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Archive for November, 2011

The simple answer is because they are short of water. But its not always that simple. Plants can be short of water for many reasons and its not always that there isn’t adequate water around, sometimes it’s because they can’t get to it. Or get as much as they need.

So lets assume you have a wilting plant and the soil appears moist. First thing to check is – is it really moist? Remember sand doesn’t hold much moisture – so it may be moist but and actively growing plant may still struggle to extract all it needs. And in clay – especially fine clays, the soil may feel moist but may be bound so tightly the plant can’t get to it.

If you’re on sand it always pays to have a check around the root zone. I’ve seen wilting plants amongst a well watered garden that are dry as a bone because there’s a big lump of non-wetting sand right underneath them. So they appear to be being watered but a little dig reveals the truth. Non-wetting sand and the soil is as dry as a bone. So out with some wetting agent or at least a hose and some slow diligent watering, making ‘mud pies’ to throughly mix the sand and water. Once wet, if it stays wet it will keep wetting up, it’s when it dries out you have problems. Which is exactly why infrequent watering in sands is an issue.

If you’re digging around make sure you don’t just look at the top 5-10 cm either. If the plants roots are mostly at 15-30 cm then it needs to be wet down there as well. With infrequent and inadequate watering its quite common to have the top few cm wet and the rest dry. and if the plant is recently planted check the root ball ie the roots in the potting mix as well as the surrounding soil. There can be quite a difference in the characteristics of each and that needs to be accounted for.

If your plant is on drip irrigation and only has one dripper then is it actually getting enough water in that one spot to provide for all its needs? I have covered this problem under previous posts on watering. and whatever you do DON’T move the dripper in mid-summer. The roots will have grown towards the water supply and if you move the dripper they will die before new roots get a chance to establish in the new dripper position.

Now we have assumed at the moment that all the plant is wilting. But what if only one side or one branch is wilting? Well the first thing to look for there is root binding. If the roots are strangling the plant that is how it often shows up. And it often show up with the first few hot summer days. Again, get down there with a trowel and have a bit of a dig. If it is the problem then you’d better get prepared to replace it. The plant might last through summer if you water it enough, and through winter, but eventually it will die.

If its something like a wattle (Acacia) then look for borers in the stems lower down. Very common. Tell-tale signs are gumming, and of course holes! And borers do attack other species too so its worth a look.

The other reason plants may start to wilt and die on one side (or both) are root diseases such as dieback (Phytophthora). Vascular discolouration will be evident (that means scraping some bark off or cutting off a stem low down). You should see some browning as a ring inside the bark or in the middle of the cross-section. This means some of the food (phloem) and or water (xylem) conducting vessels are damaged. Again its time for a replacement and you may have to think about what species if it is disease, as its successor may also succumb.

Plants also may get collar rot (Rhizoctonia). This may come in as a secondary pathogen on plants that are a bit top heavy and being buffeted by winds (eg howling easterlies). Once plants have Rhizoctonia collar rot its generally game over. Mulching too close to the stems of plants may also cause collar rot. And if you’ve been fertilising recently with granular fertilisers make sure the granules haven’t lodged somewhere down in the base of the plant and burnt it, minor damage can permit collar rot fungi to enter via the wounds, or simply the burning outright damages and kills the tissue.

Finally, plants may also wilt because they have root damage from waterlogging – which is really low oxygenation in the soil. This can happen in summer when soil temperatures are high and you are watering madly to prevent, well, umm, wilting! Once roots are damaged, it’s easy for fungi such as Pythium to enter the roots and start root rot. In fact, damaged roots exude chemicals that actually attract Pythium spores which move in soil moisture and swim towards and infect them.

Well, I think that’s about most of the things to look for. Oh, apart from simply hot days ie the midday sun. Some plants will wilt on hot days and that’s simply because their poor little roots can’t take up enough water fast enough to counteract what is being lost from the leaves, to prevent wilting. In some cases they will recover without damage eg spinach. In other cases, they will recover but the leaves will be burnt eg gourmet lettuce varieties. In other cases if the tissue is damaged as they wilt they may not recover eg gladiolous. You may be able to stake the plant but more often you need to cool it in the middle of the day by overhead watering or misting in summer or grow it under shade.

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Don’t do it!

Just a bit of a sneak preview here. We’re running some trials at the moment and in only two weeks we can see that the soil covered with fine mulch is dry – 5% moisture in sands is dry and stressful for your plants. All this rain we’ve had in the last week or two is staying in the mulch. Where we have coarse materials the rain is going through and the ground underneath is nice and moist and running at about 10% moisture at a comparable depth.. And we’re not even really into summer yet. Imagine how much worse its going to get in summer when you’ve got two-three waterings a week! This trial also doesn’t have the added factor of plants in the ground, its just mulch over sandy soil. With plants it would be so much worse.

And just a reminder to use composted materials as well – to minimise the problems of disease, non wetting and nitrogen drawdown.

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