When you are trying to grow herbs from cuttings, you may find that many of your cuttings don’t grow roots. They may fail to root at all, root but have yellow and sickly leaves, or die. In our last post, we wrote that you needed only water to grow cuttings, which is true, but in some cases your cuttings may need a little more help.
If you aren’t seeing results, or if you are attempting to grow herbs from woodier cuttings (such as rosemary), then you should try using rooting hormone.
What are Rooting Hormones?
Plants use hormones to grow just like humans and animals, though their hormones are different. Various hormones cause a plant to focus on roots rather than side buds, for example, or start flowering, or drop fruit. The particular hormones we are interested in here are “auxins”, the hormone that tells the plant to root. One auxin is called Indole-3-butyric acid (IBA). There are a few synthetic chemicals that replicate IBA and they are available to home gardeners in various different forms, such as powders, liquids, and gels.
Some kinds of cuttings require rooting hormone more than others. The greenness of the stem is what determines the cutting’s rooting capability. The top part of a cutting is called “softwood.” This is the part of the stem that is green and bends easily without breaking. Softwood cuttings can often root in water alone and don’t need hormone. In most herbs, your cuttings will all be softwood cuttings. Basil, mint, cilantro, lemon balm, etc. are all mostly softwood. Other herbs that grow more stiffly, like rosemary and thyme, will have stems that have already turned brown and hardened. The middle part of the stem, where the green changes to brown, is called semi-hardwood. The hardest part of the stem is called hardwood. These older parts of the stem contain less rooting hormone than the softwood part of the stem. If you take a hardwood cutting, you will definitely need to apply growth hormone.
How To Apply Rooting Hormone
Dosage is important. Too little hormone will have no effect on the plant, but too much hormone will cause the plant to yellow and wither. Just like real medicine, it’s important to get dosage right.
Tip: To prevent contamination, always remove a small amount of hormone first and put it into a separate bowl or dish. Throw away any unused powder at the end. This will prevent you from carrying over any diseases from one plant to another.
For powdered hormone, dip the end of your cutting into a shallow plate of hormone powder. Then tap the end of your cutting on the table or the edge of the plate to shake off excess powder. You should have a thin film of hormone left over on the skin of the cutting, no more than a quarter inch away from the base of the stem. You can put the cutting into a glass of water to start it, or you can plant it directly in a pot of potting medium (more on that below). If you do use a potting medium, don’t shake off the stem first. Just push the stalk firmly down into the soil. Any loose hormone will rub off into the soil, which is fine. It’s better to start out with too little than too much.
For liquid hormone, dip the end of your cutting into a cup or bowl containing the hormone. Only hold the cutting there for a second or two, not more. Too much time can cause the plant to absorb too much hormone, which may cause the leaves to yellow or burn the plant stem. Liquid hormone is powerful and the results can be better than average, but the dosage can be difficult to get right. Beginners often prefer powdered hormone instead because it is harder to make mistakes.
Rooting hormone also comes in gel form, which is the easiest to apply because it’s easier to measure dosage, and the gel tends to stay on the plant stem better than powdered hormone does. Gel works best when you are planting your cuttings in a rooting medium and not in a glass of water. Dip the cutting in a bowl of gel according to the instructions. Typically, the gel should come up about a quarter of an inch on the stem. Push the cutting directly into the rooting medium afterward. Discard any unused gel when you are finished.
With rooting hormone, you should see a better response from your cuttings than if you used water alone, especially if you have more semi-hardwood or hardwood cuttings. Rosemary and thyme, for instance, are unlikely to root in water alone, but with rooting hormone and a dry rooting medium, they can do very well.
Using a Rooting Medium (Instead of Water)
A quick note on rooting mediums. Rooting medium is not dirt or soil: in fact, it usually contains no dirt or soil at all. Instead, it is often a lighter material that is less abrasive to the plant cutting, and which tends to hold water better. Rooting mediums can be fine gardener’s sand, perlite, vermiculite, sand, sphagnum moss, and other similar materials. You can pick any of these up at a gardening store.