When to Apply Fertilizer and What Kind Explained

Q. When during the year should you start and stop fertilizing landscape plants and what kind of fertilizer is best for them all? It seems to me that with acid loving plants, cacti, palms, roses, fruit trees and annual flowers they might all require different kinds of fertilizers and different times to apply them.

A. You could go crazy trying to follow all the different rules when fertilizing for different types of plants. Keep it simple. Let me give you a few simple rules to follow when applying fertilizers.

This is an easy fertilizer for homeowners because it tells them what it's for. It has 10% nitrogen, 5% phosphorus in the form of P205 and 10% potassium in the form of K2O. But it should really tell us that it's primarily for frond and stem growth (nitrogen), half as much for roots and flower production (phosphorus, we don't want much of that anyway) and a similar amount of potassium as nitrogen. What other plants would this be good for besides palms?

            If plants are winter tender, in other words they might get hurt or die when temperatures dip below freezing, stop fertilizing these plants in July. Our citrus trees fall into this category.

            Lawns, bedding plants, such as annual flowers, and vegetables should be lightly fertilized once a month. Lawns that are expected to remain dark green during the winter should have fertilizer applied around Thanksgiving before freezing weather.

This is 21% nitrogen in the form of ammonium sulfate. There is no number for on this label but it also delivers about 20% sulfur as a fertilizer as well. Some people are very negative about ammonium sulfate but the plant doesn't really care where the nitrogen is coming from. We should pay attention to any contaminants, such as heavy metals, that might be in this bag of fertilizer.

            For light fertilizer applications, reduce the amount applied to half the rate recommended on the bag or container. Light applications of fertilizer can be applied every month and immediately watered in if applied early in the morning. Get in the habit of applying fertilizers early in the morning or late in the day.

            The most highly prized landscape plants should be fertilized three or four times during the year; January/February, April/May and September/October. These include plants like roses, gardenias, and Jasmine for instance. Again, use half rates when applying fertilizers.

Has a lot of phosphorus in it. The numbers tell us that. This fertilizer would be used for new plants that need to create a lot of new roots and those that flower or produce seed. Marijuana producers use this type of fertilizer when the plant is getting older and is close to flowering and producing seeds.

            Most landscape plants are fertilized only once, just before new growth begins in late January or early February. This includes all landscape trees including palm trees.

            Which fertilizer to use? You can get by with 2 or 3 fertilizers in your arsenal. That’s all. Fertilizers have three numbers separated by hyphens somewhere on their label. They represent three different plant nutrients; nitrogen, phosphorus and potassium and in that order.

            When growing plants that are primarily important because of their leaves and stems, the first number, or nitrogen, should be the highest. The middle number, phosphorus, should be about one fourth of the value of the first number. The last number, or potassium should be somewhere in between the first and second number.

Obviously then when growing roses, fertilizers with a high middle number are important for good flower production. Be careful of adding high phosphorus fertilizers over and over because phosphorus will begin to accumulate in soils unlike nitrogen and potassium.

            When growing plants valued for their flowers or fruit, then the second number or phosphorus becomes critical. It needs to be the highest. When fertilizing these plants, the second number should be highest while the first and third numbers lower. Exact numbers are not critical but the ratio of these three, or their proportions contained in the fertilizer, is more important

            To be healthy, plants need more nutrients than supplied by only these three numbers. But these three numbers represent nutrients needed in massive amounts by plants. The other important nutrients are supplied by the soil. For this reason, I frequently mention the application of compost. A compost application, once a year to landscape plants, would be extremely beneficial.

Olive Tree Suckers Easily from Many Locations

Q. An olive tree on the property of our homeowner association is sending up suckers from its base and along the trunk. I am thinking it’s because the tree is not getting enough water. Our landscaper continues to remove them and thinks otherwise. Who is right?

A. Suckering from the base can be a sign of a lack of water in some trees but olive trees also sucker from the base and along the trunk easily. If you look at the base of older olive trees you will see some “knots” or swellings attached to the lower trunk, trunk limbs and root flares as they get older. There can be so many of them the tree becomes disfigured. It gives olive trees a great deal of character in their old age.

Olives sucker easily from clusters of immature or unopened buds hidden on the trunk. You can spot them as bumps or gnarls. A limb was removed from this olive tree which encouraged the suckers to grow.

            These swellings along the trunk and limbs develop from clusters of immature buds embedded in woody growth. Suckers can originate from these “knots”. These knots or “burls” can get quite massive in older trees.

            Burls are common in other trees as well particularly trees that are prone to damage from fire or animals like coastal redwoods. Burls are valued by many woodworkers but despised by the construction lumber people.

            Suckering from the base of some trees, however, can be in response to drought. There may or may not be obvious swellings at the base of these trees. The tree finds it difficult to deliver water to its top when water is scarce.

This tree rose suckered from the rootstock after the top of the tree, or scion, died back.

            These clusters of undeveloped buds, previously asleep, begin growing from the base. Some are scattered through the wood and others are in clusters. Growth from the bottom is easier to support when water is scarce then growth at the top.

            Some trees like many ash trees don’t have that survival mechanism. When water is scarce, their leaves begin to scorch, push very little new growth and limbs dieback particularly during hot weather.        

            You could still be right. The tree may not be getting enough water and that just makes suckering even worse. It’s best to look at the tops of the trees to make a drought determination. When water is scarce, the canopy growth suffers and when water is really restricted there is leaf scorch and dieback by the tallest limbs.          

            If the tree is growing nicely and has lots of leaves then I would say it's getting enough water. The suckering at the base of the tree is probably normal. However, if the tree is sparse in its canopy and growth is poor and it is suckering from the base then I would worry about enough water.

Acacia Dropping Its Leaves

Q. My young acacia tree has abundant growth but only on the top half of the branches. Each of these branches are losing many leaves half way up the branch. There is a lot of growth at the top of the tree but not much below. Am I watering too much, too little? I water every five days during the summer.

A. Acacia trees are desert plants. Most desert plants are opportunists when it comes to using water. In other words, when water is present they grow like crazy. When water is absent, their growth slows and they then try to use as little water as possible. Desert plants may even stop their growth and drop their leaves when water is not available!

This is not Acacia but Palo Verde. Boring insects, or borers, may feed on a variety of trees and shrubs or very specific ones. Borers, like the flat headed apple tree borer, has a variety of trees they attack including the desert trees. Sometimes they attack trees with sun damage and other times they seem to attack trees without any cause at all.

            All plants are tremendous competitors for water, nutrients and light. They want to be “top dog” in their plant community by taking as much water, nutrients and light as possible when it’s available. By doing this, they take away these building blocks of growth from other plants.

            When water is present, trees try to get as tall as possible as rapidly as they can before they start to fill out. They grow upward first and then put energy into horizontal growth once they’ve established some height. This growth in height takes away light and shades competitors. This early growth in height, when there is a plenty of water and nutrients, oftentimes is at the expense of putting on lower growth .

We commonly see borers attack fruit trees and many different landscape plants. This flat headed Appletree borer infested a young Apple tree recently after was planted. The tree was so young that extensive damage was done by a single borer found feeding in the tree.

            Watering schedules take two different forms; how much water is applied and how often water is applied. It’s difficult to say with certainty without seeing the tree, but it sounds like it is receiving water too often.

            Watering every five days means nothing to me. I can take a sip of water hourly and someone might think I am drinking plenty of water. But another person might ask, how big are your “sips”? One teaspoon or 1 pint?

            How much water to apply? When watering trees, give them enough. Apply enough water to wet the soil at least 24 inches deep. Apply this water to at least half the area under the canopy of the tree. Once it enters the soil, the water spreads horizontally further than this.

Use 3 eighths inch rebar to estimate how deeply water has penetrated into the soil after an irrigation. Check the soil in 3 or 4 locations.

            Use 3/8-inch diameter rebar that is three feet long. After irrigating, push this rebar in the soil in three or four locations to check the watering depth. Wet soil allows the rebar to slip in easily to the same depth as the wet soil. Dry soil makes it hard to push further.

            It should slip into the soil at least 24 inches deep. Once you know how many minutes this takes, the amount of time you water won’t change. Each irrigation will be 24 inches deep.

            If using drip irrigation, space emitters about 2 feet apart. If using a basin or moat under the tree, the basin should be as wide as half the area under the canopy. Trees grow. This means the basin must expanded every three years. If using drip emitters, add more emitters every three years.

Basin under a tree used to capture the water for irrigation. If using a hose or some other delivery method that releases a large quantity of water rapidly, a basin is required to keep the water from going everywhere else but around the tree.

            How often to apply water? Look at the tree canopy. It will tell you. When the canopy of the tree starts to thin out, it’s time to irrigate! Desert trees tell you when to water when their canopies begin to thin out.