By Susanne von Rosenberg, UC Master Gardener of Napa County
Some gardening advice seems so sensible or obvious, or has been repeated so often, that we “know” it to be true. As gardeners, we need to be careful about falling prey to myths. In this column, we'll explore some widespread gardening myths, although perhaps I should call them confusions.
We are usually told that wilting leaves indicate that a plant needs water. This is often the case, and you can check to see if your soil is dry. But there are other reasons that a plant's leaves could be wilting.
If the soil is waterlogged, the oxygen that roots need to grow is replaced with water. This happens often to plants in containers without drainage. Plants may also become waterlogged in compacted soil that doesn't drain well. When roots lack oxygen, they can't supply water to the rest of the plant. The leaves continue to photosynthesize and to use water. By forcing roots to shut down, waterlogged soil leads to wilted leaves.
Fungal or bacterial disease can also cause leaf wilt. Fusarium and verticillium are common in local soil. These fungi block the plant's water transport structures, causing leaves to wilt even when there is plenty of soil moisture. Often the entire plant dies.
Keeping your soil too moist can promote fungal disease. Another culprit for wilting leaves is animal damage to the roots. Gophers, nematodes and other insects feed directly on roots, and other animals may create tunnels that drastically reduce root contact with surrounding soils.
Another possible cause of wilting is overfertilizing or excess salt in the soil. Always follow the directions on any packaged fertilizer, and be aware that steer manure is relatively high in salt.
You may have heard that bark chips are a better mulch for your perennial woody plants than tree chips. Some say that tree chips could contain disease-causing organisms or compounds that inhibit the growth of other trees.
Neither of these concerns has been documented as a significant issue. Conversely, bark chips, while a more attractive mulch than tree chips, have poor water retention. Tree bark is designed to prevent water loss. It contains a waxy compound, suberin, that repels water.
Tree chips, on the other hand, consist mostly of inner wood, which has the capacity to absorb and hold moisture. Depending on where the logs were stored before the bark was collected and chipped, bark chips can also contain weed seeds. Tree chips are more environmentally friendly, too, because they are local. By using them, you are reducing the amount of material that needs to be processed in a composting facility.
While we're talking about mulch, let's explode the myth that there is any permanent solution for weed control other than paving over your yard. Landscape fabric, which differs from black plastic mulch in having small holes for air and water exchange, is often touted as a permanent solution. It is used in commercial vegetable and ornamental plant production, where it is successful at keeping weeds out. It can also be easily removed and replaced each year.
However, landscape fabric is not good for permanent weed control around perennials. If left exposed, the fabric can degrade in a year. If you cover it with mulch or allow plant debris to accumulate on top of it, weeds will colonize the plant material or mulch and grow through the fabric, making it difficult to remove.
You can cover the fabric with gravel, but you'll still need to keep removing the weeds that manage to colonize gravel-covered fabric. Your plants may also grow into the fabric, so when you remove the fabric, you damage their roots.
As the fabric degrades, you'll have a clean-up and aesthetic headache, with lots of shredded bits. (I know this from personal experience.) Gophers can exert so much pressure building mounds under the fabric that the fabric rips. If it doesn't rip, you end up with exposed landscape-fabric-covered mounds.
Unfortunately, there is no magic bullet for weed control. You can reduce them, but you still need to remain vigilant no matter what method you use. Organic mulches are best as they improve soil texture and health and can be replenished as needed. Couple them with targeted watering.
If you want to learn more about garden myths, Dr. Linda Chalker-Scott, an extension horticulturist and associate professor at Washington State University, has a wonderful website that discusses all sorts of common Horticultural Myths.
Food Growing Forum: Napa CountyMaster Gardeners will present a discussion of “Perennial Vegetables, Garlic and Alliums” on Sunday, October 10, from 3 p.m. to 4 p.m., via Zoom. Register here to receive the Zoom link.
Free Guided Tree Walk: Join Master Gardeners of Napa County for a tree walk in Fuller Park in Napa on Tuesday, October 12, from 10 a.m. to noon. Limited to 12 people per walk. COVID safety protocols will be followed. You will be asked health questions and asked to sign in. Face masks and social distancing are required. Register here.
Got Garden Questions? Contact our Help Desk. The team is working remotely so please submit your questions through our diagnosis form, sending any photos to email@example.com or leave a detailed message at 707- 253-4143. A Master Gardener will get back to you by phone or email. For more information visit http://napamg.ucanr.edu or find us on Facebook or Instagram, UC Master Gardeners of Napa County.
By Susanne von Rosenberg, UC Master Gardener of Napa County
Gardening myths can come from many sources. A fair number arise when information applicable to agriculture is misapplied to home gardens. Others are based on what seems like a logical extension of science.
Let's take the myth of disinfecting or sealing pruning cuts. You can still easily find wound-sealing compound in garden-supply centers and online. At first glance, it seems like a really good idea to disinfect or cover an open pruning wound. After all, we know that disinfecting and covering our own wounds to keep out germs and dirt leads to better healing.
However, in the vast majority of cases, trees and other woody perennials heal better if the wound is left untreated. They form a natural callus that repels pathogens. Disinfecting the cut damages the tissue further, and studies show it leads to greater loss of tissue than not disinfecting. Sealing the cut seals in moisture and pathogens and prevents the callus from forming.
Another logical-sounding myth is that mulching with wood chips from tree trimmings could spread disease because the chipped trees may have been diseased. A related myth is that the chips might inhibit plant growth if they came from trees that exude allelopathic chemicals, compounds that inhibit other plants.
Wood chips should only be used around perennials. They are not suitable as mulch for annual plants or vegetable gardens. In my research I was only able identify one relatively common local tree, black walnut, that might present a problem when used as wood chip mulch.
Many trees contain allelopathic compounds, but in mulch, this is actually a benefit. The chemicals are unlikely to affect established plants, but they will reduce weed-seed germination and weed growth.
Wood chip mulches may create a nitrogen deficit where the mulch meets the soil, but that also helps reduce weed growth.
There is no evidence that pathogens in wood mulch can transmit disease to established plants if you keep the mulch three inches away from the trunk or base of the plant. Applying four to six inches of wood chip mulch to your garden is an inexpensive, sustainable practice. In highly disturbed or eroding soil areas, or when working to control perennial weeds, eight to twelve inches is recommended.
Another common gardening myth that sounds logical is that you should amend the soil in a planting hole. Some sources recommend adding 25 to 50 percent organic matter or other amendments. It makes sense that improving the soil and adding nutrients would improve the growth and survival of the plant.
Initially, this may be true. But by radically changing the soil in the planting hole compared to the native soil, you are effectively creating a “pot” in the ground. When the roots encounter the harder, finer native soil, they will circle back to the material in the planting hole. With access to only the water and nutrients in the planting hole, the roots won't be able to support the canopy. Also, the planting hole will act as a reservoir for rain, depriving roots of oxygen. In the dry season, irrigation water will leach out of the planting hole into the native soil and become unavailable to the roots. Finally, most of that organic matter will eventually decompose and your plant will sink.
Sometimes a gardening myth is only partly untrue. Advice may be valid for certain climates or soil types but not for other locations. That why it's important to make sure that recommendation are appropriate for our climate.
Before I became a Master Gardener I was fascinated by the idea of “sunken beds,” a gardening technique for conserving water in dry climates. The idea seemed applicable to my area but it wasn't. Sunken beds were developed for the arid southwest. By planting seedlings in an area excavated six inches or more below ground level, they are more protected against wind, and the surrounding soil provides some cooling. In addition, the beds can capture and retain water from monsoon rains.
But the Napa Valley rainfall pattern is different. Compared to the arid Southwest we get a lot of winter rain. Unless your soil has good percolation, your plants will drown in sunken beds, and your soil will be oxygen deprived.
Successful gardeners are always learning and thinking critically. When you hear or see gardening advice, make sure that it has a basis in science and is applicable to our climate.
Napa Library Talks: First Thursday of each month. Register to get Zoom link. http://ucanr.edu/wildlifehabitat2020
Got Garden Questions? Contact our Help Desk. The team is working remotely so please submit your questions through our diagnosis form, sending any photos to firstname.lastname@example.org or leave a detailed message at 707- 253-4143. A Master Gardener will get back to you by phone or email.
For more information visit http://napamg.ucanr.edu or find us on Facebook or Instagram, UC Master Gardeners of Napa County.