Deciduous Trees Course notes

Extract from the course

ACER (MAPLE)

Aim:  Develop knowledge in classification, identification and culture of plants, from the genus Acer.

Acer Maples

Family: Aceraceae            

• There are approximately 200 species in this genus. 
• They are deciduous shrubs to trees, cultivated commonly for their attractive foliage (particularly autumn colour), and for shade and in windbreaks. Some taller species provide timber & maple syrup. 
• Leaves are normally palmately lobed, sometimes entire or compound; opposite, and the autumn foliage is normally very attractive. The bark may also be attractive, with colour or markings on many species.  
• Insignificant flowers in clusters; normally yellow or greenish in colour. 

CULTURE

Acers are hardy in cool and temperate climates. Some species tolerate warmer climates, but they are best grown in cooler areas. They prefer a position in full sun to semi-shade. They are tolerant of frosts, but not dry conditions. 

Most maples need cool, moist, well drained soils for best results. Many maples prefer soil with a     higher pH, so the addition of lime to acidic soils is beneficial They have a spreading, often shallow, root system and small species are unlikely to cause any major problems in terms of root damage. Some species will tolerate extreme cold and dryness, but few if any tolerate hot humid climates. Protect from hot winds. 

Mulch and feed annually. Well rotted organic fertilisers or manures can be applied in early spring. Irrigate during dry periods, particularly during drought times. If possible provide protection from strong winds. Light pruning might be undertaken to shape a plant or remove dead wood; however, regular pruning is generally not needed, and may detract from the natural shape of the plant. 

Leaves dropped in autumn make excellent compost. Acer psuedoplatanus has become an environmental weed in some areas (e.g. cool, protected sites in Victoria's Dandenong Ranges), competing vigorously with native trees (eg. Eucalypts and wattles). 

PROPAGATION

Usually propagated by seeds, although the seed are generally only viable for a few months so it is necessary to sow them early, either freshly harvested or after a moist stratification for 2-3 months and then sown. Seeds may be stored in moist sand for some months. Budding/grafting of selected varieties/cultivars is commonly carried out onto seedling rootstock.

PESTS & DISEASES

Diseases are not commonly serious but can include anthracnose, leaf spots, powdery mildew, wilt and bleeding canker. Hygiene, good ventilation and drainage will usually minimize problems.

Pests are rarely serious but may include mites, thrip and borers. 

SELECTED SPECIES AND CULTIVARS OF ACER 

Acer buergeriunum (Trident Maple) – fast growing, to 5m, young tips copper colour, bright red autumn foliage

A. grosseri –to 6m tall, hardy shrub-like tree from China, attractive white markings on bark, prefers cooler areas, rich red autumn foliage

A. tegmentosum (Manchurian Maple) – Fast growing hardy, frost resistant to 6m tall, tolerates light shade, bright orange red autumn foliage.

Other commonly grown Acers include: A. rubrum, A. saccharum and A. platanoides, but these are taller and better suited to large gardens or parks.

Acer palmatum (Japanese Maple)

This is both one of the most diverse and widely cultivated species; normally a small tree (occasionally shrub like), 3‑10m, occasionally taller. Some are pendulous, almost weeping habit. They are all long lived, and can be slow growing in cold climates but have a medium to fast growth rate in mild temperate climates.

Essential to avoid hot, dry conditions. There are many different cultivars including some of the best shrubs or trees available for small gardens in temperate climates.

It has been cultivated for at least 400 years in Japan, where it is also known by the common name, “Mountain Apple”

Acer palmatum is indigenous to Japan, Korea, Taiwan and China, at altitudes to 1,100 metres. It is botanically variable…the divided leaves can have anything from 5 to 11 points and the lobes can be cut very deeply or only shallow. Serrations along each lobe can be mild or severe. Colour can also vary greatly, both normal colour and foliage colour during senescence. Habit can also be variable –some are very upright growing and do not spread greatly; while others are very spreading.

Taller varieties (to 10m) tend to be very sturdy and hardy plants in temperate climates, with either green or purple foliage; turning to brilliant autumn colourings (some of the best autumn colour comes from these larger growing cultivars).

Though pruning is not generally necessary, Acer palmatum does tolerate quite heavy pruning.

A. palmatum varieties/cultivars include:

A. palmatum ‘Aureum’ –to 3m tall, fine foliage, yellow to orange autumn colour.

A. palmatum ‘Dissectum’ –This refers to a large group of cultivars that have fine, heavily dissected leaves (ie. lobes are cut deeply to the centre of the leaf), lobes of the leaf are each one-serrated. The heavy dissection makes these cultivars appear a softer texture, almost fern like. The shape of these plants can be variable, but is commonly more bush-like, getting no taller than 2m.

A. palmatum  var. dissectum atropurpureum – can get to 4m tall, but may be shorter. Very fine foliage with reddish colourings all year, becoming deeper claret red colour in autumn.

A. palmatum  var. linearilobium - This is another major group of cultivars; distinguished by lobes of the leaf are not serrated, however they are long and narrow, creating a soft but different foliage effect. Most of this type grow to around 2m tall.

Foliage colour can vary amongst ‘linearilobiums’.

A. palmatum var. linearilobium 'Red Pygmy' has purple foliage.

A. palmatum var. linearilobium 'Kinshi' has green foliage.

A. palmatum var. linearilobium 'Shigitatsu-sawa' (syn A. p. var. reticulatum) has prominent darker coloured green leaf veins with paler cream-greenish spaces in between. This cultivar grows to around 3m tall.

A. palmatum var. linearilobium 'Kasagiyama' has reddish tips to the young growth, which develop to red-green and finally almost black coloured leaves. This cultivar reaches around 2m tall.

Growing Acer palmatum in Pots

Cultivars of A. palmatum are one of the most common and traditional plants used for growing bonsai. Smaller growing cultivars are also well suited as permanent tub plants, however for best results, any plant should be periodically removed from a tub and have the potting mix refreshed, and the plant trimmed. (In a very cold climate (prone to regular snow) this might only be done every 5 years or so; but in milder climates where annual growth can be stronger, rep0otting is best undertaken every 2-3 years. 

When you re-pot; trim back 25% of the foliage, remove 10-15% of the soil from the root ball, trim around 30% of the exposed roots, then re-pot into the same or a slightly larger container, using top quality potting media. Fertilise and water in. This re-potting is best done when the plant is dormant in winter (at least a month before the growth re-shoots.

Even if you do not re-pot; fertilise tub plants every spring (early spring) with a slow release fertiliser.

PROPAGATION

Propagated readily by seed, but cuttings, and grafting onto seedling rootstock, are also commonly used commercially. Seeds are only viable for a few months so it is necessary to sow them early. Seeds may be stored in moist sand for some months. Success rates with all methods vary greatly according to the procedures followed (from 0% to almost 100%).

Seed Propagation

Most Acer seed are produced in autumn and will generally have one or more types of dormancy. Seed can be collected before it reaches full maturity (before it darkens in colour, dries and begin to fall from the tree), to reduce drying which results in a harder seed coat. Such seed can be sown immediately.

Seed of A. rubrum and A. saccharinum sets seed in spring. This seed should be collected as soon as it is mature and sown immediately before it dries out. It will usually germinate in about 7-10 days.

For the Acer species that aren’t sown fresh the seed can be stratified (cold moist storage) at 2-5 degrees C (about the temperature on the bottom shelf of standard fridges) for around 90 days. The seed is then sown (early spring), into either standard seedling trays, directly into the ground, or into individual containers (2 or 3 seeds can be planted and then thinned out to the strongest).

Mature seeds can be sown in Autumn outdoors if the seeds are soaked for a week in water that is changed daily, or if they are placed under running water.

Acer palmatum seeds have been successfully germinated after soaking in water at a temperature of around 43 degrees C for two days, followed by stratification at 2-5 degrees C for 2-4 months. Soaking of seed of A. rubrum (5 days), and A. negundo (14 days) may improve germination. Cold – warm – cold stratification of seeds of A. negundo, A. barbatum, & A. floridanum may improve also germination.

For Cuttings in General

Cuttings should be ideally 15-20cm long, wounded and dipped in a 1000‑2000ppm IBA talc powder mix or IBA quick dip solution. Use a well drained medium with mist for best results. Common media include peat/sand, peat/perlite and straight perlite.

When rooted leave undisturbed until they have gone through a dormant period before transplanting.

Cuttings from young trees usually strike more readily than those from older trees.

Select strong, thick vigorous shoots of current season’s growth as cuttings. Keep leaves moist at all times. Gradual hardening off is best, making sure humidity is kept high.

Some growers keep the plants actively growing through the first normal dormant period, as many of the cuttings may otherwise fail to re-shoot in spring if allowed to go dormant.

Optimum bottom heat is 21‑24 degrees C.

Cuttings should normally be over wintered at normal air temperatures, unless being deliberately kept in active growth.

Some growers feel that cutting grown Acers are not as vigorous in the long term as those budded or grafted onto vigorous seedling root-stocks.

Many Acer cultivars can also be successfully propagated from Hardwood cuttings taken in early to mid winter. Misting is also not necessary when taking hardwood cuttings, so the facilities required are usually much simpler. Avoid over-watering;good drainage is critical as early roots develop. Newly developing foliage should be protected from bright sunlight, particularly when it is wet, as it can burn readily.

Cultivars

Acer palmatum and some of it’s many varieties have often been grown by cuttings, even commercially. Cultivars of A. palmatum will give strike rates varying from 0 to 90% or higher.

A. palmatum var. dissectum cultivars often give low strike rates, however, hardwood cuttings of A. palmatum var. atropurpureum have proved quite successful, and generally are hardier, requiring less care while handling than softer material.

Cutting propagation is rarely used commercially for other Acers: Many, including Acer pseudoplatanus, can be grown from cuttings, but are difficult and have low strike rates. Acer negundo cultivars (Box Elder Maple) have been successfully struck from dormant tip cuttings in winter. The cuttings should be a bit less than pencil thickness and about 12-20cm long. A. callipes treated with IBA has given strike rates over 90%. A. cissifolium is one of the easiest maples to strike by cuttings. A. japonicum cultivars can give strike rates up to 90% or higher. A. saccharum can be difficult, but has been known to give strike rates as high as 90% occasionally.

Grafting

Grafting is widely used to propagate selected cultivars of Acer, particularly weepers. Acer palmatum cultivars are grafted or budded onto vigorous seedling root stocks of A. palmatum using a variety of techniques

Other Acer graft combinations:

Scion                                     Rootstock

A. campestre                          A. campestre

A. cappadocicum                    A. cappadocicum

A. davidii                                  A. davidii

A. griseum                               A. nikoense

A. heldreichii                            A. pseudoplatanus

A. japonicum                           A. palmatum

A. pennsylvanicum                  A. pseudoplatanus

A. platanoides                         A. platanoides

A. pseudoplatanus                  A. pseudoplatanus

A. rubrum                                A. rubrum

A. saccharinum                       A. saccharinum

A. velutinum                            A. pseudoplatanus

Seedlings used for budding/grafting of Acer cultivars are generally direct seeded into prepared beds, or into containers and grown for around 8-12 months before being budded grafted.

Acer cultivars are commonly grafted using side (A. palmatum) or side veneer (A. palmatum, A. pennsylvanicum, A. psuedoplatanus, A. saccharinum), or whip (A. platanoides, A. pseudoplatanus, A. rubrum), or apical splice (A. palmatum) grafts. The time of grafting varies according to the climate or propagators preferences. Some propagators graft as soon as root stocks show signs of growth in spring, Many graft in summer, with the grafts taking in as little as three weeks. Late winter grafting has the advantage that the plants go straight into an active growing season, however, the rapid sap flow may flood the cut areas and hinder callusing. For this reason summer grafting is usually preferred by most growers.

 Acers are also commonly budded using T-budding and Chip Budding techniques in mid to late summer. With T-buds the xylem (woody part) of the bud is removed leaving only the bud and the bark. The tops of the rootstock are cut back to just above the inserted bud in the following spring.

For more information on propagation refer to the following texts:

·         Hartmann, H.T. et al, (1997) Plant Propagation: Principles & Practices (6^th ed.), Prentice Hall.

·         Macdonald, B., (1986) Practical Woody Plant Propagation for Nursery Growers, Timber Press.

WHAT MAKES FOLIAGE CHANGE COLOUR IN AUTUMN?

Deciduous plants shed their leaves in autumn or early winter, and are fully or partially devoid of foliage over the colder months of the year. This is an adaptation that allows the plant to better survive unfavourable conditions (such as extreme cold).

Prior to leaves dropping they undergo a period of senescence.

Senescence is the period during which leaf cells progressively die.

Over this senescence period, tissue at the leaf base progressively dies, until finally a complete section of tissue between the leaf and the stem is dead (At this point there is nothing left to hold the leaf to the stem; so it detaches and drops to the ground).

As senescence occurs, the amount of chlorophyll in the leaf (which gives it the normal green colour) reduces. Chlorophyll is actually only one of many pigments that generally occur in leaves; but it is usually the strongest pigment, and for that reason alone, most leaves usually appear green if the plant is healthy.

Other types of pigment chemicals commonly found in leaves include:

• Anthocyanins –Reds, Blues and Purples
• Carotenoids –Yellows and Oranges

Generally Carotenoids also decompose rapidly in Autumn, but Anthrocyanins break down much more slowly.

Often Anthocyanins can still be at close to 100% normal levels when only 40% of normal chlorophyll and carotenoids remain.

Anthocyanins are produced through chemical processes, from excess sugars in the leaves, particularly in the presence of bright light. In view of this fact; the level of anthocyanins will be stronger if the plant has been actively photosynthesising (producing sugars) over summer, combined with lots of bright autumn days (if weather is frequently overcast and dull in late summer and autumn; the production of anthocyanins is decreased).

Lower temperatures in autumn reduce the movement of sugar around the leaf, so if the weather changes from warm to cool fast, the leaf sugar remains high and anthocyanins build up; otherwise the levels of these pigments might not be so high.

High levels of anthocyanins will generally result in more vivid autumn foliage colours.

Blue Colouring

Some types of anthocyanins can react with metals (ie. for a chelate) to produce a different coloured pigment (eg. the anthocyanin delphinidin is normally a violet-mauve colour, but when it reacts with tin, it produces a vivid blue coloured pigment.

Variations

Autumn colour can still vary from plant to plant within a species. Variations include:

            -The time at which colour occurs (some produce colour earlier, others later)

            -The duration of colour (some maintain good colour for longer periods)

            -The intensity of colour

Such variations can be affected by:

            -Duration of seasons

            -Severity of seasons

            -Whether climatic changes are gradual or more abrupt

            -Aspect (whether it faces north or south, east or west.

-Degree of protection (whether it is exposed or protected by walls or other plants)

-Genetics

-Nutrition (eg. Acer rubrum fertilised with low levels of nitrogen for the

 summer developed better autumn colour than an Acer rubrum fertilised with

 high levels of nitrogen).

-Sex (eg. parentage of cutting/grafting material can have a significant

 effect in some plants. This is an important factor with Acer rubrum)

Leaf fall appears to be hastened by various environmental triggers. (ie. if the environmental conditions are appropriate, the autumn leaves may be held on a plant for longer; but under certain other conditions, the leaves conclude senescence quickly and drop (shortening the duration of the autumn display). e.g. Leaves will drop faster from an Acer pseudoplatanus at the onset of a period of overcast (low light) and cold days.

More on Nutrition

If the soil has higher levels of nitrogen, sugars that would normally form into pigments can end up reacting with the excessive nitrogen to form proteins. The plants then have higher levels of protein but lower levels of anthrocyanins –hence poorer autumn colour. Nurserymen sometimes use an understanding of these processes to obtain better autumn colour on plants, earlier in the season.For Acer palmatum, autumn foliage colour is known to be effected by temperature, shade and leaf age

SET TASK

1. Mix up your own propagating mix. Obtain materials...propagate at least 2 different species of Acer from seed. Keep a record of progress...send a report when you pot them up. Propagate a range of Acer plants (at least five different types) using other techniques. If time is available take photographs of the germinated seedlings.

2. Study at least two different species of Acer. You might see these in a garden, as street trees, or in a nursery. Study the wood and foliage. If available study the flowers and/or seeds. Observe any differences in shape or branching habit between species. Preferably study any parts under a magnifying glass, to observe fine differences more closely. Take note of any differences. Draw/sketch any characteristics that seem unique to a particular species, and label the drawing (ie. write a description of the characteristic you sketch, and the name of the species it relates to).

ASSIGNMENT

1. What experience have you had with Acer (propagating them, caring for/growing & identifying them, etc)? What Acer (if any) do you currently have growing in your own garden (or nursery), if any? List them. What difficulties, if any have you had in growing these types of plants? Describe the difficulties. How did you overcome these problems, or didn't you? Write up to a page on this.

2. Submit the drawings from your set task (number 2 above). Indicate the genus and species name of each of the plants you submit.

3. Make a list of what you consider to be the 10 most commonly grown varieties of Acer. Create a poster/chart to provide information on these different plants and how to grow them in your garden. You may need to do some research to create this. If there are a few gaps you cannot fill in, where you have difficulty finding information, ask your tutor for assistance.

Example…you might create a chart such as the following (but on a larger scale, and filling in the blank spaces):

4. List four Acer species that could be grown successfully in your locality, if treated carefully. For each, indicate how you would grow these, outlining any special treatment that would be required for healthy growth.