They are like time release fertilizers that will not cause root burn.  The down sides 1) they usually cause a layer of black crust on the soil surface, 2) depending on the recipe they can smell bad and, 3) animals like them better than trees do.  As far as chemical time release fertilizers; they are ugly and you can’t remove them when you need to.  I do not use them except in nursery pots with young plants and seedlings.

Below is my recipe for fertilizer cakes.  I make them in the Summer and store them in large coffee cans for the next year.  Once they are dry they can last years.  I use cottonseed meal which is readily available but is quite acidic.  That is why the lime is added.  I used to put insecticide in the mix (you see this in many recipes) but I now recommend you do not use it.  I have pets that could get into them.  You also never know when a visiting small child will want to sample one.

Okami Cakes
6 cups     Cottonseed meal
2 cups     Dolomite lime - powdered
1 cup       Bone meal
2 cups     Dry roots 2
1 cup       Fish emulsion (some like Neptune’s Harvest don’t smell)
1/2 cup    Soluble fertilizer powder
1/2 cup    Superthrive
2 cups     Blood meal (optional, this smells bad!)

Add enough water to make a doughy mixture and kneed for a few minutes to be sure everything is mixed well.  For into 3/4 – 1” round, 1/2" flat patties and lay them out on trays in the sun to dry.  When one side is dry, turn them over to dry the other side.  This can take a few days so bring them in at night.  I also let them stand out in my garage or shed a few more days to be sure they are very dry.  I use old discarded cafeteria trays lined with aluminum foil to dry them.

Good luck and feel free to e-mail me if you have any questions.