forest-trees

What Is the Best Tree Fertilizer?

Fertilising your trees might seem like a daunting task, but it doesn’t have to be! There are many types of fertiliser on the market. Which one should you use? Well, that depends on what type of tree you’re planting. This article will help walk you through the steps and explain which is best for each type of tree. Let’s get started! 

There are many types of fertilisers out there, so it can be hard to know which ones work well for certain plants or trees. In addition, different plants require different nutrient levels in order to grow properly and healthy – some need more nitrogen while others need more phosphorus than others. So how do you figure out which plant needs what kind of fertiliser?

Trees, particularly native trees, in gardens need care and attention so that they stay healthy and safe. A tree that is living in your backyard is under a lot of stress compared to the same species in the bush.

To highlight this, Don contrasted a large blue gum (Eucalyptus saligna) growth in a suburban backyard and the same tree in the nearby bush.

The tree in the backyard had soil built up over its roots from when the house was built. Nearby was a cement slab and incinerator. As well, lawn was being grown near the tree, which would be competing with the tree’s roots system for nutrients. The lawn was also raked regularly to remove fallen leaves, bark and twigs.

Further afield, the surrounding houses, roads and footpaths have altered the amount of natural run-off the tree receives, compared to what it would have had before the area was subdivided and houses built.

In contrast, the tree growing in undisturbed bushland had a rich mulch of leaves built up around its base. This mulch nourishes the micro-organisms in the ground and helps to feed the tree. In the bush, soil levels had not been altered, and the tree appeared to be strong and healthy.

A closer examination of the backyard tree showed that it was stressed. In addition, there was evidence of a bullseye borer in the main trunk (caused by the larvae of one of the longicorn beetles).

Borer attack is a sure sign of stress as a healthy tree would be able to repel such an attack. Signs to look for in your tree include holes in the trunk or branches (especially holes that look like a bullseye), oozing gum, a build-up of frass or fine wood shavings, or die back of branches.

Fertilising Trees & Shrubs

Trees and shrubs are living investments that grow in value with each passing year. When properly selected and planted, trees and shrubs can be expected to thrive with the right care, including watering, fertilising and pruning. Just as certain established drought-tolerant plants may not require water during dry spells, mature trees and shrubs growing in favourable soil conditions may require little or no fertiliser.

Fertiliser is often misunderstood and misused. Fertiliser is not “food.” Plants produce their own food in the form of sugars through photosynthesis. The minerals or nutrients supplied by fertiliser provide the ingredients needed for photosynthesis and growth. When minerals are lacking or absent in the soil, fertiliser can be added to maintain an adequate supply.

The fertiliser should not be considered a cure for ailing plants when unadapted or unhealthy plants are chosen, carelessly planted or improperly watered.

When fertilising trees and shrubs, keep these two points in mind: (1) Fertilizer is beneficial when it is needed; but (2) Use it in the right amount, at the right time and in the right place.

Establish a Need for Fertilizing

The backyard gum tree Don looked at needed immediate attention. Don’s main concern was to nourish the sick roots and allow the soil to re-establish a healthy population of micro-organisms. Achieving this would get the tree growing again and hopefully help it resist the insect attack.

To do this, Don treated the tree with a liquid called Seasol. Research has shown that this particular product can help to invigorate the roots, possibly by encouraging the growth of beneficial soil micro-organisms, particularly mycorrhizal fungi normally associated with the tree’s root system.

When the soil micro-organisms are present, they help the tree roots absorb nutrients needed for growth (including phosphorus, zinc, manganese and copper).

Don recommends that the Seasol be applied regularly for several months before any fertilising is done. Straight applications of chemical fertiliser can harm micro-organisms and make the problems worse. Once the tree is starting to respond to the Seasol treatment, Don suggests applying an organic fertiliser such as Dynamic Lifter or blood and bone, which will not harm the new population of soil organisms.

As an alternative, he recommends the use of slow-release tree tablets, which deliver nutrients to a small area directly where they are needed by the tree. The use of sulphate of ammonia or complete lawn food is considered bad and should be avoided.

Consider the following conditions to help you decide if you should fertilise your trees and shrubs:

Soil Test: Have your soil tested through the Clemson Extension Service. A soil test determines the acidity or alkalinity (pH) of the soil, along with the levels of nutrients that are present. Depending on the results, you may need to add nutrients to make up for any deficiencies in the soil. 

Growth: Look at shrubs and trees for signs of poor growth: poorly coloured leaves (pale green to yellow); leaf size smaller than normal; earlier than normal fall colouring and leaf drop; little annual twig growth; or twig or branch dieback. 

These symptoms of poor growth are not always related to low levels of nutrients in the soil, nor should you assume that fertilisers would cure these problems. For example, heavily compacted soil; stresses induced by insects, diseases and weeds, or adverse weather conditions can cause these symptoms. Before fertilising, determine the cause of the problem and correct it.

Planting Age: Fertilizer applications in the early years of established, transplanted trees and shrubs can speed up top growth and help young trees fill their allotted space in the landscape. Slow-release fertilisers are well-suited for recently planted trees and shrubs.

Location: If shrubs or trees are growing in a regularly fertilised lawn, there is no need to fertilise them separately. The roots of trees and shrubs will absorb some of the fertiliser applied to the lawn. However, trees and shrubs growing in planting beds may need to be fertilised, especially on sandy soils with little or no organic matter.

Commonly Applied Nutrients

tiny-tree

The most commonly applied nutrients are nitrogen (N), phosphorus (P) and potassium (K). Other plant-essential nutrients used in fairly large quantities are calcium, magnesium and sulfur. However, it is usually unnecessary to apply magnesium and sulfur because they are generally sufficient in South Carolina soils.

Micronutrients such as zinc or iron are added to many fertilisers. If your shrub or tree has a micronutrient deficiency, either apply the recommended rate of the deficient nutrient or uses a fertiliser containing the micronutrient deficient in the soil.

Kind of Fertilizer to Use

A complete fertiliser, such as 16-4-8, 12-6-6 or 12-4-8, is generally recommended unless the soil test reveals that phosphorus and potassium are adequate.

Two kinds of fertilisers are available: fast-release and slow-release. Fast-release or water-soluble fertilisers are less expensive than slow-release products, which release nitrogen over an extended period; however, the nutrients in a fast-release fertiliser may leach quickly through the soil. 

In sandy, well-drained soils, the soluble fertiliser may move past the root system after only a few inches of rainfall or irrigation. In fine-textured clay soils, leaching will be slower, but runoff may be greater.

Slow- or controlled-release fertilisers have extended release periods compared to fast-release fertilisers whose nitrogen is water-soluble and readily available to the plants. The nitrogen in slow-release fertilisers may be sulphur-coated or a form such as IBDU or urea-formaldehyde. 

One-half or more of the total amount of nitrogen in controlled-release fertilisers should be “water-insoluble” or slow-release nitrogen. For newly planted shrubs and trees, or in areas where the potential for runoff is very high, such as slopes or compacted soil, slow-release fertilisers are a good choice. Since the nutrients are released slowly, the potential for fertiliser damage (“burning”) and water contamination is less.

Natural fertilisers, like composted sewage sludge, cow manure, or complete fertiliser blends, slowly provide nitrogen and other nutrients. An advantage of these natural “nutrient suppliers” is that they provide minor nutrients – minerals required in small amounts such as iron or zinc – not usually found in synthetic fertilisers. Natural fertilisers also improve the soil structure.

A disadvantage of natural fertilisers is that the concentration of nitrogen, phosphorus, and potassium is usually lower. Therefore, a greater amount of a natural fertiliser must be applied to provide the same amount of nutrients that can be obtained with a lesser quantity from a synthetic nutrient source.

Many fertilisers are formulated for use on lawn grasses. Some, known as “weed-and-feed” fertilisers, may contain a herbicide that can damage groundcovers, vines, shrubs and trees. Read the labels and carefully follow the directions.

Amount of Fertilizer to Apply

Similar to lawn fertiliser applications (HGIC 1201, Fertilizing Lawns), the recommended rates for fertilising shrubs and trees are based on actual pounds of nitrogen. Shrubs and trees can receive 2 to 4 pounds of actual nitrogen per 1,000 square feet of root spread area per year. 

The root spread area occupies 1½ times the area of the crown spread (3.14 x radius²; see Figure 1). Thus, generally, younger shrubs and trees should receive higher rates of nitrogen than mature plants.

For shrubs and trees in lawns, follow the fertiliser recommendation rate and timing for the turfgrass. Depending on the formulation, applications exceeding 2 pounds of actual nitrogen can overstimulate or burn the grass. 

If trees or shrubs growing in fertilised lawns show nutrient-deficiency symptoms indicating a need for additional fertiliser, space the fertiliser applications a few months apart, not exceeding the total yearly amount of nitrogen required by your lawn grass (follow the rate and timing for the lawn grass).

Avoid adding too much fertiliser, which can harm the plant and the environment. Excessive fertiliser produces rank, weak growth that breaks easily and is susceptible to injury from cold, drought and pests. Also, fertiliser not absorbed by the plant roots may contaminate groundwater and surface water.

Don recommends following the dosage instructions on the package. He applied one cup of Seasol to a standard nine-litre watering can of water (250ml to 10 litres of water). He filled up each of the forked holes with this liquid and recommended the treatment be applied every month for six months.

Once the tree is growing (usually after two to three months), apply a granular fertiliser or tree tablets at the rates recommended on the packet, which will depend on the size of the tree. As a guide:

  • Trees up to 5 metres (small trees ) – 1kg of fertiliser
  • Trees 5-10m (medium trees) – 2 to 5kg of fertiliser
  • Trees over 10m (tall trees) – 5-10kg of fertiliser

The granular fertiliser or tree tablets should be placed into a hole in the ground. The hole will need to be larger that the forked holes. Don recommends making a hole about 25cm deep by using a crowbar. Holes should also be made under rocks and paths if they are in the root area.

Once the tablet has been placed in the hole, backfill. In addition to regular fertiliser applications, the tree should also be given a deep watering once a week in summer and around once every three weeks in winter.

If you are in any doubt about the health or safety of a large tree in your garden, contact a registered tree surgeon.

Fertiliser Application Methods

Plants may be fertilised by either indirect or direct methods. With either method, apply the fertiliser to the entire root zone area. Because of the naturally high oxygen concentrations near the soil surface, a plant’s principal feeding roots are usually within the top 10 to 14 inches of soil. 

Therefore, many roots of mulched plants are located just beneath the mulch on the soil surface. Apply fertiliser to the soil’s surface or mulch; rainfall or irrigation water will carry it to the roots.

Whatever fertiliser or method of application you choose, irrigate soon after applying fertilisers to wash any fertiliser from the leaves and help nutrients dissolve and penetrate through the mulch and soil to the roots. Without irrigation or rainfall, some of the nitrogen applied may evaporate and be lost to the atmosphere without benefiting the plants.

Indirect Fertilization: Shrubs and trees growing in lawns are fertilised indirectly when the property is fertilised.

Direct Fertilization: The cheapest and most effective method of directly fertilising trees and shrubs is broadcasting. Using a cyclone or drop-type spreader, scatter a prescribed amount of fertiliser over the entire root zone area. To obtain the best coverage, split the total amount of fertiliser to be applied in half. 

Apply one-half of the total amount in one direction and the other half in a direction perpendicular to the first for excellent coverage. When fertilising over the top of shrubs and groundcovers, make certain the leaves are dry and use a leaf rake or broom to brush fertiliser off the leaves and onto the ground after application. Some plants, like liriope and azaleas, can collect fertiliser granules in the whorls of their leaves, and injury may result.

If the soil in a lawn is compacted, aerate the soil, then fertilise. Watering the fertiliser in afterwards will reduce the chances for injuring any groundcover or lawn grasses.

Fertiliser can be applied in liquid form to the leaves of shrubs and trees. Liquid application is commonly used to correct micronutrients such as iron chlorosis or yellowing in azaleas (the youngest leaves are yellow leaves with green veins). 

Foliar applications provide a temporary solution that controls deficiencies in existing leaves, with the best results achieved in the spring. However, applying fertiliser to the leaves will not cure the real reason for the micronutrient deficiency, which can be the result of an improper soil pH. To find the underlying problem, refer to the soil test. If the pH will not be corrected, then the foliar application will have to be repeated.

A liquid or dissolved dry formulation of fertiliser can also be applied in the irrigation water. This practice will place nutrients in the upper soil surface where most of the absorbing roots are located. Use care to get even coverage and the proper dilution rate. A backflow preventer should be installed on the irrigation system.

When to Apply

tree service melbourne

Apply fertiliser when plants need it and when they can readily absorb the nutrients with their roots. Time your application to coincide with active root growth and adequate soil moisture. Trees and shrubs should be fertilised in early spring, and a light fertiliser application can be made in early summer if conditions are conducive to plant growth (that is, reasonable temperatures and soil moisture). 

Avoid fertilising trees and shrubs stressed by drought during the summer months. If water is unavailable, do not fertilise at all because plants will be unable to absorb the nutrients.

For shrubs and trees in lawns, apply the fertiliser at the appropriate time and rate for the turfgrass. Always be sure that adequate moisture (supplied by either rainfall or irrigation) is available.

Calculating Area & Fertilizer

Shrubs and trees growing in lawns should be fertilised at the appropriate time and rate for the turfgrass (see Amount of Fertilizer to Apply section). When trees and shrubs are growing in beds or natural areas, you need to calculate the amount of fertiliser required.

Calculating the area where fertiliser should be applied

Trees: Apply the fertiliser to the area occupied by the tree’s roots or root zone area. The root zone area is roughly a circular area with the tree in the centre. The root zone area extends beyond the tree’s drip line or outermost branches, with the roots extending 1½ times the distance from the trunk to the drip line or outermost branches.  For example, if the distance from the trunk of your tree to the drip line, which is called the crown radius, is 8 feet, the “feeder” or mineral-absorbing roots can extend an additional 4 feet beyond the drip line. So, the root zone area can occupy an area up to 12 feet away from the trunk.

Tree cultivars that have a narrow canopy, such as Fastigiata English oak (Quercus robur ‘Fastigiata’) or columnar Japanese pagoda tree (Sophora japonica ‘Columnaris’), or trees with small canopies, or trees that were pruned into unusual shapes, have a root zone area that can be much more than the drip line.

In these cases, make your fertiliser calculation based on the trunk diameter. First, measure the diameter in inches at 4.5 feet above the soil level (dbh) and multiply it by either 1 or 1.5 to get a number expressed in feet. This number will be used as the radius measurement for the fertilisation area. For example, the radius of the fertilisation area of a 12-inch diameter tree would be 12 to 18 feet, depending on the multiplication factor that was used.

Follow these steps to determine the amount of fertiliser needed to supply 2 pounds of nitrogen per 1,000 ft²:

1. Calculate the root zone area (assuming it occupies a roughly circular shape), using the following formula where Pi = 3.14:

Pi x (radius)² = 3.14 x (root zone radius) x (root zone radius)

In the example given above, the root zone area would be:

3.14 x 12 x 12 = 452.16 square feet

2. To calculate the amount of fertiliser required per 1,000 square feet, use the following equation:

Lbs N desired x 100%

%N in bag

= Number of pounds of fertiliser required per 1000 square feet in order to apply the desired amount of actual nitrogen

To deliver 2 pounds of essential nitrogen per 1000 square feet, the equation would look like this:

2 Lbs N x 100%

%N in bag

= Number of pounds of fertiliser required per 1000 square feet in order to apply 2 pounds of actual nitrogen

Assuming you have a 16-4-8 fertiliser, the equation for this example would look like this:

2 Lbs N x 100%

16% N

= 12.5 pounds of 16-4-8 required per 1000 square feet

3. Calculate the actual amount of fertiliser to apply using the following equation:

Root area ft²

1000 ft²

x Pounds fertiliser per 1000 ft² = fertiliser to apply over the root area

In our example, calculate the amount of 16-4-8 fertiliser required in order to apply 2 pounds of actual nitrogen to 452 square feet:

452 ft²

1000 ft²

x 12.5 pounds fertiliser per 1000 ft² = 5.65 lbs fertiliser to apply over root area

Apply 5.65 pounds (about 11 to 12 cups; 2 cups of 16-4-8 is equivalent to 1 pound) of 16-4-8 evenly over the root zone area. Since most of a tree’s roots can be found in the top foot of soil, broadcast the fertiliser evenly with a rotary or drop-type spreader over the root zone area to fertilise the tree. Water after application to make the nutrients available to the roots. If the tree’s root zone area is confined by a sidewalk or driveway, reduce the root zone area accordingly.

Shrubs: When fertilising individual shrubs, follow the directions given above for trees. However, when several shrubs are grouped together in a bed or natural area, it is easier to measure the entire area to determine how much fertiliser to apply. Measure the area of the entire bed, making an allowance for the roots that extend beyond the branches of the outermost shrubs. To determine the bed area, use this formula:

Length x width = root zone area

Let’s assume the bed is 30 feet long and 10 feet wide. The bed (root zone) area is 300 square feet.

Calculate the amount of fertilizer required to apply 2 pounds of nitrogen per 1,000 square feet using the same equations from the tree section. Assuming you have a 16-4-8 fertilizer, the first equation would look like this:

2 Lbs N x 100%

16% N

= 12.5 pounds of 16-4-8 required per 1000 square feet

Since the root zone area is 300 square feet, the actual amount of 16-4-8 fertilizer to apply is calculated as follows:

300 ft²

1000ft²

x 12.5 lbs fertilizer per 1000 ft² = 3.75 lbs fertilizer to apply over root area

Apply 3.75 pounds (about 7 or 8 cups) of 16-4-8 evenly over the mulched bed. Sweep fertilizer off the branches and water afterwards to make the nutrients available to the roots. If the shrub’s root zone area is confined by a sidewalk or driveway, reduce the root zone area accordingly.

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