Citrus Questions & Answers by Ian Tolley

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Q. At the hardware chain store I observe many citrus trees budded onto Swingle citrumelo rootstock. Is this an appropriate rootstock for Adelaide Plains & Hills soil?

A. No! Refer to my review report. If you want to, it can be put on the website.

Q. In your citrus trees you recommend the grafting / budding union to be 25-30 cm above the soil / potting media level. Why is this important?

A. Avoiding Phytophthora & splashing infections up from the soil and to retain a smooth bud union.

Q. In your writings on citrus varieties & rootstock combinations you mention that budding/grafting Imperial mandarin onto Poncirus Trifoliata is incompatible.

A. Causes over cropping and severe benching. Use an Inter-stock for P.tri.

Q. Are there any other variety/rootstock combinations to avoid?

A. Too many to mention actually. Refer to my booklet Citrus – A Gardener’s Guide (Page 31)

Q. Why do the majority of citrus trees that come into Adelaide garden centres ignore the identification of rootstock i.e. no tick in any box?

A. Sloppiness & ignorance!

Q. If my soil were basically a clay loam of average pH between 7.5 & 8, what would you recommend I use as a rootstock for citrus trees?

A. Clay loams-citranges. pH7.5 & 8 is alkaline – Cleopatra mandarin.

Q. If my soil comprises of deep sands pH of 8, what rootstocks would you recommend for my citrus trees?

A. Without limestone, there is no limit to the range on deep sands.

Q. A well-known horticulturalist asserted that Citrange & Swingle rootstocks, being non-dwarfing, were not appropriate for use as rootstocks for growing citrus varieties in large tubs. Is this correct?

A. No!

Q. Does it matter what rootstocks you use for large pots/tubs?

A. No! The tub limits the rootstock growth. It is more important to relate rootstock/scion compatibility and produce good fruit quality.

Q. The citrus trees in macro pots in your collection are irrigated by pulse irrigation. This maintains soil/potting media moisture levels at close to field capacity, which for surface rooted trees is no doubt ideal. If you had your citrus trees in Adelaide & were relying on mains water with its associated restrictions how would you go about irrigating them? Is there a way to arrange emitters so to optimise irrigation for citrus trees whether in the ground or in tubs?

A. In ground, use a simple probe. With tubs, watch for the amount of drainage to decide. This is a very large question area requiring far more detail.

Q. Many of the citrus trees in your collection are in macro pots. From the time you pot them up how many years on average do citrus trees continue in that potting media before you need to re-pot them again?

A. 10 to 40 years depending on the quality of the substrate. (Fertilization can always be mediated from the surface. eg substrate + fertilizers = media).

Q. In potting up citrus trees into macro pots I understand you use a 9-month slow release fertiliser completely mixed in the potting media plus zeolite.

A. Yes. The Zeolite binds fertilizers in the media allowing sustained root uptake.

Q. Obviously you again apply fertiliser at the start of the growing season. Is this still a 9-month slow release prill this time applied at the surface?

A. No, the roots are now developed, so I assess the nitrogen level by leaf size and colour, use appropriate amounts of chicken pellets + foliar mixes. (See my list emailed to the RFS.)

Q. If planting a collection of four citrus trees in the ground; for example a Lisbon lemon; a Washington Navel orange; a Lemonade tree & an Emperor mandarin, what distances would you be planting these apart?

A. My questions are – how much space is available, are you determined to practise a regular pruning programme for the first year, and what is the direction of the sun to the area? Email the question & I can provide more solutions.

Q. In your opinion what are the highlights/favourites of your collection?

A. None really, I like each of the eating ones when they are ripe, and all of the blossom varieties in their seasons.

Q. Who has the best Desert Lime (citrus glauca)?

A. That sounds like a competitive question like ‘mine’s better than yours.’ I have worked with Paul Tardieu out of Pt. Augusta for some years now and I think he has selected one that he says comes from years of testing and it is good for condiments. It is available from my collection as a grafted plant in a 70mm tube.

Q. The main citrus rootstock seeds are polyembronic. How do you, in practice, distinguish nucellar seedlings from sexual seedlings?

A. I have brought a selection of zygotic or off-type seedlings as an illustration for one rootstock variety only. Everyone is different and I am including more detailed comments about this issue in my forthcoming citrus manual “Commonsense citrus”.

Q. What effect has last November’s heatwave had on this year’s citrus crop?

A.  In terms of fruit numbers, disastrous, in remaining fruit sizes, fantastic. It illustrates an important story about regular fruit thinning, particularly for home gardens.

Q. On many of your citrus trees in macro-pots I observed inarching with Flying Dragon (Poncirus Trifoliata var. Monstrosa) seedlings. These from memory were citrus varieties that originally were budded/grafted onto Trifoliata rootstocks. How old were these citrus trees before they required these inarching grafts?

A. I think you are referring to the Kumquats. They were mistakenly budded in my previous nursery next door some six years earlier and were about to be thrown out, so I was “given” them by the new management as I thought I had a solution. I had spare 6year old Flying Dragon seedlings in one litre tubes, so I planted them next to each stock in the tubs and a year later, inarched above the now very restricted union. This has been totally successful and the combination is thriving. I first read about the technique 50 years ago in a small illustrated original report I treasure, from horticulturalists bringing new cultivars back to the USA in the early 1900’s. I tried it successfully some 40 years ago and have enjoyed using it successfully ever since.

Q. Does this suggest a delayed incompatibility with citrus varieties budded/grafted onto Poncirus Trifoliata? Does this happen on other rootstocks?

A. By their very nature, physical mismatches develop slowly and I have observed many examples over the decades. There are examples where disease sensitivity causes incompatibility such as oranges on sour orange rootstock where Tristeza virus is present. Sensitivity to Exocortis virus can also be a problem. General disease problem solutions are to use treated and indexed material and avoiding known sensitive combinations. Wayne Wyett lives at Yundi close to Mt. Compass. He reports very little frost on a sloping site next to a dam, but he does have cold winters.

Q. Can I grow Blood Oranges and Limes on this site?

A. Blood oranges require high summer heat and a lot of accumulative heat units to develop good colour internally and externally. They need cold winters as well as dry humidity. There are only a few places on earth that produce an ideal climate like Geraldton in WA and the areas surrounding the Mediterranean within a very few kilometres from the sea. On one of my visits to this region I asked an Italian grower about even colour each year and he smiled and said “I wish” So try, but don’t be disappointed about the colour, and remember the cultivar was a mutation from a Valencia so the fruit is still edible. Tahitian lime is tropical and should not be grown in cold climates. I have Rangpur, Kusaie and the NT lime which are far more adaptable, much more cold tolerant, crop year round, and don’t drop their crop as soon as they colour like Tahitian, and all are good for fish garnishing.

Summary from Ian:  My answers here are, of necessity, very brief. If they don’t provide sufficient detail to satisfy your need, email me on and I will endeavour to satisfy your interest, given a little leeway in time.

Sheryl:   Ian lives in South Australia and is about to publish a book called “Commonsense Citrus”.  Will let you know when it is available.