Fertilizing Basics for Healthy Rooftop Gardens

Design and InstallationGarden Caregarden TriviaPlant Health Care
Written By Peter Morris

Simple Definitions

Fertilizing is the addition to the soil, by people, of essential elements for the better nutrition of the plant. 

However, more specifically there are 17 essential elements that plants need to live. Without any one of these substances plants fail to thrive.

One of these is in the air, the Carbon in CO2.  Another two are in water, Hydrogen and Oxygen.  The remaining 14 are either found in soils or need to be added as a fertilizer. They are needed in tiny amounts and sometimes referred to as trace elements.

In most rooftop gardens, the soil is constantly leached by the irrigation systems.  Since there is no place to go other down the drain, most garden soils get depleted of essential elements very quickly.  Of course, in ground gardens do not have this problem as anything unused or leached has no place to go but back in the soil !

Now that is a well fertilized garden !

Three Magic Numbers

Most fertilizer products label their packages with a combination of 3 numbers and letters.  This NPK specification relates to the elements N = Nitrogen, P = Phosphorus and K = Potassium. The numbers refers to the proportions of these elements in the product.

Commercial fertilizers are the salt molecule containing these elements.  The different combinations and mixes of these quantities are used by people to stimulate different responses from plants.  N stimulates foliar growth and helps the plant produce a lot of sugar.  P stimulates root development, blooming and helps recovery after stress.  K stimulates and helps the overall metabolism.

For most garden applications a similar spread of the three NPK numbers is sufficient.  For example,  NPK 5-5-5 is balanced and not too strong.  Likewise a 20-20-20 is balanced but very concentrated. Only when there are nutritional issues should one use products with unbalanced NPK numbers. An example of a specific fertilizer made to boost foliage growth and used only when a plant is looking poorly is Tree 27-9-9.  The high Nitrogen (27) is like a shot of expresso to the plant!

A well fertilized garden !

Sulfur for Acidity

Most fertilizers that are labelled for acid loving plants also add the essential element Sulfur.  As this substance will help acidify the soil.

How it works is quite fascinating. The Sulfur is absorbed and consumed by special bacteria in the soil called Thiobacteria.  These in turn will excrete extremely small amounts of sulfuric acid as a waste product back into the soil.  The sulfuric acid acidifies the soil and helps release minerals back in to the soil for absorption by the plant.  It does so by breaking large molecules into smaller ones.

Once again, watering and irrigation systems will leach out the sulfur from the soil rendering it alkaline.  Alkaline soil will hurt acid loving plants. It also binds the essential elements making them unavailable to the plant even if they are present in the soil.

Acid loving hydrangeas loving the spot !

Organic vs. Synthetic

Synthetic fertilizers usually contain essential elements in the form of large salt molecules.  These have to be absorbed, broken down and then metabolized by the plant order to utilize them. 

Organic fertilizers on the other hand contain the same essential elements but as smaller organic molecules.  These can be used immediately by the plant without having to metabolize them any further.  Like Malic or Fulvic acid - Plant candy ! 

The trade off between using either is usually concentration.  Organic fertilizers tend to have very low NPK numbers and are are best suited for weekly use.  This is usually not feasible or too expensive except for  homeowner with ample gardening time.

Different plants have different fertilizer needs 

Microorganisms and soil health

The two main microorganisms needed for plant health and proper mineral absorption are Mycorrhiza and Nitrogen Fixing Bacteria.  They are absolutely essential and rarely spoken of.  These organism bind to the plant roots and help them absorb the minerals.  Without them the plant is most certainly at a disadvantage.

Many of them also consume the raw minerals and metabolize them into final products the plant can use directly.  Many fertilizer products have all these essential organisms in addition to the essential elements - but not all!  Price is a big determining factor - as the ones with mycorrhiza are more expensive and have a limited shelf life.

It is important to note that many of these microorganisms die in winter and they must be reapplied every Spring.

A lovely acid loving Rhododendron in bloom in a back garden !

Fertilizing can be confusing but we at Plant Specialists have the know!

For the prettiest (and well fertilized) garden ever - Call Plant Specialists TODAY !

Don't delay – the sooner the better !

GREENING NEW YORK FOR OVER 52 YEARS !

Article written by our Staff Horticulturist, Peter B Morris, BSc, MSc, MBA

All photographs used with permission from our own gardens !

Peter Morris

Peter was born and raised on a beautiful green island in the midst of a tropical rainforest. He was introduced into the world of plants at the age of six when his grandmother, an avid Spanish gardener herself, asked him to help her grow seeds for her pepper garden. He was hooked! By the time he was a teen, he had his own rose and orchid collection numbering in the hundreds. Botany was in his blood, and that is what he set out to study.

His passion brought him to NY in the late seventies to further his education. His tenacity allowed him to work full time at Plant Specialists while he completed a MS in Plant Biology. As a manager at the time he felt unsatisfied with his knowledge of business and business processes. Peter felt compelled to learn, so he then pursued and completed an MBA in Quality Management within a few short years.

Peter’s other passion is teaching. His natural ability is quickly consumed by our staff in all subjects in Botany, Horticulture, and Landscaping. He created an immense reference library of more than 3,500 plants providing an invaluable resource for our staff.

Peter’s breadth of knowledge and wisdom allows him to effectively diagnose the needs of plants. Sometimes just by walking into a garden he can create a prescription that fixes even the hardest issue. He is our Staff Botanist, Diagnostician, and all around Mentor. Recently, he has put his immense knowledge and skills into developing a new department that focuses on Plant Healthcare. As he puts it, “Magic through Science”. The PHC staff that surround him have avidly consumed his teachings. Substantially developing their own plant wisdom, many have taken on difficult plant health issues with spectacular results.

Plant Healthcare has been an instant success with customers! The proper treatment of insects and diseases including Organic methods has made pest control a necessity for every plant. Correcting hormonal imbalances caused by planting in containers or refurbishing soils leached of nutrients by irrigation systems are big challenges PHC has become quite comfortable addressing. The scientific approach to the complex demands of keeping plants healthy in our harsh city environment has made many a customer say WOW!

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