What is a root stimulator? Think of it like a cup of strong coffee or a can of Red Bull for your plants. It gives them a burst of energy to overcome stressful conditions such as transplant shock and help them quickly establish in a new spot.
Like the name suggests, root stimulators promote faster root growth. Longer roots and lots of them means a plant can more efficiently absorb water and nutrients. This fosters vigorous growth and improved overall health, including better pest and disease resistance.
Also called root promoters, root stimulators are widely used by commercial growers to encourage fast, healthy and uniform root systems when growing cuttings. They can be used on vegetable crops, woody plants, perennials and annuals. Pretty much any plant type may benefit from root stimulators if conditions merit it, whether in the seedling stage, as a cutting or when a plant is in mature growth.
Root stimulators are best used as a pick me up, not as a regular feed. So when should you consider using one? When you've dug up a plant for transplanting and didn't get it back into the ground right away. Or when a plant has a long taproot – taprooted plants are notoriously difficult to transplant, and root stimulators seem to help. Maybe you've bought a plant at the end of the season that's struggling but can be saved with some TLC. Plants that have been neglected or straggly annuals that could use a boost are good prospects, too.
Root stimulators can be made from seaweed, algae, humic acid, or plant extracts, or a combination of them. They are most often sold in a liquid solution that is diluted with water and watered into the soil around the plant. They can contain macro- and micronutrients, trace elements, minerals, and beneficial microbes.
The active ingredients in root stimulators are auxins. Auxins are hormones that plants make to regulate cell growth and division. Some brands of root stimulators contain natural auxins, while others are synthetic-based.
Root stimulators may contain phosphorus, the "P" in NPK (nitrogen, phosphorus, and potassium). Phosphorus also stimulates root growth, so any plant fertilizer containing it is a de facto root stimulator. Some brands add mycorrhizae, "good" fungi that work symbiotically with plants to promote growth.
Then there's Vitamin B1 (thiamine), touted as a miracle ingredient in some well known root stimulators, such as SuperThrive and Rootone. Early research indicated that vitamin B1 stimulated root growth, helping plants better establish in new soil. (Plants naturally make Vitamin B1, as do fungi and bacteria. Healthy soil will contain adequate amounts of it.)
Subsequent research has refuted these results, indicating that it wasn't the B1 that was initiating root growth, but the auxins also present in the solutions. It's not that these products are ineffective, it's that their success can't be attributed to Vitamin B1 alone.
You can also make your own root stimulator from willow water. Willows and other fast-growing trees such as poplars and silver maples are loaded with auxins. That's why you can cut a willow switch, stick it into the ground, and it will grow roots. It's made by cutting the newest branch growth, boiling the pieces in water and then watering plants with the cooled mixture. This article has more about making willow tea: https://www.gardeningknowhow.com/garden-how-to/projects/making-willow-water.htm
So are root stimulators necessary? Not under ordinary conditions. As long as the soil is good, with nutrient levels and pH appropriate for the types of plants growing there, plants are healthy, and receive the proper amounts of sunlight and water, root stimulators shouldn't be necessary. Consider root stimulators an emergency "medicine" to give a plant that's struggling.
The bottom line? Plain water is the best root stimulator of all. When planting or transplanting, be sure the plant is adequately watered beforehand, water deeply at planting time and afterwards as it settles into its new spot. Take care to ensure that the water soaks past the rootball and into the existing soil. This encourages the plants' roots to grow into the soil past the planting hole.
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By Delilah Onofrey, Suntory Flowers
Photographs courtesy of Suntory Flowers
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