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A Propagation Primer

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A new succulent plant emerges from an existing leaf through the propagation process. 

For amateur gardeners, the idea of propagation, the purposeful act of reproducing plants, can sound daunting, something that should be left to the professionals.   

But Beth Hall Hoffman, the Paul J. Kramer plant collections manager at Duke Gardens, reassures us that propagating plants is easy with the right tools and information in hand. We sat down with Beth to discuss the basics of plant propagation and the many ways these methods can be used to transform gardens both large and small. Thank you, Beth!

Thanks also to propagation team volunteer Karen Webbink for documenting the team's work in these photos.

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There is a plethora of information about plant propagation available on the internet, but it often includes concepts and terminology that can baffle laypersons of the gardening world. To simplify and summarize propagation, it’s best to think about the process in terms of three methods: cuttings, seeds and division.


Cuttings

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A variety of cuttings in Duke Gardens’ greenhouses.   

Cutting propagation is the most widely used form of asexual propagation. Essentially, you take a section of a plant that is healthy and growing, and you encourage new roots to form on that section, thereby creating a clone of the parent plant. In horticulture speak, this method relies on the formation of adventitious roots, or roots originating in an unusual place, such as the stem.   

Cutting propagation has many advantages, including:

  • plants are true to type because they are identical to their parent;
  • production is faster and the time to flowering decreases;
  • there is more uniformity among plants;
  • cutting propagation works with almost every plant, although it is harder to do with some woody plants and cacti.

So how do you start? First, identify the kind of plant you’re planning to take cuttings from. Cuttings have a better chance of success at different times of year for different plants. Should you take a hardwood, semi-hardwood, or softwood cutting? We have included a more detailed guide below to help you figure out the what, when and how of cutting based on plant type at the end of this article.

Once you have cuttings, there are a number of strategies you can employ to maximize the formation of adventitious roots. First, it’s important to keep the cuttings wet in order to minimize water loss. Without roots, plants need to stay hydrated. As soon as you have made the cuttings, keep them covered with a damp paper towel or place them in a bucket of water.  

Next, plant your cuttings by inserting them into a potting medium, remembering that constant moisture is essential. At Duke Gardens, we generally use a mix of half perlite, a glassy silicate of volcanic origin that provides aeration, porosity and fast drainage to soil, and half sand because we have a misting system that supplies continuous water. At home, you could use a standard potting mix and keep the cuttings covered with a plastic bag to retain moisture.  

For some plants, you may also want to add auxin, or root hormone, to the base of the cutting to induce adventitious roots. The two most useful synthetic auxins, which are widely available to consumers, are indole-3-butyric acid (IBA) and naphthalene acetic acid (NAA). While they don’t form roots themselves, these hormones signal the natural auxins in the plant to form new roots at the base. Lighter concentrations of root hormone should be used for easy-to-root species and/or small cuttings. Medium concentrations should be used for intermediate cuttings, and high concentrations for hard-to-root species and/or thick cuttings.

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Succulent leaves sprout new roots in the Duke Gardens greenhouses. 

Finally, try to keep the base of your plant cuttings warm. New growth forms where the temperature is warmest, so bottom heat is crucial for getting those roots to grow. Propagation heating mats are available at specialty gardening stores. Without warmth, you risk new growth only developing at the top of the plant. With that same logic, it’s important to remove your cuttings from bottom heat once roots have formed, as you’ll want more uniform growth throughout the new plant. You’ll know you have roots when you tug at the plant (gently!) and feel a little resistance.

Your cuttings do not need full sun during their initial formation. It’s better to put them in a place where you’ll pay attention to them. Once they have established roots, plant them in a container with potting a medium. You can plant them outdoors as long as you remember that they require a little bit of extra TLC and watering because they are young and tender.

Voilà! You have created a treasure trove of new plants for your garden.


Seeds

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New growth from a seed pod. 

You might be surprised to learn that a large number of the plants found at Duke Gardens are grown from seed. Duke Gardens generally plants seeds for annuals for the Terrace Gardens and vegetables for the Discovery Garden anywhere from 4 to 8 weeks before the last frost date (in mid-April in Durham – Google your location for your average frost date), which gives the plants a good head start before they are planted outdoors.     

There are many different types of plants and many different ways of growing them from seed, so it’s always good to take time to research or read the back of the seed packet to learn more. For example, some seeds require a period of cold stratification to break dormancy; some need light to germinate and others need dark; and some seeds need to be soaked in water for 24 hours before sowing.  

In general, though, there are some helpful tips to help pave the way for seed success no matter what the seed. At Duke Gardens, we use a seed planting mix made of finely ground peat, perlite and vermiculite. Fill a flat or pot to nearly full with seed mix and moisten the media by sprinkling water over the top. Depending on the seed size, you may then want to use your finger to create an indentation in the soil, creating a well to hold the seeds. The rule of thumb is that seeds should be planted at a depth of 2 to 3 times the diameter of the seed. If you are working with very fine seeds such as lettuce, you should skip this step; it’s better not to cover fine seeds with a medium.

Now you’re ready to sow the seeds thinly in rows or scatter them lightly over the seed mix. Depending on the weather, you could also sow them directly into your garden. Once the seeds are in place, cover them with a thin layer of seed mix. Once your seedlings have their first leaves, transplant them to a pot that is only one size bigger than the previous pot. Remember that you should always dig rather than pull your seedlings out of the media when replanting to ensure the roots are not damaged.

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Plants growing from seed in the Duke Gardens greenhouses.

If you have any leftover seeds, you can store them in a cool, dry place such as your home refrigerator. You want to make sure that you eliminate any moisture, which can cause mold.

If your gardening ambitions propel you to go that extra mile, you can also cultivate your own seeds from existing plants. Look for seeds to collect in late summer or fall. This is where you want to do your research, as some plants have different cleaning methods, storage requirements and time when they need to be planted. It is very satisfying to see a seed you collected germinate!


Division

Division is essentially chopping up a plant into several sections, with each section growing into a new plant. It's probably the easiest form of propagation, which makes it a good starting point for beginners. But it can also be terrifying. Can you really chop your plant in half without hurting it? With the right information in hand, your efforts can be fruitful.    

Division works best with perennials. To start, dig up the plant you wish to divide, roots and all. You will then want to brush off as much dirt as you can so you can see the roots and the crown of the plant. Then either pull or cut the plant in half to form two distinct plants, and plant both back in the ground or in separate pots. Because division causes some stress to the plant, you will want to pay a little more attention to both sections after they have been split, giving them more water until they establish stronger roots. It’s best to divide plants in the fall to minimize stress from heat.    

You can also use division to propagate a number of specialized stems and roots. We have included more detailed information below to guide you through the process of dividing rhizomes, tubers, bulb scales, bulblets and corms.

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Beth Hall Hoffman with members of Duke Gardens’ volunteer propagation team.  

If you are able to join us for the Duke Gardens Spring Plant Sale, you will find approximately 300 different types of plants for sale, with more than half propagated from plants growing at Duke Gardens. The sale will be Saturday, March 30, from 9 a.m. to noon, with a members-only preview sale on Friday, March 29, from 4 to 6 p.m. We hope that you will be inspired not only to add new plants to your garden, but also to bravely delve into the world of propagation to grow your garden to new heights.


MORE INFORMATION ABOUT CUTTINGS

Stem Cuttings

Segments of shoots containing lateral or terminal buds are used. The goal of the treatment is to create adventitious roots at the base of the shoot, thus creating a new plant. 


Hardwood Cuttings  

Wood is taken when it is matured, dormant and leafless.  Newsletter-0319-9-hardwood-cuttings-diagram.jpg

Ideal sources: Most deciduous shrubs, including Abelia, Buddleja, Cornus, Forsythia, Philadelphus and roses; trees such as Platanus, poplar and willow.

When:  Between late fall and early spring.

How:  Cuttings should be taken from the previous season’s growth. Hardwood cutting material is usually ready when you can remove the leaves without tearing the bark. Wood of moderate size and vigor works best. There are three different types of cuts commonly used for hardwood cuttings: mallet, heel and straight cuttings. 

  1. Straight cutting
    A type of cutting that does not include any old wood. This is the most common and gives the best results for most plants.
  2. Heel cutting
    A type of cutting that has a bit of old wood attached at the very base of the cutting. This type of cutting can form a heavier root system faster in certain plants.
  3. Mallet cutting
    A type of cutting that includes a small section of older wood attached to the current season’s growth. This type is commonly used on vines and sometimes hardwood cuttings.

Deciduous hardwood cuttings

This type of cutting can be done in four ways:

  1. Direct fall planting
    In areas with mild winters or reliable snow cover, cuttings can be planted directly in the ground.
    When: In fall.
    How: Once a plant is stuck in the ground, rooting may occur over winter, or roots and shoots can develop simultaneously in the spring.
     
  2. Warm-temperature pretreatment for root initiation
    When: In fall while the buds are in or entering dormancy.  
    How: The cuttings should be treated with auxin and stored in warm (65° to 70°) and moist conditions for 3 to 5 weeks to initiate root formation. The cuttings are then placed in the field over winter (in mild climates) or held in cold storage (35° to 40°) to simulate a mild winter. In spring the cuttings should be transplanted to the field.
  3. Initiating roots with bottom heat
    Cuttings are placed on top of heat for rooting, and their tops are left exposed to the cooler temperatures. This method is successful for hard-to-root cuttings and it works best in areas with mild winters.
    When: In fall or late winter
    How: The cuttings should be treated with IBA at a concentration of 2,500 to 5,000 ppm. Then place the cuttings in a rooting medium that contains bottom heat (65° to 70°F). To prevent rotting of cuttings, do not water a lot, and keep cuttings under shelter to protect from rain.
  4. Direct spring planting
    This method is used for easy-to-root species.  
    When: During winter
    How: Wrap cuttings in newspaper, place in a polyethylene bag, and store them in 32° to 40º temperatures until spring. If buds start to develop prematurely, lower the storage temperatures or plant cuttings in the field immediately.

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Narrow-leaved evergreen hardwood cuttings

What: Juniper, Chamaecyparis, Thuja

These cuttings should be taken while they are dormant, but the foliage should be left on the stem. Mature terminal shoots of the previous season’s growth are usually used.

When: Between late fall and late winter

How: The cuttings should be placed in a greenhouse in order to give them lots of light and a high humidity. Very light misting can replace high humidity, but heavy wetting of the leaves is detrimental. This type of cutting is very slow to root, sometimes taking several months to a year. Taking cuttings from a young mother plant, applying auxins and giving bottom heat (75º to 80ºF) facilitates rooting. A rooting medium of solely sand or a 1:1 mixture of perlite and peat moss is satisfactory. Dipping the cuttings into a fungicide is also recommended, considering the time it takes for some to root.


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Semi-Hardwood Cuttings

What:  Woody, broad-leaved evergreen species (e.g. camellias) and leafy cuttings of deciduous plants.

These cuttings are made from woody plants with partially matured wood.  

When: For broad-leaved evergreens–usually during summer, but they can be taken during late spring through early fall in warm climates. For deciduous, summer and early fall. The cut should be made early in the morning when the plant is turgid (hydrated).

How: In general, semi-hardwood cuttings should be about 3-6 inches long with the top portion of leaves retained. The leaves below this section should be removed to prevent substantial transpiration losses. The cuttings are usually taken from the shoot terminals, but the intermediate and basal portions of the stems will usually root as well. The cut should be made just below a node, because most of the roots will form from this bottom node. Once the cutting has been taken, apply wounding and auxin treatment for best results. The cutting should be placed in the shade with either high humidity or high misting. Bottom heat is also beneficial to rooting.


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Softwood cuttings

What: Deciduous or evergreen species such as crape myrtle, lilac, maple, magnolia and apple.

These cuttings are prepared from the succulent, new spring growth of deciduous or evergreen species. Softwood cuttings generally root more easily and quickly (2 to 5 weeks) than other types, but they require more attention and sophisticated equipment.

When:  Just after a new flush of growth in the spring. The softwood condition for most woody plants lasts from 2 to 8 weeks. It is important to take softwood cuttings early in the morning because they stress easily.

How:  This type of cutting is a much more precise science than other types of cuttings. Very fast growing tender shoots are not desirable, because they are likely to dry out before rooting. And thick or heavy shoots should be avoided as well, because they rot. Taking cuttings of average growth from portions of the plant in full sun should work well. The best cutting material is from the side branches of the plant, has some degree of flexibility, and is mature enough to break when bent sharply. The cuttings should be about 3 to 5 inches long and should contain at least two nodes. As with most other cuttings, the cut should be made just below a node. After the cut, the leaves on the lower portion of the stem should be removed and auxins should be added. Difficult-to-root species benefit from a high leaf-to-stem ratio. In general, all flowers or flower buds should be removed on cuttings. Once the cuttings are taken, they should be placed under mist in a rooting medium supplemented with bottom heat (75°-80°F).


Herbaceous cuttings

What:  Succulent, non-woody plants and many foliage crops.

These cuttings are made from succulent, non-woody plants. They tend to root easily.

When: Year-round.

How:  Cuttings should be 3 to 5 inches long, with the upper leaves retained. The cutting should be made just below a node, and auxins should be applied. The cuttings should be placed under mist in a rooting medium supplemented with bottom heat (75° to 80 ºF).


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Leaf cuttings

What: Only a small number of species can be propagated this way. Eucomis, African violets, begonias and peperomia can be propagated using this technique.  

The leaf blade and petiole, just the leaf blade, or a section of a leaf containing a midrib is used in this type of cutting. Adventitious buds, shoots and roots form at the base of the leaf and develop into a new plant while the original leaf dies off.  

When: Year-round.

How: When sections of leaves are used as the cutting material, the leaf should be laid flat on the surface of the rooting medium. Once flat, the leaf is pinned down and put under high mist or high humidity. A new plant will form from the cut midrib and the old leaf will disintegrate. The other types of cuttings should be put in a similar environment, either under mist or in high humidity tents. Most leaf cuttings root readily, but most do not form adventitious buds or shoots readily. To correct this, cytokinins can be added at the leaf base to induce shoot formation.

Cytokinins are plant growth hormones that stimulate cell division. In tissue culture, cytokinins initiate shoot sequences.


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Leaf-bud cuttings

What: Black raspberry, blackberry, boysenberry, lemon, camellia, fig, maple and rhododendron.

A leaf-bud cutting consists of a leaf blade, a petiole, and a short piece of the stem with an axillary bud attached. These cuttings differ from leaf cuttings because only roots need to form. New shoots form from the attached axillary bud.

When: Year-round.

How:  Each node can be used as a cutting. Once the cut has been made just below the node, the cutting should be treated with an auxin and inserted into the rooting medium with the bud a half inch to an inch below the surface. The cuttings should be placed under mist or high humidity and supplied with bottom heat.

 


 

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Root cuttings

What:  Kiwifruit, phlox, trumpet vine, fig, St. John’s-wort, American sweet gum, sassafras, primrose and lilac.  

Short sections of roots are used in this type of cutting. It is one of the simplest forms of propagation unless getting to the roots of the plant is a problem. It is crucial to take cuttings from juvenile plants.

When: Late winter or early spring, while the roots are still supplied with carbohydrates but before new growth starts.

How:  What type of roots a plant has is important for deciding how to propagate it. Most roots can be divided into three groups, based on their appearance.

  1. Plants with small, delicate roots – These roots should be cut into 1- to 2-inch segments and placed horizontally in a small container filled with rooting medium. The cutting should be placed on top of the medium and covered with about a half inch of soil. Water in the containers and place them in a shady location. The group of containers or container should be covered with plastic to prevent drying out.
  2. Plants with somewhat fleshy roots – These roots should be cut into 2- to 3-inch segments and stuck vertically in a flat. It is important that the end of the cutting closer to the base of the plant points upward in the medium. Otherwise the polarity will be reversed: roots will form at the top of the cutting and shoots will form at the bottom. The cuttings should be covered with about a half inch of medium and watered in. The flats should be covered with plastic and placed in shade.
  3. Plants with large roots – These roots should be cut into 2- to 6-inch segments. After cutting, the roots should be placed in a box of damp sand, bark sawdust, or peat moss and held in a 40° environment for about three weeks. During this handling process, it is important to remember the polarity of the cuttings so as to avoid burying the wrong end of the cutting later on. After the three weeks of cold storage, plant the cuttings with the tops just below the surface.

 

Polarity: A piece of a plant has two ends, proximal (nearer to crown of plant) and distal (farther from crown). For root cuttings the distal end forms roots and the proximal end forms shoots. For other types of cuttings, it is the opposite: proximal forms roots and distal forms shoots.


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Cane cuttings

What:  Dumb cane, Chinese evergreen, dracaena and similar plants with thick fleshy stems.

This form of stem cutting is mainly used to propagate several types of houseplants.

When: When the buds are dormant.

How: First remove the upper portion of the plant and cut this upper portion into 2- to 3-inch sections. Each section must contain at least two leaf scars and one dormant bud to be viable. Once cut, the cane should be placed horizontally just below the surface of the rooting medium, with the dormant bud facing upward. The cuttings should be watered in and kept moist by covering with plastic. Keep the cuttings in bright indirect light. Direct sun will cook the cuttings.


MORE INFORMATION ABOUT DIVISION

Specialized Stems and Roots

Rhizomes

  • What: Many monocots, such as bamboo, sugar cane and banana, and many grasses. Rhizomes are fleshy underground stems that serve as food storage. Planting rhizomes of a sufficient size can create new plants.
  • When: At beginning of a growth period (spring) or toward the end of a growth period (late summer or fall).  
  • How: One rhizome can be broken into several pieces, each producing a new plant, but make sure that each piece of rhizome contains a lateral bud. These rhizome pieces should be covered with soil and watered in. Covering with plastic helps to prevent drying out.

Tubers

  • What: Potato, caladium, and Jerusalem artichoke. A tuber is a modified stem that functions as an underground storage organ. The potato is a classic example of a tuberous plant. The tuber has all the parts of a normal stem, but it has “eyes,” which function as nodes. Each eye contains one or more small buds.
  • When: Dig up tubers in fall and propagate two months later.
  • How: Tubers can be propagated by either planting it whole or cutting it into many pieces and planting each separately. If planting sections of tubers, make sure each section contains at least one “eye” and weighs at least 1 ounce to provide ample energy reserves. After they are cut into sections, the tuber pieces should be placed in room temperature at a high humidity (90 percent) for about two to three days. This period allows for the cut surfaces to heal before they are planted. After they have healed, they are ready to plant in a basic medium such as Fafard 3B.

Bulb Scales

  • What: Almost any lily species. Scaly bulbs are composed of several thin fleshy segments that are attached to an axis. These fleshy segments are known as scales.
  • When: Soon after flowering.  
  • How: Scales can be pulled by the base and then placed in a plastic bag filled with moist soil to grow into new plants. If the soil in the bag stays warm (70°) and moist, bulblets and roots will form at the base of the scales in about 6 to 8 weeks. Once the bulblets are well developed, the bag should then be transferred to a cold (35° to 40°) environment for at least eight weeks in order to overcome dormancy. After this the small bulblets can then be potted up.

Bulblets

  • What: Tunicate bulbs such as tulips and daffodils. Certain bulb plants produce offsets, which are daughter bulbs that are still attached to the mother bulb.  
  • When: Fall
  • How: These daughter bulbs can be separated from the mother and then planted.

Corms

  • What:  Gladiolus and crocus. A corm is a swollen storage structure located at the base of the stems of certain ornamental plants. Corms have nodes and internodes, and they are enclosed in a dry, membranous tunic.  
  • When: Two months after blooming or until frost kills the tops.
  • How: Dig them up and place them in a warm (90°) and moist (80 to 85 percent relative humidity) environment for about a week. If the corms are still tough to separate from each other after this treatment, then the temperature should be increased to 95° for a few hours. Once the corms are separated, they should be treated with a fungicide and put back in the 95° environment for a week. This process helps the wounds on the corms heal. After this treatment, the corms should be stored in a cool (40°), moist (70 to 80 percent relative humidity) environment until you wish to plant them.