Pollination is an important topic when growing fruit trees because many – but certainly not all – varieties require pollination from a compatible donor tree before they can set fruit. However it is a natural process that almost always “just works”. Some simple rules of thumb:
- If you are in an urban environment you probably won’t need to worry about a pollination partner for your apple tree – there will usually be compatible apple trees or crab apple trees in neighboring gardens and hedgerows. Pears, plums, and cherries are a bit less widely-planted though, and you can’t assume there will be others nearby, but try asking neighbors.
- For varieties which are not self-fertile, and require a pollination partner, the partner has to be a different variety of the same fruit species. Two trees of the same variety will not pollinate each other.
- If you are in an isolated area and only want to plant one tree, choose a self-fertile variety.
- If in doubt, and you have space for more than one tree of the same species (e.g. 2 apple trees or 2 plum trees), plant two compatible varieties. (If doing so, it is a good idea to choose varieties that have different picking times so that you have a spread of fruit through the season).
- You can always contact us for specific advice and we will be glad to help.
So having reassured you that pollination is not such a big issue when choosing what fruit trees to grow, here are some of the factors that can affect pollination:
Species
In general terms each species can only pollinate others of its own kind – apples will only pollinate other apples, pears will only pollinate pears, and so on.
Amongst apples there is generally no distinction between crab apples, cider apples, and mainstream apples – they can all potentially cross-pollinate each other.
Things are less clear with plums. European plums (
Prunus domestica) can inter-pollinate with closely-related species such as damsons, mirabelles and cherry plums. European plums cannot generally cross-pollinate with Japanese plums (
Prunus salicina). Many European plums are self-fertile, but most Japanese plums are not self-fertile.
Sweet and Acid cherries are also different species but can cross-pollinate each other – but usually cannot be pollinated by ornamental flowering cherries.
Blossom time
For most fruit varieties, pollination is carried out by insects, often bees. Since pollination happens in early spring, good weather which will encourage bees can be a factor.
Pollination also depends on having blossom to be pollinated – which is why the risk of late frosts which can damage blossom is sometimes a concern. Frosts just after pollination can also damage the first stages of fruit formation.
Temperatures at blossom time are also very significant for effective pollination. Pollen germination in apples works best at temperatures in the range 60F-70F (15C-20C). If you are in an area where spring temperatures are less than this (say around 50F) then you will need lots of pollinators and/or varieties that can germinate pollen at lower temperatures.
While bad spring weather can prevent effective pollination, it is useful to know that you only need 1-2 fine warm days during the bloom period for pollinating insects to come out and for blossom to be successfully pollinated.
Flowering groups / Pollination groups
One of the easiest and simplest ways to see if two varieties could pollinate each other is to check their pollination or flowering groups. The flowering groups are not the only factor in determining compatibility between varieties, but they are a good starting point.
These groups are somewhat arbitrary (there is no official definition) but the concept is simple – each group contains varieties that flower at around the same time. Groups may be given letters or numbers, but they typically run from the earliest-flowering to the latest-flowering varieties in each species. This works for apples, pears, and most plums. Pollination is most likely to be successful with two varieties that are in the same group.
In cool temperate climates where spring lasts many weeks, such as the UK and northern Europe you can assume that varieties in neighboring flowering groups will also be compatible because the flowering will overlap. In continental climates where the transition from winter to summer happens very quickly, such as much of the USA and southern Europe, you can assume that varieties even two groups apart will probably still overlap and therefore have the potential to cross-pollinate.
Our variety pages automatically show you compatible varieties based on these flowering groups.
Blossom day – best ignored
Some authorities record precise dates for the peak blossom day of each variety. This sounds more accurate than flowering groups but in practice this data is potentially misleading.
The problem is that flowering dates are different from one region to another i.e. trees in more southerly or sheltered regions will usually start blossoming earlier than those in more northerly climates.
The seasons are also different from one year to the next, depending on the severity of the winter and the weather during spring. The early spring of 2012 experienced across many of the northern states of the US brought the usual bloom period forward by several weeks.
A more subtle point is that in continental climates such as much of the USA, spring is often compressed – the transition from winter to summer happens very quickly. In contrast in temperate climates such as the UK – where much of the original blossom data was first recorded – the transition from winter to summer is more gradual and less prounounced, with the result that the blossom season is relatively longer.
For all these reasons, knowing an exact day can be misleading. The flowering groups, by virtue of being less precise, are much more helpful when comparing different varieties.
Flowering times
Some authorities, including the UK National Fruit Collection, publish flowering time data in the following form:
10% – 6th May, 80% – 12th May, 90% – 18th May.
This means 10% of the flowers are open on 6th May, 80% open by 12th May, and 90% have fallen (10% still open) by 18th May.
In this scenario the 80% figure is equivalent to the peak blossom day mentioned above, when it is most useful as a pollinator and to be pollinated, so exactly the same caveats apply. (Whilst 90% of the flowers are open on 18th May, the majority are past the stage where they can pollinate or be pollinated). However because a spread of dates is provided, the flowering times data is more useful than just knowing the blossom day, as it allows comparisons to be made with other varieties. It also explains a key point about the flowering groups mentioned previously – i.e. that varieties in neighboring groups are likely to overlap in their flowering times and therefore have the potential to cross-pollinate.
Rootstocks and flowering groups
Another complication is that the rootstock can affect the flowering times. For example, any apple variety grafted on the MM106 rootstock will tend to flower a few days ahead of the same variety on most other apple rootstocks, whilst the M9 and M25 rootstocks tend to delay flowering by a few days.
Good pollinators and poor pollinators
Some varieties naturally tend to produce a lot of blossom over a long period, and/or are genetically highly compatible with a lot of other varieties – this makes them good pollinators for other varieties. Most crab apples fall into this category and commercial apple orchards sometimes inter-plant them for this purpose.
Some varieties are very poor pollinators. Bramley’s Seedling is a particular case in point, because it is a ‘triploid’ variety which means its own pollen is ineffective at pollinating other varieties – see below.
Ploidy
This strange word refers to the number of chromosomes found in the cells of all living things, including fruit trees.
Most fruit trees are diploid (just like humans), which means they have two sets of chromosomes, one set inherited from the mother (the tree where the fruit subsequently forms) and the other from the father / pollinator. However some varieties of apples and pears are triploid, which means they have three sets of chromosomes. This is relevant to pollination because triploid varieties cannot cross-pollinate other varieties. Although some triploid varieties display a considerable degree of self-fertility it is perhaps best to assume they need another apple tree to pollinate them. In fact, if you plant a triploid variety you will usually require
two other trees nearby, each of different varieties, which can cross-pollinate each other as well as the triploid tree.
This might put you off growing triploid varieties, but they have many advantages including often very good disease resistance –
more details here.
Self-fertility
The vast majority of apple varieties are self-infertile but there are a few exceptions such as Alkmene which are self-fertile – they do not require a pollination partner. However, fruiting and fruit quality is usually improved with a suitable partner.
In other species such as apricots, peaches, nectarines, the rule is the opposite – they are invariably self-fertile so you can safely plant just one example. However even self-fertile varieties still need the pollen to be transferred from one flower to another and if bad weather deters pollinating insects the pollination may be poor and you will get a reduced “fruit set”.
A number of apple (and pear) varieties are also listed as
partially self-fertile. This suggests they should still set some fruit even if there is no pollinating partner nearby, but this is not necessarily the case. In practice partially self-fertile varieties tend to be fully self-fertile if the spring weather is good when the blossom is open, and not self-fertile at all if the spring weather is bad. It follows that if you generally have cold wet spring weather, you should assume even partially self-fertile varieties will be self-sterile in your conditions.
As an aside, self-fertile apple and pear varieties, if not pollinated by a different variety, can be prone to a fruit disorder called bitter pit which makes the fruit rather unsightly. This seems to be related to the lack of pips and / or small pips which occurs in self-pollinated apples. Good quality apples tend to have larger and / or more numerous pips – the result of good pollination. This is the reason why (in the case of apples and pears) it is often best to plant at least two trees (of different varieties), rather than relying on one self-fertile variety.
Relationships
Even if all the other factors are taken care of, some varieties are still not compatible. This is often because there is a family relationship. Thus Golden Delicious – which is an excellent pollinator for many apples because of the duration and quantity of its blossom – will not pollinate Jonagold or Crispin and is a poor pollinator of Gala, mainly because these varieties are closely related to it (very closely related in the case of Jonagold and Crispin).
These relationship incompatibilities operate at a genetic level and are difficult for the non-scientist to follow. However a useful rule of thumb is that you can usually assume traditional varieties from the USA are unlikely to be related to traditional varieties from Europe and vice versa. Thus Golden Delicious, which originated in the USA, is a good pollinator for many heirloom European varieties. This rule breaks down for varieties developed from the late 19th century onwards though, because by then transport and communication links had developed and new varieties were increasingly raised by research stations and knowledgeable amateurs using varieties from both continents.
This self-incompatibility is a particularly important issue with the pollination of sweet cherries, and very complicated to work out. For this reason it is often best to begin your cherry orchard by planting a self-fertile cherry variety, as this will usually pollinate most of the other cherry varieties.
Fruit bud formation
In order to have pollination you have to have blossom … and in order to have blossom some of the buds must be fruiting buds rather than leaf buds. Perhaps surprisingly, this spring’s fruit buds are formed the
previous summer. Therefore if you have good spring weather but little blossom, the cause is often incorrect pruning the preceding summer or over the winter.
Conversely, you can encourage a tree that is not producing much blossom to create more fruit buds by tying new branches to the horizontal in early summer – this fools the tree into thinking that it is fruiting, and in turn causes it to set new fruit buds (which will hopefully blossom
next spring).
