Spring is a great time to prune trees in the orchard or in the landscape. Most of us think of pruning as the removal of branches and twigs. That is true but pruning also stimulates new growth and how we do it will determine the direction of that growth.

We prune landscape trees to improve their shape, remove potentially dangerous growth and to try to limit size. Fruit trees are pruned to improve the yield of fruit. Each type of pruning has its own goals and techniques.

When pruning apple, peach, pear and other fruit trees we want to establish several strong, fruit-bearing branches called scaffolds. We also want to open up the canopy so that sunlight and sprays may penetrate into the interior of the tree. A well pruned fruit tree may appear “butchered” to the average homeowner. In fruit trees, beauty is found in an abundant crop of well formed fruit next fall.

Landscape trees are pruned for aesthetics and safety. If you plant the right tree in the right site, you should have to do very little pruning. Given enough space and separation from houses, driveways, streets and power lines, most trees will grow just fine on their own.

Pruning trees is both an art and a science. The science consists of understanding how trees grow and how to guard their health. Art involves being able to imagine what the tree will look like six months after you prune it.

Following a few general rules will help guarantee success in pruning trees. One of these is to avoid leaving stubs. When using hand pruners, cut small branches back close to the larger stem. Small tubs will only die and provide a place for insects and diseases to enter the plant. Plus they will interfere with your enjoyment of the tree. Stubs are ugly.

Speaking of pruners, one key rule is to keep all blades sharp. A wide range of  hand pruners, loppers and saws are available. The most important factor is to buy good quality equipment which will hold a sharp edge. Dull pruner blades will cause ragged cuts which do not close easily and look awful. Cheap equipment tends to be made of lower quality steel and is more difficult to keep sharp.

In the olden days, we used to apply a thick, tar-like compound to the larger wounds created by pruning. Research has shown that these compounds may actually cause more harm than good. They seal moisture next the wound and this may encourage rot and other fungal problems. So, the general recommendation is to avoid using pruning sealants.

There are two exceptions to the rule. One is when pruning peach trees. Since peaches are susceptible to an insect borer that enters through wounds, sealants are often used to protect them. The other exception occurs when pruning red oak trees.

Oaks and more specifically, primarily red oaks, are susceptible to a serious disease called oak wilt. This fungal disease is spread by small beetles visiting an infected tree and then flying off to another tree. The insects are attracted to fresh, open wounds so the recommendation is to avoid pruning oaks during the growing season. If you absolutely must prune them when the leaves are present, use a sealant to cover the wounds. Otherwise, prune oak trees during the dormant season i.e. when the leaves are off, and the insects are not active.

Finally, avoid pruning spring flowering shrubs such as lilacs, forsythia, rhododendrons, magnolias and others until after they bloom. Shrubs that bloom in April and May develop flower buds the previous autumn. To prune them now will only reduce the number of flowers for your enjoyment.

Ice Covered Stem

Those of us who live in the temperate zones i.e. those that routinely get at least one killing frost, often have to deal with injury to our landscape plants from cold temperatures. When we venture out into the garden in the early spring, we begin to see dead or damaged plants that seemed to be in good shape the previous fall. What happened?

Of course, the first thing and for some people, the only factor we think about is cold temperatures. However, there are several other weather related impacts that need to be considered.

1. Low Temperature Injury – Most gardeners are familiar with the USDA Hardiness Zone map. This divides each area of the country into units based on the typical lowest temperatures expected. For instance, if you are in USDA Zone 5, you can expect to experience winter low temperatures between -10 and -20 degrees Fahrenheit.

Plants that are not winter hardy for your Zone will suffer damage to buds, twigs, branches or, in extreme cases, the entire plant will die. Basically, this is due to the fact that the cells within the plant tissue will freeze and cause its solid cell walls to break. The dead cells will turn brown and, of course, will never return.

Desiccation on Rhododendron

2. Desiccation Injury – Cold air can hold less moisture than warm air. So, cold winds flowing over buds and other plant tissue in the winter may slowly pull moisture from the plant. Since the soil is frozen and the roots cannot replenish the moisture, the plant cells may lose all their water. This will cause death and browning of that tissue. This is especially a problem on broadleaf evergreens such as rhododendrons and boxwood.

3. Frost Cracks – When the wood on trees or shrubs is frozen but the sun shines on a January or February day, there may be enough heat generated to begin to expand the exposed bark. Since the wood beneath is unable to move, the bark may develop a split. Often, but not always, this split will be on the south or southwest side of the tree since that is the direction of the most intense winter sun.

4. Snow or Ice Damage – The weight of ice or snow may cause flexible trees such as birches or evergreens such a pines bend over. In some cases, this will be heavy enough to actually break off the branch or the entire tree. In others, however, it merely bends the plant and it will return to its formal position. Unfortunately, the bending may have cracked the bark allowing fungal organisms to enter the plant and cause the development of open wounds called cankers.

5. Salt Damage – Although special anti-icing products are now available that do not damage plants, plenty of people still use good old salt. Whether the salt is regular table salt or a granular fertilizer, it has the same effect on plants. When salt comes into contact with plant foliage or roots, it forms a powerful attraction to water. It will pull the water out of the plant tissue causing it to die and turn brown.

6. Heaving – Soils that contain a lot of clay are plagued by a process called heaving. Clay soils are poorly drained and retain too much water. During the winter, when that water freezes, it expands. This can cause damage to roots and, in some cases, may actually push plants out of the ground. The effect is greater on plants that were installed late in the fall and have not had a chance to establish enough roots.

7. Rodent Damage – Unlike bears, mice do no hibernate in the winter. Therefore, they need to continue to eat. If the winter has a lot of snow and other food sources are denied, mice may turn to chewing on the bark of trees. Often, they will burrow under the snow and much and eat the bark at the base of the tree. If they destroy enough of it, they will have girdled the tree resulting in its loss.

8. Early Spring or Early Fall Frosts – Plants that are hardy for cold winter areas have developed one or more techniques for surviving. Generally they build up hardiness slowly in the fall, reach peak hardiness in the height of winter and then slowly lose hardiness into the spring. For example, a plant buds that will survive in -20 degrees in February, may be damaged or killed by 20 degrees in October or March.

So, what can you do about all these problems? Check back next week for How to Avoid Winter Injury in Landscape Plants.

1. Low Temperature Injury – The key to avoiding this problem is to not “stretch the zone” with your plants. This means that, for instance, if you are in USDA Hardiness Zone 6, avoid using plants that are only hardy to Zone 7 or 8. You may get away with this for several years if the winters are warmer than average. However, all that it takes is one night below the “typical” low temperature for your zone and the plant may die. That is a fallacy of people when they think that global warming automatically changes their climate zone.  You may, in fact, experience above average temperatures for 364 nights but, if that one night in the dead of winter drops below the hardiness level of the plant…it will still be dead.

The other factor here is the condition of the plant coming off the previous growing season. There is some evidence that plants that were stressed by drought or excessive heat or other factors may not be able to attain their normal level of winter hardiness. Thus, even though the plant is technically hardy for a particular zone, if it comes into the winter under stress, it may still be damaged by “normal” low temperatures.

Burlap Wind Protection

2. Desiccation Injury – Generally, the key factor in desiccation injury is the type of plant and the direction of the predominate winter winds. As we mentioned before, evergreens, especially broad leaf evergreens like rhododendrons and boxwood, are most likely to sustain this type of damage. Therefore, these types of plants will benefit most from protection from winter winds. This can be done by a temporary wind shield such as burlap fences for rhododendrons and fences, buildings or other more permanent windbreaks may also be used.

Plastic Tree Wrap for Both Rodents and Frost Crack

3. Frost Cracks – Preventing the penetration of winter sunlight is the object to preventing winter cracks. Smooth, dark, young bark is most susceptible to this damage. So, on newly planted, small trees such as a maple, it might pay to wrap the bark in a light colored material that will reflect the rays of the sun. The key is to remove this wrap after a season or two and either leave it off or replace it if the bark has not yet become rough textured and more “mature”. If the wrap remains on the tree too long, the expansion of the girth of the tree may eventually cause it to cut into the bark and girdle the tree.

4. Snow or Ice Damage – First and foremost, don’t shovel the snow from your driveway or sidewalks onto the top of your landscape plants. Generally, they can handle the weight of a normal snowfall but the added snow may cause damage. If you have tall, upright arborvitae trees, you might need to tie them together with ropes in the fall  when they are older and have become very tall and weak. Gently knock off excessive snow cover as soon as the storm is over.

Wrap Left on Too Long

5. Salt Damage – If you must control the ice on sidewalks or driveways with other than mechanical means, always use products labeled as not damaging to plants. Remember that granular fertilizers are also salts and could damage nearby plants if applied excessively for ice control throughout the winter.

6. Heaving – This problem takes a little more work than some of the others. If you have clay soil, you need to incorporate large amounts of organic matter such as compost, leaf mould, clippings, etc. This will help to improve the drainage of such soils. However, this needs to be done before you install your plants. It is pretty tough…though not impossible…to amend the soil after landscape plants, other than annuals, have been put into the ground.

7. Rodent Damage – This is most commonly a problem during extremely cold winters with large amounts of snow on the ground for long periods. Since you can’t predict which winter is going to meet these criteria, you need to take some general steps. One is to avoid piling mulch up against the trunk of the tree. Often mice will burrow in the mulch right up to the tree and chew away in the comfort and security of the mulch. Also, wrapping the trunks of young trees that still have immature bark can also be a deterrent.

8. Early Spring or Early Fall Frosts – Unfortunately, on a practical level, there is not always anything you can do about this type of damage. If you have plants small enough to cover with a tarp or other covering, that can help for a night or two. However, this won’t do much for large shrubs or trees. Remember that the goal of any covering is to keep the temperature of the plant material at least one degree above that needed for the cells to freeze. Unlike animals, plants don’t “feel” the cold. Their cells either freeze and die or they do not freeze and survive. It is that simple.

Hostas have 30 chromosomes in each of the male i.e. pollen, and female i.e. egg, parts. During the normal fertilization process 30 chromosomes come from the mother and 30 from the father for a total of 60. This is called a diploid or 2n plant which represents 2 sets of chromosomes that make up the genetic information in the resulting seeds and seedlings.

Probably the greatest thing for the Hosta Rookie to understand in all this is that sometimes something happens in the cells of a plant which results in more than 2 sets of chromosomes being present. The most common variation you will encounter is when you hear of plants called tetraploids.

Instead of having 2 sets of chromosomes, tetraploids have 4 sets. This occurs occasionally as a natural mutation so there are plants in the wild who are tetraploids such as the species Hosta ventricosa or triploid i.e. 3 sets, like Hosta clausa. However, people have now also figured out ways to treat hostas with chemicals to create plants with extra sets of chromosomes.

Perhaps the first way was an unintended exposure of hosta plants to the herbicides named Surflan (oryzalin) and Treflan (trifluralin). Following this exposure, the nursery person noticed a change in the physical characteristics of hostas.

A second, much more common way that tetraploid hostas have been induced is through exposure to certain plant hormones during the tissue culture process. We will cover tissue culture in greater detail later but suffice it to say that part of the process involves the use of plant hormones such as indole butyric acid (IBA) and naphthalenic acid (NAA). For some reason, certain hostas exposed to this treatment will develop tetraploid plants.

Daylily (Hemerocallis) breeders have been developing tetraploids for several decades. They often use a chemical called colchicine which is an extract from the fall crocus, Colchicum speciosa. It appears that this chemical does not have the same effect on hostas, however.

So, why are we so interested in tetraploid plants? Well, not only do they have a unique genetic makeup, tetraploid plants also generally have some unique physical characteristics including:

1. Plant Size – Tetraploid plants tend to be a bit smaller than diploid plants of the same cultivar and have shorter leaf petioles. However, this more compact structure makes for a denser plant habit.

2. Substance – Compared to a diploid plant, the tetraploid has thicker cell walls and larger stomata guard cells resulting in a leaf with thicker substance. The petioles are thicker also.

3. Flowers – This is the area where the daylily people have made big advances with tetraploids. The flowers of these plants are larger and have thicker petals or, in the case of hostas, tepals. Also, flowers of tetraploids produce larger pollen grains than their diploid counterparts.

4. Flower Scapes – The flower stalks will be shorter and thicker on tetraploids. Often the flowers on the scape will be compacted together in a denser arrangement.

5. Seed Pods – On tetraploid hostas, the seed pods will be shorter in length and thicker.

6. Growth Rate – In general, tetraploid plants will have a slower growth rate than diploids of the same cultivar.

7. Roots – In keeping with the general trait of these types of plants, the roots of tetraploids will often be shorter in length.

8. Leaf Color Variegation – Tetraploids will generally have wider marginal variegation but narrower medial or center variegation.

9. Chloroplasts – Chlorophyll is, of course, the green pigment. The chloroplasts are the structures where chlorophyll resides and where photosynthesis takes place. In tetraploid plants, there may be up to twice as many chloroplasts as in the diploid plant.

10. Leaf Surface – For cultivars with corrugations or ripples on their leaves, tetraploids tend to be more pronounced.

CAUTION: Now that we have told you all the characteristics that may indicate a plant is a tetraploid, be aware that it is not that simple. Even if a plant has ALL of these traits, the only way to know for certain that the plant is a tetraploid would be to subject it to a bunch of different laboratory tests. Also, it would take a person well-trained in the science to examine the morphology of the plant and, perhaps, do some cross-breeding to tell for sure. Most of what you hear about a plant’s tetraploidy will be a matter of opinion. It may well be truly tetraploid but not proven yet by analysis.

Clematis ‘Barbara Harrington’

Over the years of dealing with the gardening public, I realized that we often throw around terms and names that could be a bit misleading. Eventually, I jotted some of these tricky terms down and came up with the following:

The mountain ash tree (Sorbus) may come from the mountains but it is not really an ash tree (Fraxinus). It is actually a member of the Rose Family and is a close relative to apples, pears and roses. Mountain ash trees are not bothered by the Emerald ash borer either although, since mountain ash have their own serious disease and insect problems, that might not be a bad thing.

Boxelder tree seedlings are often mistaken for poison ivy when they first emerge from the seeds. They have the same three leaf structure but they soon form a tree-like stem and not a vine like poison ivy.

Douglas fir is not a true fir tree. Kirk Douglas was the star of Sparticus.

Blackberry plants are less winter hardy than raspberries. When you pick a blackberry, the little white pit comes along with it while the pit stays on the plant when you pick a raspberry.

Dutch elm disease killed primarily American elms (Ulmus americana). It might have come to the U.S. from Holland, thus the name.

Oregon grape holly (Mahonia) is not a holly (Ilex). It may be from Oregon but it does not produce grapes either.

The roots of black walnut trees (Juglans nigra) produce a substance called juglone that is poisonous to many other plants. Tomato plants growing near walnut trees will often wilt and die in late July.

Hostas used to be called Funkia. There was a rock group called the Funkadelics back in the 1960s but I don’t think they had anything to do with hostas but might have been familiar with grass

All azaleas are actually rhododendrons. Only a small number of rhododendrons are azaleas.

Spider mites are not insects. Insects have six legs. Spiders and their relatives have eight. Eight is Enough was a television program back in the 70’s.

Staghorn sumac has red berries borne at the tips of the branch. Poison sumac has white berries borne along the stem. Chuck Berry is one of my favorite singers.

Grape hyacinth are not hyacinths. They are in the genus Muscari. Their fruit makes lousy wine.

Onions form bulbs in response to day length. Some that will grow in the North will not form bulbs in the South where day length in the summer is shorter.

Bats have their young during the first two weeks of June. The runt of the litter is always named Robin.

Yellowjackets, wasps and hornets live and use their nest just one year. A few queens survive the winter and re-establish completely new nests the following spring. The rock band, “Queen” maintains a nest in England.

As always, if you have landscape gardening questions or have a topic you would like to see covered, please let me know.

Also, I have recently published a couple of eBooks that you might want to check out. The titles are, “A Rookie’s Guide to Designing Beds and Borders” and “A Rookie’s Guide to Implementing Beds and Borders“.

I am currently working on the next eBook, “A Rookie’s Guide to Hostas, Hostas, Hostas” which should be ready by the end of November, 2012.