Mums, Shrubs, Cannas, Walnut Leaves

Q&A with Carol Reese Ornamental Specialist, University of Tennessee Extension

Mum Winter Survival
Fall Shrub Fertilization
Cannas Overwintering
Walnut Leaf Concerns
Perennial Mexican Bush Sage

Question: I finally moved into my own home, and last fall I planted dozens of colorful mums in the beds along the driveway. None of them survived the winter. What should I do this year?
-RB, Ottumwa, Iowa

Answer: The mums you planted were meant to be used for a brief floral display. Commonly called pot mums, they are not bred for hardiness, but for compact habit, color range, and ease of culture. Some of them prove hardy in some parts of the country, but usually they don’t have time to get well established before the onset of cold winter weather.

If you want hardy perennial chrysanthemums, it’s hard to beat some of the heirloom varieties. One that has stood the test of time is ‘Sheffield Pink’, which has soft peachy pink flowers with yellow centers. It is similar to (some say identical to) ‘Hillside Pink’.

Other good pinks include a form called ‘Ryan’s Pink’, which may actually be ‘Country Girl’ or even ‘Clara Curtis’. All of them are great beauties and tough. Ryan Gainey, noted garden designer and author in the Atlanta region, began using this mum after it was given to him as a pass-along plant, so he is not claiming to know the plant’s origin or real name. He simply popularized it when he began using it in many of his designs. He also popularized another mum, one commonly called ‘Ryan’s Yellow’.

These mums are lovely, easy to grow, and make good cut flowers. However, they do tend to flop unless their stems are pinched back several times in early summer, up until mid-July. They are also somewhat late bloomers. Although mums will tolerate some frost, you may wish to find some cultivars that bloom earlier, such as the Minnesota Hardy Chrysanthemum series.

The University of Minnesota has an 80-year-old breeding program producing mums for northernmost gardeners. These are extremely hardy, very tolerant of frost, and exhibit a range of early-, mid-, and late-season bloom. The first "cushion" mums-hemispherical plants with a solid blanket of bloom-came from this program. Among their most recent introductions is the My Favorite mum series, plants that will grow to a height of more than three feet and a width of four to five feet. (UMN’s hardy mums)

Question: Local nurseries often have ads promoting fall fertilization. Should you really fertilize shrubs in the fall?
-KL, Owensboro, Ky.

Answer: Technically, you may not need to fertilize any of your plants. The plants in field and forest get along without any supplemental fertilization, and so will yours if you have good soil. However, in the wild there is a constant replenishing of nutrients from natural decomposition. This is often not the case in suburbia, where the nutrient-rich topsoil was scraped away to level the building site, and leaves and grass clippings are raked and bagged.

Though it may seem counterintuitive, fall is actually the optimal time to fertilize shrubs. It is prime time for root growth, and plants are storing reserves in their woody tissue for next springs growth. Research has shown that shrubs fertilized in the fall have better foliar color and larger leaf size, as well as superior growth in the following season.

However, timing is very important. Fertilize too early in the fall, especially if there is plenty of rainfall, and you may stimulate the plant to put on a new flush of growth that will not have time to harden off before winter. This could result in a depletion of the plant’s reserves. Timing will vary by region, but application should be when all new growth has ceased, daytime temperatures have begun to moderate, and the soil holds adequate moisture. Two to three weeks before your average first frost is a good target date.

Typically there is plenty of phosphorous and potassium in the soil, so your fertilizer application can be solely nitrogen or a formula high in nitrogen, such as 21-4-4. Plan to apply between two to six pounds of actual nitrogen per thousand square feet. For flowering shrubs and trees, use the lesser recommendation to avoid excessive vegetative growth at the cost of flowers.

To calculate actual nitrogen, divide the number of pounds desired by the percentage of nutrient in the fertilizer. For example, to get 3 pounds of actual nitrogen using 21-4-4, divide 3 by .21. This equals 14. You should apply 14 pounds of the fertilizer, broadcasting it evenly on the soil surface.

Remember that over fertilizing is not only costly and detrimental to plants, but runoff can make it harmful to our water supply. It is always better to err on the side of restraint.

Question: I bought some of the new cannas with variegated foliage and enjoyed them all summer. I know I must dig them before the cold weather sets in, but I’m not sure about storing them. I also have some caladiums I’d like to overwinter. Please advise.
-BB, Cincinnati, Ohio

Answer: Caladiums, or elephant ears (Caladium spp.) are very intolerant of cold earth and will begin to show signs of decline as the soil cools. When they begin producing leaves that are small and pale you should gently dig them up, being careful not to damage the tubers. Spread the plants on paper in a well-ventilated indoor area to dry for a week or two. Then remove the foliage, pack the roots in sphagnum moss, sawdust, or vermiculite, and store them in a location that will not drop below 50°F.

Cannas are not as tender as caladiums. They can be left in the ground until a frost or two has blackened their foliage, but be sure to dig them before soil freezes. Then treat and store them by the same method as for caladiums.

If the cannas have been grown in a container it may be easier simply to move the entire container into a cool location after you have cut the tops back. Cease watering and wait until spring to clean and divide the fleshy rhizomes for replanting.

The difficulty in storing either cannas or caladiums is finding a spot with the right temperature. An unheated basement or storage room that doesn’t get too cold is ideal.

Question: I pick up bags of leaves left curbside and use them in my compost bin or spread them as mulch. I’ve been told this is a bad idea because black walnut leaves might be in the mix. Is this really a concern?
-GP, Cave Springs, Ga.

Answer: The fallen leaves of black walnut trees (Juglans nigra), and all of the plant’s parts as well, produce a chemical called juglone that can inhibit the growth of-and even kill-certain other plants. Some plants are extremely susceptible to juglone. Tomatoes are an example; in them the phenomenon is known as walnut wilt.

That said, there is perhaps more to-do made over black walnut’s allelopathic ways than they warrant. The tree doesn’t discourage some plants at all; Virginia’s Cooperative Extension publishes a thorough list of these plants. If your garden plants are on this list, you have no worries.

Even if you do have plants sensitive to juglone, the compound is broken down by microbial action, so composting the leaves should ensure that your leafy cache is good for all your plants. If you are still nervous about any potential toxicity of your compost, try testing it on a tomato plant.

Bagging and removing leaves is costly and a waste of nutrients and organic matter that are of value to the landscape. You are to be commended for recycling them.

Question: I have several gardening friends who successfully grow Mexican bush sage as a perennial, but I have to replant it each year. I’ve tried a heavy covering of mulch, but that didn’t help things. What am I doing wrong?
-JV, Millington, Tenn.

Answer: Mexican bush sage (Salvia leucantha), native to the mountains of Mexico, often succeeds as a perennial in the southern states. It is worthy of your devotion; as you know, the fall display is spectacular. Throughout summer, it gives little hint of the coming show as it forms a large gray-green mass. But as the days grow short, the flowering begins. Spikes of velvety purple flowers shoot upward-spikes that continue to form new flower buds at their growing tips. They become longer and more dramatic until a hard frost stops the show. The individual flowers may have white or purple petals, but they always have purple calyces. Migrating hummingbirds love to visit Mexican bush sage, as do butterflies and bees.

When this plant fails, it is usually because of drainage issues. Soils in your area are likely to have a strong clay component, and you have wet winters. Excess moisture around your plants’ roots is probably the trouble, not cold temperatures. Heavy mulches only compound this problem.

Try planting this salvia in the sunniest, driest part of your garden, in a bed that is higher than the surrounding soil. Some gardeners have found that leaving the stems of Mexican bush sage standing through the winter, instead of cutting them back, improves the survival rate.

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