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By David Graper

SDSU Extension Horticulture Specialist

Woody plants are frequently propagated using cuttings. The challenge is that there is a wide diversity of plant material and each species can have different propagation protocols to successfully root them. Softwood cuttings work best for some species, which are harvested early in the growing season. Semi-hardwood cuttings that are harvested later in the summer, after new buds form on the twigs, work better for some other species. In addition, some species are best propagated by cuttings that are harvested after the shoots have dropped their leaves and gone dormant, which are known as hardwood cuttings. Evergreens, like arborvitae and junipers can also be propagated as hardwood cuttings, even though they still have their leaves (needles) intact.

Like herbaceous cuttings, woody cuttings can be taken from different portions of stems or roots. Care must be taken to prevent desiccation before the cuttings are stuck into the growing medium. When you are collecting cuttings from the trees or shrubs in your yard, put them into a plastic bag to keep them from drying out. Also, when you harvest hardwood cuttings, it is generally best to just harvest from last year’s growth, making the cut at an angle. If you later decide to make smaller cuttings from the material that you harvested, cut the top of the cutting end square off, then recut the bottom the remaining portion at an angle to help you to keep straight which end is up. Most cuttings will not root if they are stuck into the rooting media upside down.

In general, the best cuttings are from the current season’s growth. But, some deciduous woody plant species root better if a small portion of the previous year’s growth is left attached to the base of the new shoot. Heel cuttings, for example, are removed from the parent plant by peeling back a current season’s stem from the 2-year old stem from which it is growing so that a small portion of the older stem is attached at the base of the current season’s shoot. Mallet cuttings are harvested by using a sharp pruning shears or knife to remove the current season’s stem from the older stem by cutting the older stem about 1/2” on either side of the current stem, where it is attached. Therefore, the base of the cutting looks like a small mallet.

As with succulent plants, most woody plants need to form callous tissue at the wound site before new roots can form. Callous tissue is comprised of undifferentiated cells that can multiply and then later differentiate to become roots and/or vascular tissue. The actual wound that was created when the cutting was removed from the parent plant can stimulate callous tissue to form. However, in some cases, additional wounding is done to enhance callous formation. This wounding is most often accomplished by stripping off some portion of the bark or making vertical cuts down the lower portion of the cutting. Some references might even suggest smashing the end of the stem with a hammer but this often causes too much damage to the vascular tissue to be helpful.

Hardwood cuttings are often treated with supplemental rooting hormones called auxins which contain indole butyric acid (IBA) or indole acetic acid (IAA) or both. Home gardeners can often find small packages or bottles of these rooting hormones at local garden centers or discount outlet stores in the gardening section of the store. Usually they are sold as a dry powder that contains the IBA or IAA but occasionally you can find liquid formulations which can be more effective. Higher concentrations of the rooting hormones are generally used for woody cuttings than for herbaceous cuttings. But, even if you cannot find a higher concentration powder or liquid, they will still help speed the rooting process and usually stimulate more roots to form. The liquid formulation of rooting hormones may be applied as a timed dip of the base of the cuttings or the entire cutting may be soaked in the rooting hormone solution for a specified amount of time. The concentration used, usually 3,000 to 16,000 ppm and treatment timing will vary with the species and type of cuttings being treated.

Coarse sand, peat moss, perlite, vermiculite or a mixture of components are generally used as a rooting media to provide a good healthy root system. Ordinary, clean, plastic or clay pots, greenhouse flats or other containers can be used. Since woody cuttings will generally be heavier and longer than herbaceous cuttings, there should be at least 2 to 3” of media in the containers.

Bottom heat can be quite beneficial in speeding rooting. Small electrical heating mats with thermostats are commercially available that are large enough to accommodate several greenhouse flats or pots or racks of conetainers, which are frequently used for rooting woody plant cuttings. Generally, a temperature of 70 to 80°F is sufficient. When rooting hardwood stock harvested in the spring, it is also beneficial to keep the air temperature cooler to slow bud-break on the cuttings until roots have formed. Hardwood cuttings are often rooted in a cooler with bottom heat on a commercial basis. But, homeowners can still use a heating mat and place it and the cuttings in a heated garage where the temperatures will stay around 40 to 50°F.

Commercially, hardwood cuttings are usually rooted under intermittent mist or in a propagation room with fog to maintain humidity. But home gardeners can mimic those conditions by simply sticking your cuttings into a pot of propagating media, water them in well, then cover them with a plastic bag with a few holes punched in the top. A small amount of pruning sealer or grafting wax can also be applied to the top of cutting is they were pruned there, to help slow desiccation.

Cuttings need to be exposed to moderate light but generally not full sun during rooting. Check the cuttings regularly to look for callous formation which will appear at the base of the cutting, usually at the wounded area. The callous tissue will often appear as a creamy white growth of cells at the cut surfaces. Once callous is observed, media moisture content should be gradually reduced which will usually encourage quicker rooting. Some cuttings also have pre-formed root initials as are often seen in some herbaceous plants. Willows (Salix) are a good example of this. They can root in just a few weeks. They are also known to be good producers of naturally-occuring auxins, which also help to speed rooting. In fact, early plant propagators would soak willow bark in water, then soak other kinds of cuttings in this solution before sticking them into a rooting medium to speed rooting. However, rooting will still often take from 6 to 8 weeks up to several months, depending on plant species and other conditions.

A light application of plant fertilizer once a month or so can help cuttings from being leached of nitrogen which can happen during extended periods under mist. Degree of rooting can be checked periodically by gently tugging on a cutting to see if it is easily pulled from the rooting media. If it does, then you know that it hasn’t rooted yet. Also, cuttings will often begin to grow again when roots have formed. Cuttings should be left covered in their plastic bags or in the misting area until roots are at least about 1” in length. Then they can be carefully dug up and potted up.

Woody plant cuttings may also be rooted outdoors, usually in a narrow trench dug into the soil. Use a spade or similar implement to make the trench about 1” wide and 4 to 6” deep. Generally, this process is used for hardwood cuttings that are harvested in the fall of the year. Take longer cuttings, usually 8 to 12” long, keeping in mind that the bottom 4 to 6” will be buried in the trench. Space the cuttings about 4 to 6” apart in the trench. Firm the soil around the cuttings then water thoroughly. Irrigate again if the soil dries before it freezes in the fall. Once the soil is frozen, apply a couple inches of a light mulch around and over the cuttings. Ideally, by the time buds break in the spring and new growth appears the cuttings should be rooted. Then, carefully dug up the individual cuttings and transplant them to their permanent home or into containers.

Mark your calendars

Spring Fever 2018 Gardening Event will be held in the Rushmore Room at Ramkota Hotel in Rapid City on March 3, from 8 a.m. – 4 p.m. This year’s conference features two presentations by Dr. Linda Chalker-Scott of Washington State University, speaking on “Evidence Based Gardening Information” and “The Root of the Problem, When Plants Don’t Thrive.” Dr. John Ball, SDSU professor and Extension forestry specialist will present “Selecting Trees for Western South Dakota.” Short “table talks” topics include: “Growing Vegetables in Containers on Your Deck or Patio”, “Caring for House Plants”, “Conserving Water in Your Garden”, and “Using Compost and Mulch in Your Garden.” Lunch catered by Minerva’s is included in the $35 registration fee. The event includes a silent auction, door prizes and a free table. Registration limited to 200, please register before Feb. 27. Visit www.blackhillsgarden.com under the “Welcome” tab for registration blanks and more information.

South Dakota State Horticulture Society 2018 Spring Workshop will be April 21 at the Plains Dining and Recreation Center, located at 960 4th St. NE in Huron, S.D. Registration begins at 9:30 with the program starting at 10 a.m. We are extremely fortunate to have Mr. Brent Heath, of Brent and Becky’s bulbs as our speaker for the workshop. Brent is a noted authority on bulbs who has spoken all over the country and world, educating and promoting the use of flowering bulbs of all sorts. He will speak on four different topics: “Bulbs as Companion Plants”; “Pest Resistant Bulbs”; “Tantalizing Tulips”; and “Lovely Long Lasting Lilies and Awesome Alliums.” Cost for the day-long workshop, including lunch is $30 for members; $35 for non-members. Mr. Brent Heath is co-owner with his wife Becky of Brent and Becky’s Bulbs. Brent and Becky, along with their family has been growing bulbs since 1900 when Brent’s grandfather moved to Virginia after falling in love with the Daffodil. Today, they still love the Daffodil, but offer so much more! Bulbs, seeds, perennials, plants, supplements, books, gifts — they’ve really evolved! They live and work on their 28-acre farm and gardens in Gloucester, VA. They are well-known flower bulb suppliers, garden writers, photographers, lecturers, consultants and educators as they developed 8+ acres of educational demonstration gardens. You can view their website, catalog, learn more about Brent and their company and read more about the topics on which he is speaking at https://www.brentandbeckysbulbs.com/

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