The tree winters in the house. In spring it goes on the deck. It produces flowers, but they fall away or disappear and never start growing a little fruit. What am I doing wrong?
Careful not to overwater. Fluctuations in watering can affect fruit and flowers and citrus are quite sensitive to this. Use a high potassium feed at flowering this will help regulate the transfer of water through the plant.
The leaves on my lemon tree are covered in a sticky liquid which also covers the floor under the plant. Is this normal? There were a few black flies and thrips which I have removed physically. Otherwise, the tree seems very healthy. The plant is kept in my conservatory.
It sounds like you have a pest problem, possibly other than the thrips you already removed -- sticky liquid is usually the honeydew secreted by any number of pests.
This article may help you correct the issue:
https://www.gardeningknowhow.com/garden-how-to/info/what-is-honeydew.htm
And this article provides additional information about common Lemon Tree problems:
https://www.gardeningknowhow.com/edible/fruits/lemons/common-lemon-tree-diseases.htm
I have a dwarf lemon tree in a pot and have had 4-5 Meyer lemons on the tree, I picked them all and now I don't have any more blossoms. Did I do something wrong this year?
Nope, you did nothing wrong by harvesting your fruit. Your tree may not be mature enough to produce any more than 5 lemons, or it may not be getting enough water or nutrients. Here are some articles that may help you:
https://www.gardeningknowhow.com/edible/fruits/lemons/growing-meyer-lemons.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-not-fruiting.htm
We have grown lemon trees from seeds taken from lemons purchased at a grocery store. The trees are now about 1 to 2 metres tall, about 3 years old. To date, they have neither flowered nor, of course, fruited. To our annoyance, we can see smaller lemon trees in garden shops which have fruit: ours don't. They are kept inside in a sunny conservatory during the cold seasons; we put them outdoors on warm days and during the summer. We give them Baby-Bio for citrus plants. Still nothing. Can you tell us what we need to do to get fruit? Many thanks for your help Glenn SAMPSON
Your trees are probably simply too young to flower; see this article for more information:
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-not-fruiting.htm
The lemon trees you see with fruit in garden stores are probably Meyer Lemons, which are dwarf trees that often flower earlier than other lemons.
You are doing the right things in terms of bringing the tree indoors and outdoors depending on the weather. With time, your tree will get there. These articles will help you identify ways to promote flowering and fruiting:
https://www.gardeningknowhow.com/plant-problems/environmental/plant-not-blooming.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm
1) What type of lemons should grow? 2) Do they need extra care? 3) Around what time should I plant?
I am not sure what varieties of lemon are best for Africa or your particular region. You could try asking at a university near you that has an agriculture or horticulture program. Someone these could probably tell you about the best varieties and the disease resistance that is needed in your area.
If growing from seedlings or saplings, it is important to find a reputable source to make sure the plants will be disease-free. Lemons can also be grown from seed. These articles discuss how and when to plant:
https://aggie-horticulture.tamu.edu/citrus/lemons.htm
http://ucanr.edu/sites/VCMG/Planting_and_Care_of_Young_Citrus_Trees/
http://aces.nmsu.edu/ces/yard/2000/040800.html
These articles have advice on growing and caring for lemon trees:
https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-pruning.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/learn-about-fertilizer-for-a-lemon-tree.htm
The branches of my lemon tree have cracked open. The bark is full of lichens, it has fewer leaves and less lemons. What can I do? Is it going to die? Will it contaminate the orange tree and the tangerine tree?
Could there have been frost damage, lightning or mechanical damage at any point? This could cause the dying and cracking branches you see.
Otherwise there could be an infectious disease present. Check for any more spots with sap leaking out of the tree, since this is a symptom of several diseases. See the following articles:
https://www.gardeningknowhow.com/edible/fruits/lemons/common-lemon-tree-diseases.htm
http://ipm.ucanr.edu/PMG/C107/m107bptrunkrootdis.html
Is it safe to add dirt a few inches higher on the tree trunk?
No, if this is a potted Lemon Tree you will want to repot and add soil to the root system.
If this is in the ground, avoid piling soil or mulch up against the trunk; this can cause disease or death of the tree.
https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm