I was mentioning the other day we used synthetic auxins as rooting powders, and today, walking about the greenhouse and the yard, I found more examples of auxins’ influence on plants, which I will share with you.

Auxins are one of the families of plant hormones, still rather elusive substances, synthesised in small quantitied by plants somewhere in their tissues (i.e. root tips) to send growth messages across to other tissues either locally or elsewhere .

Of these substances, auxins were the first to be discovered, the most common form they take in the family being IAA (indole-3-acetic-acid). Once they were discovered, we tried to replicate their effects, synthesising substances in the lab: those, together with the natural plant hormones, are collectively known as plant growth regulators.

The current hypothesis is that plant hormones are difficult to trace because they are active in such small quantities, and possibly different mixes of the same substances have different effects. If you read a range of books, they will all say slightly different things about plant hormones, but they generally agree that auxins:

• affect cell elongation in stem and roots, in particular, they are behind tropic responses. For example, take phototropism, the plant’s growth response to light. Positive phototropic plants grow towards the light (i.e. Kalanchoe stems bend towards the light: mind where you put your plant! Tropic responses are irreversible). Negative phototropic plants grow away from the light (i.e ivy-leafed toadflax – Linaria cymbalaria – which grows on walls, has stems that, after flowering, bend away from the light to direct seed pods towards wall crevices, where the seeds will get a chance to survive).

  The way auxins work is by accumulating in the shady side, stimulating elongation of the cells, so that they get longer than the ones in the sun, and as a result the stem bends away from it.

  
  Today I was watering a Rhododendron and noticed a broken stem. At first I thought I might have broken it, and was thinking how clumsy of me… when I noticed the stem’s tip had moved upwards towards the light: a phototropic response which would have taken some time to take place… so not my fault! 

• Auxins are mainly produced in the stem and root tips and move directionally (top to bottom and bottom to top respectively). When produced in the stem tip, they help maintain apical dominance, which means that the uppermost bud grows tall and the other ones beneath it, in the leaf axils, do not grow. Once the uppermost bud is removed, the other buds start growing. Because of that, pruning generally makes plants bushier. I noticed that quite strikingly in a Euonymus cutting.
  

  Apical dominance suppresses  the growth of lateral buds

  Once the apical bud is removed, buds in the axils start to grow

• Auxins promote root initiation, that is why synthetic auxins (for example α-Naphthalene acetic acid or NAA and IBA or Indole-3-butyric acid), or willow water, are used to help rooting.
• Because of the way they make cell grow, some synthetic auxins (for example 2,4-Dichlorophenoxyacetic acid  or 2,4-D for short and dicamba – 2-Methoxy-3,6-dichlorobenzoic acid) are used as broad-leaf herbicides: the auxins cause the plant to grow abnormally and consequently die. Sigh.

The polytunnel needed a tidy up, to make room for more plants, so we moved a few pots around.

We use regular plastic and terracotta pots, but more and more we use air-pots, which have some advantages over the usual pots, mainly:

• they don’t have a flat, circular surface, against which the roots start circling and become pot-bound; they have the same in-out structure of egg cartons;
• where the pot walls are sticking out, the cones end in a hole; roots are funnelled into these outward cones and reach the holes where they get air-pruned: they encounter air and die back, so the plant sends out new roots in response.

Fascinating things as they are, however, plants sometimes respond to air pruning by sending shoots out of the holes, like the Deutzia in this picture.

Another advantage of air pots is they unroll open, so they are quite convenient to pot on plants without disturbing the roots.

Also, they can be rolled closed again, cleaned and reused, which we do, after keeping them a few hours in a sterilising solution.

Just out of curiosity, I have shaken clean of compost a rooted cutting that we were throwing away, and it is really amazing to see the amount and ramification of roots – they were obviously happy in the pot.

We also got a chance to make some new pots from scratch, as a new batch arrived for us to trial. It’s a new size, tall and narrow, meant to contain rootstock plants.

In the pictures below you can see how it is done step by step. The pots come in sheets, which I guess are easier to transport. You can tell what the top is because the first two rows of cones do not have holes, so they work as a water-retaining rim, that allows you to water the plant (otherwise the water would flow out of the holes sideways instead of downwards towards the roots)

There is a bottom disc, which is designed as a grid so that the roots grow downwards and do not get stuck inside the pot, and a little cone at the centre directs them outwards. You place the bottom some 3 rows up, so that roots will never sit in water and rot.

Then you roll the pot closed and firm it with a screw handle. Job done!

Unpacking the pots

Bottoms 3 rows up

Roll the pot

Screw close

I learned how to mix compost today, that we then used for potting on. It was fun!

We get our compost components in bulk basic mixes in massive canvas bags. Depending on the use to which the compost will be put to, we will then mix those basic mixes together to give the desired texture: shovel and buckets at the ready….

The compost we were making today was for general potting on and had to be made up of 3 parts of a coir + woodchips + controlled-release fertilizer mix to 1 part of a sand + gravel mix.

Add an electric mixer, like the ones builders use… et voila…

… our potting on compost is ready for the bench.                                                                                                                                                                                  

So today was my first day in the arboretum nursery. July is the month when the Kew Diploma students in the 1st year finish their classes and go back into the gardens, so with me was also a student, new to the nursery.

We were shown around, and then assigned an area to take care of.

For the next two weeks, I will have responsibility for temperate greenhouse 30A. Inside, some rootstock plants, bedding plants (i.e. Euonymus fortunei) that will be used around the Palm House, some propagated Ilex from the holly walk that need replacing, and the collection of rhododendrons that will be used to populate the new area when it is ready (as the soil is getting impoverished in the current area and the stock needs rejuvenating).

I am also looking after the two polytunnels (one here portrayed with peacock and plane, two of Kew’s most characteristic sightings!) where some plants are grown on before going off to their final destination.

The day starts with taking the min/max temperatures from all the houses and polytunnels, and checking that the mist unit and hot pipe are working. Then it’s watering of those plants that need it. I find this task quite daunting, as I am very conscious that overwatering can kill more than letting plants dry out. Roots cannot get oxygen from soil that is waterlogged, so they cannot perform respiration, they die and rot.

In order to check that a plant needs watering I am looking at three things:

• the surface of the soil is dry; that is not enough however, because the surface is what dries first and underneath the soil might still be wet;
• probe to see if soil is wet underneath the surface, looking under those pots that have grids instead of bottoms or even opening those pots that can be unrolled (if in doubt);
• weigh the pots: if the soil is dry they will be rather light compared to those with wet soil in.