Archive for the ‘Flowering’ Category

During summer, growers experience a lot of problems with tomatoes. This article deals with the effects of temperature on tomatoes – on pollination and fruit set and also on ripening.  I will deal with diseases in another post.

Tomatoes are affected by high temperatures in a number of ways. Some sensitive varieties are affected when average daily temperatures exceed 25°C, whereas more heat tolerant cultivars are not impacted until daytime (maximum) temperatures exceed 32°C. There are even some cultivars are able to set fruit at temperatures above 35°C.

Under marginal conditions fruit may set without adequate pollination but the internal fruit segments will contain few seeds and the tomato will be flat sided and puffy. Irregular pollination can also cause ‘cat facing’ (http://vric.ucdavis.edu/veg_info/catface.htm).

In general fruit set is adversely affected when temperatures fall below 10°C or rise above 27°C. Optimum temperature for fruit set is 18° to 24°C. Even moderate increases in mean daily temperature (from 28/22°C to 32/26°C day/night) result in a significant decrease in fruit set.

As a general rule, the 8 to 13 day period prior to flowering is the most critical phase. If the average maximum temperature in that time exceeds 29°C, pollination and fruit set are impacted. However as pointed out earlier, this does vary according to cultivar.

Why aren’t my tomatoes ripening?

In hot weather people expect fruit to ripen faster. But with tomatoes the optimum temperature for ripening is 21 to 24ºC. When temperatures exceed 29 to 32ºC, the ripening process slows significantly or even stops. At these temperatures, lycopene and carotene, the pigments giving the fruit their typical orange to red appearance cannot be produced and so the fruit stays green.

For tomatoes light has very little to do with ripening. Light is not needed for ripening and fruit exposed to direct sunlight can heat to levels that inhibit pigment synthesis (As explained above). Direct sun can also lead to sunburn. Do not remove leaves in an effort to ripen fruit. Also, soil fertility doesn’t play much of a role. High magnesium and low potassium can cause blotchy or uneven ripening or yellow shoulders. But slowness to ripen is generally not due to poor nutrition and adding more fertilizer won’t help.

You can remove fruit which are just showing the first colour changes (mature green), and store them at 21-24ºC in the dark, preferably in an enclosed space or in the presence of fruit that give off ethylene gas such as bananas. This may speed up the process by up to five days.

References and further reading





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It has been a long held belief that Rosa x fortuniana Lindley is the only rootstock suitable for local conditions in Western Australia. Rosa x fortuniana Lindley has produced outstanding yields under conditions in Florida (McFadden 1962). The reasons for its superior performance include better adaptation to warm weather and sandy soils. Resistance to soil borne pathogens such as Pythium, Phytophthora,  Rhizoctonia and crown gall has been found in Florida trials. Hybrid vigour is also a possibility – Rosa x fortuniana Lindley is believed to be a hybrid of R. banksiae x R. laevigata. Superior uptake of iron during hot weather could also be a factor.

Rootstocks found in Western Australia include R. multiflora, R. x fortuniana, R. indica major, R. ‘Dr Huey’, R. manetti and R. canina inermis. Some of these are being used for inground cutflower production whereas others are used in the home garden as well.

Characteristics of a rootstock which are important include:
1) Ease of propagation
2) Lack of suckering
3) Disease resistance and/or tolerance to nematodes
4) Vigour
5) Tolerance of local conditions eg salinity, heat and drought.

Multiflora is noted as being more salt sensitive and more cold tolerant. It is less tolerant of alkaline conditions. It also picks up virus infections from the scion material very easily (however in Australia there is no virus free material, I can write separately on this topic). There are numerous lines of multiflora used internationally and at least two lines have been found Western Australia. One line is greatly lacking in vigor and displays a multitude of trace element deficiencies. Even the other line of multiflora seems susceptible to trace element deficiencies, especially copper and iron. Studies, both at the Department of Agriculture and overseas have shown it to be an ideal host to both root knot nematode and to lesion nematode, but particularly, root knot.

‘Dr Huey’ appears to perform quite well especially on heavier soils. In both McFadden’s study and in that of the Department of Agriculture, it came second to fortuniana. I have not seen any obvious problems with ‘Dr Huey’. It is reported to be susceptible to black spot which may be a problem in more humid climates.

R. manetti, used commercially, appears to have some degree of resistance to nematodes and has solved grower issues with trace element deficiencies. The growth is far superior to R. multiflora and on a par with ‘Dr Huey’.

R. canina, also used commercially, also appears to have some degree of resistance to nematodes. Studies overseas have supported this. R. canina is extremely tolerant of root knot nematode and reasonably tolerant of root lesion nematode (Coolen and Hendrickx, 1972). Growth is superior to multiflora and trace element deficiency symptoms not evident.

R. x fortuniana is planted extensively in home gardens and to a lesser degree in commercial inground production. It is definitely superior to multiflora. It does seem to have some issues with trace element deficiencies. One disadvantage is that it is more difficult to propagate. It also suckers more freely.

Trials in Florida (Gammon and McFadden, 1979) compared flower production between bushes on fortuneana, odorata, multiflora and manetti. Odorata produced the highest yields, followed by fortuneana, manetti and multiflora. They also found large differences in the accumulation of trace elements. Fortuniana accumulated five times more manganese than odorata but this was not related to flower yield. Odorata was a superior accumulator of potassium and under low nutrient conditions both fortuniana and odorata were good accumulators of nitrogen and potassium and this was related to flower yield.

The results of the trial at Medina Research Station (1980 to 1982) did seem to support the superiority of fortuniana, especially for bloom counts. However the following factors should be borne in mind:

• High pH water and soil (both around pH 8),
• Medina soil is a Spearwood sand unlike most of the soil in the metropolitan area with is much poorer in nutrient status, and
• Climate – Medina is recognised as being a particularly cold spot in winter.

All these factors could have a significant bearing on the performance of any rootstock. Finally the experiment at Medina lasted for only three years when the normal lifespan of a bush in the average home garden is many times that.

In Western Australia which has a hot climate and nutrient poor sandy soils prone to nematodes and with poor water holding ability, R. x fortuniana is the logical choice. However home gardeners often modify their soils to varying degrees which may decrease this advantage. In areas with colder night temperatures multiflora may perform better than on the Swan Coastal Plain. In the clayier soils of the scarp, ‘Dr Huey’ also does well.

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There are many myths about potassium. The biggest one being you need it for flowering.  WRONG!  You need potassium no more or less than any other nutrient for flowering.  Sure, some commercial growers might change the ratio of nitrogen to potassium as the plant moves from vegetative to fruiting but it has nothing to do with the formation of flowers and fruit, it’s more about flavour.  Too much nitrogen can produce fruit that is big but watery, tasteless and low in sugar and other flavour components

Potassium is important in that it helps plants tolerate stressors such as cold/hot temperatures, drought, and pests.  It is a catalyst for many plant enzymes and helps regulate water use in the plant by affecting the opening and closing of the stomata in the leaves and water movement in and out of cells.

Potassium is a funny nutrient in some respects because we often see little response to it in trials.  But palms and other plants that clump, may respond to it by increased clumping and branching.  We’ve seen that response in some native plants as well, such as Stirlingia.  Too much potassium can make stems very brittle so they snap easily.

Signs of potassium deficiency can be quite dramatic and also species specific.  Sweet corn gets a sort of burning – a band around the leaf margins which is dried out and dead looking.  Carnations also get necrotic spotting at the tips of older leaves.  Hoyas end up with necrotic spots all around the leaf edges.  The symptoms are always on the older leaves because potassium is mobile and will move to the place of greatest need – we call that a sink and is often the fruit or flowers, or at the very least a growing point.

Potassium is supplied in many products as potassium nitrate – 36-38% potassium and 12-13% nitrate depending on formulation and purity.  Potassium sulphate can also be used but is more acidifying – an effect we often wish to avoid in our soils.  The cheapest source of all is muriate of potash or potassium chloride which we don’t really recommend at all because of the high chloride (salt) content.

Good organic forms of potassium are wood ash, dried seaweed and blood and bone, though the phosphorus content of blood and bone is rather high relative to the nitrogen (N) and potassium (K) and plants fed with blood and bone will need both N and K supplemented.

Potassium is readily leachable in our sands but is held on slightly better than nitrate in the presence of clay or compost as it is a positively charged ion.  Nevertheless, we see high rates of leaching of potassium from newly applied compost.

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A frequent cry from gardeners relates to lack of flowering and very often the response is to add potassium. But potassium is NOT responsible for flowering, any more or less than any other nutrient and the real reasons for flowering problems can be many.

No flowers at all? Ask yourself how old is the plant? Is it a seedling which may not flower and fruit for several years? Many eucalypts are in this category. I have an Illyarrie (E. erythrocorys) which is now seven years old and hasn’t yet flowered – I’m not happy but there’s not much I can do about it! Passionfruit from seedlings are similar and may not fruit for several years.

Sometimes pruning is the problem. Particularly with fruit trees, cutting off the wrong wood means no flowers and no fruit. Pears, plums and cherries produce fruit on spurs, lateral growth is often minimal. Japanese plums fruit on 1-3 year old spurs. European plums fruit on lateral spurs on 2-3 year old wood. Peaches and nectarines produce fruit on the new season’s fruiting wood. Apples produce their best fruit on fruit on wood two-years and older, though some varieties such as Sundowner™ also bear fruit on one-year wood. Figs produce their main crop on current seasons growth and an early lighter crop on the previous years wood.

Sometimes the weather is responsible. A cold snap at a critical time can prevent flowers from initiating or abort them very early. Drought can do the same. Occasionally plants don’t flower because the light hasn’t been right. Some plants such as Zygocactus will not flower if their night time darkness is interrupted by light from a street light or inside a house.

The mechanisms for flowering in plants can be complex and are generally a combination of temperature and daylength and one of these may be more important than the other. So anytime you grow a plant out of its natural habitat, you run the risk of problems depending on how pernickety it is. Banksia coccinea is one plant like this that tends to not flower well, if at all, around Perth. Banksia coccinea is also prone to the ‘pruning effect’ because it actually is initiating the next years flowers around the same time as it is flowering. So prune after flowering and you’re apt to be pruning off all the next years flowers!

It needs to be understood that flowers may have quite separate requirements for flower initiation as compared to flower development. Obviously flower development can’t happen without first having flower initiation but you can have flower initiation and no flower development afterwards. Most European bulbs require a period of chilling during winter to initiate flowers but the soil temperature during spring can also be too high and those flowers will abort. Conversely, many South American or South African bulbs that are autumn flowering have high temperature requirements that must be met for flower initiation and flower development to occur. Sometimes these requirements can be met by other means such as putting bulbs in the fridge for a few weeks but sometimes this doesn’t always work well because of these other factors at play.

Sometimes uneven flowering or flowering on one side of plant can happen and that’s usually for the same set of reasons as above.

Poor nutrition is seldom responsible for a complete lack of flowering, although excessive nitrogen can keep a plant vegetative. Poor nutrition though, can be responsible for some peculiar flower development such as no petals/empty flowers. Calcium or boron are most often the culprits when there are fruiting or flowering problems but first check for periods of water stress because interruptions in the transport of these nutrients can be the reason. Both calcium and boron are essentially immobile (so when in short supply they can’t be moved from older leaves to places of greater need) so that is why symptoms are seen on the growing tips and why symptoms of deficiency show up in flowers/fruit or any growing points (in palms death of the growing point will kill the plant). Bent neck (should be self explanatory) is usually a calcium problem but often more to do with water stress/periods of reduced transpiration such as high humidity) rather than actual lack of calcium in the soil.

Sometimes flowers go green (gerberas show this symptom). This is usually due to infection by a mycoplasma and is incurable. Get rid of affected plants.

Occasionally flowers go mad and start producing flowers within flowers. Or whole little plants within flowers. Usually this sort of secondary development is physiological and due to erratic weather conditions. Rarely is it due to herbicides or any other chemical agent. I have seen this on banksias, roses and carnations to name a few.

You may have flowers but they remain stubbornly closed. This is often wind damage – we call it blasting. Sometimes, it’s also disease such as grey mould caused by Botrytis cinerea.

Finally, you may have had flowers but they may have aborted or dropped for some reason. And depending on when that happened, it may not have been that evident. Most often this is due to moisture stress or a sudden increase in soil salinity (either because the soil dried out temporarily or because you threw on a heap of fertiliser). Some plants also have inbuilt mechanisms for controlling flower (and hence fruit) load to ensure that all fruit set can fully mature. All cucurbits are like this (pumpkins, melons). Other fruit trees can be biennial bearing – that is they tend to have one year when they crop heavily, the next year is a lighter load.

While we are talking flowers in this blog, don’t forget the importance of pollinators where fruit is concerned. Bees, flies, moths or other insects. Lack or, or poor or uneven pollination can also cause a range of problems from no fruit to malformed fruit. But that will be a whole other episode!

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