Being able to predict the mature height and spread of a fruit tree is always important, whether you are hoping to fit a new fruit tree into a small space in your garden, or whether you are planning a backyard orchard, or working on a community orchard project.
Many fruit tree textbooks and websites have a table showing the likely size of the mature tree on different rootstocks. The implication is that the rootstock is the only factor affecting the final size of the tree - but this is far from the whole picture.
Other factors play a significant role: the quality of the soil, the natural vigour of the scion variety, and the local climate. Trees growing on poor soils will not be as big as the same variety on the same rootstock growing on better soil. Similarly a Bramley's Seedling apple tree growing in the same soil and on the same rootstock as a Cox's Orange Pippin is always going to produce a much bigger tree, because of the inherent vigour of this variety.
Taken together, these other factors can cause the actual mature height of a tree to vary from the textbook diagrams plus or minus 25%.
Assessing soil quality
Soil quality can be ranked on a 5-point scale:
- Poor
- Below average
- Average
- Above average
- Very good
There are no specific rules for deciding what your soil quality is, but if you take into account the following guidelines you should be able to make a good guess of how your soil ranks.
- Shallow soils will generally be less than average.
- Sandy soils will generally be less than average.
- Shaded roots (e.g. due to a fence or hedge) in well-drained soil will help retain moisture and will boost the apparent soil quality due to slower drying in the summer.
- Hard (rocky) subsoils can prevent root growth and may have shallow topsoil.
- Soils with a pH that is far from near-neutral (pH 6.5 - 7.0 is ideal) may slow growth.
- Chalky soils are often shallow and raised in pH.
- Soils prone to waterlogging (unless tree planted on a mound).
- Low-rainfall regions may reduce vigour.
- You are unlikely to have very good soil unless you have basically average soil that has then been heavily worked with compost, and fertiliser over a period of several years (allotment soils are often in this category).
- Similarly, you are unlikely to have very poor soil unless nothing else grows either.
To help get a better idea of your soil characteristics it is a good idea to dig a hole, as deep as you can, to see how deep the soil is and what type of material lies underneath it - the subsoil. Try wetting some of the topsoil and then squeezing it gently in your fingers to get an idea of whether it's sandy (feels gritty) or clay (feels slimy) or whether it's the perfect middle-ground in-between, with an equal mix of sand, clay and organic material.
Another approach is to half-fill the a jam jar with water, then add a quarter jar of your topsoil. Replace the lid and mix well, then allow it to stand. After a short while the soil components will separate out so that you can see how much sand is in your soil.
Another simple method is the "bucket of water test". Dig a bucket-sized hole, pour a bucket of water into it and see how long it takes to drain. Too fast drainage (e.g. measured in minutes) could reduce vigour by losing the water and nutrients before the roots can grab it. An excessively fast-draining soil may absorb several inches of water per hour and the soil will become too dry in summer.
Too slow drainage (e.g. measured in days) could reduce vigour by waterlogging, partial suffocation of roots, rotting of roots, or by the soil remaining relatively cold. Excessive moss and algae will also be present in slower-draining soils, although moss/algae can simply be a feature of a shady location that remains damp, but not necessarily badly waterlogged. An excessively slow-draining soil may absorb less than an inch of water per hour and may remain waterlogged for long periods.
If you are not sure what your soil quality is, assume it is average.
Annual rainfall
Water availability is vital for the growth of a fruit tree (and the production of fruit) and has an important impact on the size of a fruit tree. Most commercial orchards use sophisticated irrigation systems to ensure the water needs of the tree are fully met, thereby ensuring the tree reaches its maximum size. In contrast many home orchardists probably under-estimate the amount of water a fruit tree requires, particularly during the spring and summer growing periods, and through pressure of other commitments may leave the tree inadequately watered from time to time.
Different rootstocks also respond differently when exposed to drought conditions. The MM106 apple rootstock is usually considered slightly less vigorous than the MM111 rootstock, but in dry soils it will be much less vigorous, perhaps half the size of the MM111 rootstock - MM111 is far better at coping with dry conditions. In contrast, when grown in areas where spring and summer rainfall is plentiful, MM106 can be just as vigorous as MM111.
Length of growing season
It is perhaps obvious but the longer the growing season, the larger a tree can become. The growing season can be considered, simplistically, as the time from the last routine spring frosts to the first frosts of autumn.
| Length of growing season | UK and Europe | USA |
|---|---|---|
| Short | North of Scotland, Scandanavia | USDA zones 3 and 4 |
| Slightly short | Northern UK | USDA zone 5 |
| Normal | Southern UK, northern Europe | USDA zone 6 |
| Slightly long | Central France, Germany | USDA zone 7 |
| Long | Mediterranean | USDA zones 8 and above |
When considering this factor, make an allowance for the amount of sunlight in your area. For example, the Pacific north west region of the USA and the north west of Scotland both have mild climates with quite long growing periods, but summers are cool and cloudy, which limits the potential growth of trees.
Variety vigour
The variety or "scion" vigour is the inherent vigour of the chosen variety that has been grafted on to the rootstock and is based on a 6-point scale. Other things being equal, some varieties just grow bigger than others. Here are some examples using apple varieties:
- Small, e.g. Court Pendu Plat, Greensleeves
- Slightly small - e.g. Braddick's Nonpareil, Grenadier
- Average - e.g. Charles Ross, Empire
- Slightly large - e.g. Katy
- Large - e.g. Howgate Wonder, Laxton's Superb
- Very large - e.g. Blenheim Orange and Bramley's Seedling.
In practice the scale is essentially a 5-point scale from small to large (with average being the mid-way point), but Blenheim Orange and Bramley's Seedling are exceptionally vigorous and merit a separate category. The same principle applies to plums and pears, except the range of vigour in the varieties is less.
Note that whilst scion vigour has an important effect on the size of the tree, it has no affect on the size of the fruit.
Style of training
The way you train and prune your tree will have an effect on the mature height.
Once a fruit tree begins fruiting it tends to switch some of its energies from growth to fruit production. Therefore if you encourage a young tree to start fruiting earlier in its life than it otherwise would, it will probably not reach the same size as one that has been left to develop at its own pace. Encouraging fruiting is therefore a useful method of controlling vigour if you have a bought a tree on a more vigorous rootstock than you intended - in many cases this approach is more effective than pruning.