Did you ever wonder why limes are green and every other type of citrus fruit is either orange or yellow? The reason for this, I recently learned, is that limes (and lime trees) are the most cold sensitive citrus.In their tropical lands of origin, which are mainly the archipelagos of equatorial Southeast Asia, all citrus types stay green until they are ready to be picked. This habitat does not experience the sort of winter cold that is required to change their color from green to yellow and orange. The reason lime fruit do not turn color is that the degree of cold required for this to happen would kill the tree on which they grow.I got to thinking about citrus trees the other day thanks to a letter from Alan Sarkisian, who gardens in Torrance. Sarkisian has two Meyer lemon trees, both more than 30 years old.One of the trees is doing well but the other has lost its leaves, not a good sign since all citrus trees are evergreen. Sarkisian wants to know if this defoliation could have happened due to excessive pruning, allowing unreachable fruit to rot on the tree, or spraying weeds on the adjoining driveway.Above all, Sarkisian should be congratulated for having nurtured his Meyer lemon trees for more than 30 years, especially during a long stretch of drought which only abated this winter. Stress shortens the life of fruit trees in general and of dwarf trees such as Meyer lemon in particular. When grown in a stressful environment — and the Los Angeles megalopolis would definitely qualify as such — a Meyer lemon may only live for ten to fifteen years.The fact that one of your trees defoliated is not an indication of improper care, only that the tree has reached a ripe old age and, in the manner of all living things, became more susceptible to disease . Your tree may have succumbed to a soil borne fungus that, when healthy, it would have been able to withstand.In general, defoliation of any plant is a sign of not enough or too much water in the root zone. Plants really do have a love-hate relationship with water. If there’s not enough, they wilt and die, but if there is too much they also wilt and die. This is best illustrated with indoor plants that frequently die from over watering since people just love them too much and cannot resist looking at them without doing something, which usually means watering them even when they don’t need it.By the way, should you want another Meyer lemon tree, you do not have to go to the nursery but can clone your own, although you may have to wait a few months to begin the process since you will need softwood cuttings — of 8-10 inches in length — that may not be grown out to that size until late spring or early summer.You will need to take a one gallon plastic pot, even if brand new, and sterilize it in a solution that is 1 part bleach and 9 parts water. Place fast-draining soil mix into the container and stand it up in a bucket that contains a few inches of water. Through capillary action, water will move upwards until the soil in the container is throughly soaked. After dipping your pruning shears in the bleach solution, snip your cuttings and immediately submerge them in a tall glass of warm water for fifteen minutes. After removing them from the water, detach all leaves except for the top three and then snip the cuttings from the bottom so they are 6 inches tall. Run water over the bottom two inches of the cuttings, dip them in hormone rooting powder, and insert into the potted soil mix. Before insertion, 2 and 1/2 inch deep holes should be made with a pencil. You can comfortably fit 10-12 cuttings in your one gallon container. Now insert three wooden dowels around the container perimeter and cover with polyethylene film. This will create a hot house effect and you should only have to worry about moistening the soil on an occasional basis if at all. Place the container on a sunny window sill or in bright shade outdoors. Within three to four weeks, roots should form, at which time you can remove the film and water your cuttings until they harden sufficiently, within another week or so, to be individually planted in pots.Meyer lemon cuttings were brought back from China by Frank Meyer, a USDA employee, in 1908. The tree is thought to be a hybrid between a lemon and either a common sweet orange or a mandarin. Meyer lemons are sweeter, rounder, and have a thinner skin than classic lemon varieties. In the 1940‘s, a devastating virus curtailed enthusiasm for Meyer lemons until, in 1950, a virus-free strain was discovered which was dubbed ‘Improved Meyer Lemon,’ which is the only type of Meyer lemon sold today.Tip of the Week: It’s time to think about planting the spring garden and you may wish to develop a color scheme with your design. Some people like to stay with a single color, such as lavender, or a single band of the color spectrum, such as blue, lavender, and purple. But two contrasting colors can also make an arresting display.I recently saw an orange and purple pansy and recalled that these two colors go remarkably well together in the garden. Although you may never find someone wearing an orange shirt with purple pants, these colors somehow work together in floral combinations, perhaps because of the green foliage that they share.For a sun splashed garden, select your oranges from bird-of-paradise (Strelitzia reginae), lion’s tail (Leonotis leonurus), torch lily (Kniphofia hybrids), orange cosmos (Cosmos sulphureus), orange gerbera daily (Gerbera jamesonii), orange daylily (Hemerocallis hybrids), orange Peruvian lily (Alstromeria aurea), California native leopard lily (Lilium pardalinum), California poppy (Eschscholtzia californica), bulbine (Bulbine frutescens), butterfly weed (Asclepius tuberosa), orange, kumquat, and persimmon tree, and orange marigolds (Tagetes). For your purples consider Mexican sage (Salvia leucantha), Salvia ‘Indigo Spires’, Salvia nemerosa, purple beard tongue (Penstemon spp.), purple iris (Iris germanica), California native woolly blue curls (Trichostema lanatum), blue throatwort (Trachelium caeruleum), pincushion flower (Scabiosa spp.), squill (Scilla peruviana), purpletop (Verbena bonariensis), sea holly (Eryngium spp.) and purple petunias.