It's Time For Oil

We’re now within weeks of the time when we start applying oils to some of our trees and shrubs. No, not because they are creaking and making strange noises calling for lubrication, but because these special oils can reduce insect and disease problems appreciably when they are applied just at the beginning of the growing season.

The initial oil application is in the form of what we call “dormant oils,” which have been used for over a century. Sprayed on fruit trees and some of our ornamentals, this oil spray essentially squashes scale, newly emerging aphids and other insects, while coating eggs that may be present, and can also be helpful in controlling some diseases — including powdery mildew — by smothering them, as well.

Early on, dormant oils were tricky to use. Your timing had to be perfect since the oils were not highly refined. When applied to trees with unfurling foliage, the foliage often got burned. Newer oils, however, have been refined to over 99 percent purity and are, for the most part, non-burning. The refining process has also left some of these oils so pure that they are considered usable in organic gardening — and many even have an ORMI label, so check for that.

It’s important, though, to try to use petroleum-based mineral oils for the dormant spraying because they dry more slowly and seem to be more effective in early applications than other “horticultural” oils, which we use later in the season.

These oils, no matter what type, have no residual effects and are only effective on contact. They are usually sprayed in March when the days are mild — 45 degrees Fahrenheit is perfect — and night temperatures don’t drop below freezing. Obviously, timing is critical and rain should not occur for 24 hours after spraying, and don’t apply oil to wet foliage either. You also want to avoid spraying these oils on foliage that’s begun to unfurl, as some burning can take place, though this has become rare with the highly refined oils.

The most common use for dormant oils is on fruit trees — and those who skip this treatment inevitably suffer the consequences. This type of oil can also be applied to roses that haven’t broken dormancy and on lilacs that are prone to scale. And, if you should forget to do your application in March, you may still be able to in April if it’s cool enough, or you can also use a lighter, vegetable-based horticultural oil. It won’t be as effective, but it won’t burn.

Two years ago, I noticed that three of my magnolia trees had bad scale issues, which is not uncommon. I made a note in my calendar and, the following March, I got out my small sprayer and applied dormant oil to all three. Yes, it worked and quite well. It was difficult to find any live scale on these trees last year and I’ll do a repeat application in several weeks.

Many of you will have your tree service or landscaper do this spraying and it may be part of the package you have with them. Be sure to ask what oils they use. On smaller properties, you can do this chore on your own, but take a few things into account before you start.

You may need up to four gallons of finished spray to adequately cover everything, including nooks, crannies, cracks and crevices. Those gallons add up and that’s a lot of weight, since the spray alone for four trees will total over 32 pounds. If all you’re doing is a few rose bushes, you might get by with a two-gallon pump sprayer. But how do you get higher than 12 feet or 15 feet, or spray an entire orchard, even if it’s a small one?

Like I noted, most will have this level of work done by a professional, but there are some options for the small property owner. Several brands have ready-to-use spray bottles that you simply attach to your hose, turn on the water and spray. But do you really want to be dragging a hose around when it’s 45 degrees outside? This could be a great option for later oil sprayings, but not the initial dormant spray.

Also, keep in mind that not every plant is oil tolerant. Oil sprays can damage junipers, cedars, maples — especially red and Japanese — and spruces, particularly the blue and Alberta types. As always, read the label before spraying and, if you have someone else doing this, make sure you move furniture and automobiles out of the spray and drift area.

For those of you with smaller properties, I do have one suggestion if you want to spray on your own. I don’t own a large sprayer and none of my compression sprayers would ever reach the top of my magnolias, so my solution is a small, 10-gallon sprayer with an electric motor that can be run off a tractor battery, or a small, 12-volt motorcycle battery.

This particular sprayer — a 10-gallon NorthStar ATV Spot Sprayer, which has an output that will reach about 15 feet high — comes from Northern Tools and costs around $115. At just 15 pounds, it’s easy to move around and, while not perfect, gets the job done on a small property.

The sprayer does require some easy assembly and before you fill it with dormant oil, first try it out with a few gallons of water, so you become familiar with how to use it. Needless to say, check for leaks.

With a little ingenuity, you can figure out how to power it and move it around. I put mine in a small trailer behind my Deere X380 lawn tractor and run the power cord off the tractor battery. My larger tractor has a lighter outlet at the seat, so that can be used, as well. So far, it’s the best rig I could set up for my small property and still get the spraying done without having to call in Big Tree.

That’s the dormant side of the oil story. But there are other oils you can use during the growing season that also come in handy. These are referred to as “summer oils” or “all-season oils.” For the most part, they are vegetable oil based and most are organic — although neem oil, which has various uses, comes from the neem tree. The vegetable oils are very effective contact insecticides, but they are non-selective and will smother and kill just about any insect that comes into contact with it. For this reason, they should be used when bees are not present.

Keep in mind that none of these oils have residual properties, so they only kill what they touch. Those labeled for all-season or summer use come in handy for insect control and some disease control, with neem oil seeming to be the best of the lot when it comes to both. Any of these oils need to be used with caution, though.

Never use on a hot, sunny day — you will inevitably end up burning foliage and buds — and check the label of each product to see what plants might be sensitive to it. They are also excellent to use on house plants, but again, check the label for plants that might be sensitive, like ferns. Remember, there is no residual effect and if it rains within several hours of application, your spray may be ineffective.

When used in the vegetable or ornamental garden, keep a record of when and what you spray. You may find that this becomes such an effective way to manage insects and diseases with low environmental impact that you will want to apply it on a regular schedule. But again, refer to the label.

And, yes, when sprayed on Rose of Sharon and roses, these all-season oils will kill Japanese beetles — especially one brand that has some pyrethrin mixed in — but there is a catch. Japanese beetles are most active in bright sunlight in the heat of the day. This is the worst time to apply oils to any plant. Spraying early in the morning before the beetles emerge will be useless and, remember, no residual activity. If I ever need to spray my shrub hibiscus or roses, I try to do it when the buds are closed with no bees present.

So, figure out what you’ll use as a sprayer and speak with your arborist or landscaper to see what they are using and why. Keep in mind that there are oils that can be used as dormant and all-season sprays, but the best dormant oils are only dormant oils and not all-season oils.

Keep growing.