Osmanthus – A Care Guide

Osmanthus or Tea Olive Care:

Osmanthus have become so popular that many gardeners want to grow them outside of the zones for where they are comfortable. Many gardeners look on a hardiness zone map and see that many Osmanthus are hardy to zone 7. This may be true, but what “zone 7"? This may sound ridiculous, but Long Island, NY is almost exclusively in zone 7a and 7b. Also the upper part of South Carolina is in zone 7b, the warmer part of zone 7, but many plants that thrive in zone 7 in SC may die in zone 7 on Long Island, NY. Why is this? It is best explained by an experienced gardening friend of mine who puts it like this: “I had much rather jump in and out of a freezer than sit in a refrigerator all day.” In parts of the Deep South which are in zone 7, the daytime high temperature rarely ever stays below freezing. As a matter of fact, we haven’t had a day where the high temperature has been below freezing for years, even in our two coldest winters of 2009-10 and 2010-11. Plants can take a quick dip to a very low temperature, but if it warms up rather quickly, there will be very little if any damage. But if the temperature stays below freezing for an extended period, even if it is not extremely cold, the plant may experience damage or even be killed.

Even though the Pacific Northwest is as mild during the winter as we are here in the Deep South, many have not succeeding in growing Osmanthus well. We are thinking that it might be due to the fact that one does not have high enough temperatures during the summer months. Some plants just prefer higher temperatures during the summer. So when growing Osmanthus in Washington, Oregon and northern California in an outdoor setting, I would recommend siting ones plants against a south facing wall or fence to give them as much heat as possible during the summer months. But our nights here in the Deep South are extremely hot and humid, so there may also be a factor here. 

The most cold hardy species of Osmanthus is O. heterophyllus. I have actually heard of this plant doing well in zone 6. As a matter of fact, our 'Party Lights' Osmanthus is doing quite well in Pennsylvania. But some of it’s variegated cultivars such as ‘Goshiki’ and ‘Variegatus’ might not be as hardy because of less chlorophyll in their leaves. This species and its cultivars flower in the fall, usually mid October for us. The colored flower forms of Osmanthus fragrans (more generally referred to as “Tea Olives”) such as Osmanthus fragrans aurantiacus (Orange flowering) and Osmanthus fragrans thunbergii (yellow flowering) tend to be much more cold hardy than the straight white flowering form, Osmanthus fragrans and its cultivars such as ‘Fudingzhu.’ Osmanthus fortunei is another cold hardy species. This selection is actually a hybrid between O. fragrans and O. heterophyllus and is sometimes rendered Osmanthus x fortunei (this designation is no longer acceptable) and is more cold hardy than O. fragrans but not as hardy as O. heterophyllus. We also grow Osmanthus armatus. This species is growing at Longwood Gardens, Kennett Square, PA, and is thriving. Since this species is so little known, I am sure that it is not extensively grown, but since it is growing there, I would think that it might be as cold hardy as O. heterophyllus.

Because many gardeners cannot grow these plants outside in the garden for which they are designed, they choose to grow them in containers and over-winter them indoors. This is very tricky and only an experienced gardener can do this. We have customers wanting replacements because they lost their plants by keeping them inside during the winter. You have 5 days to notify us if a plant has arrived in an unhealthy condition. We have no control after this.

I just received an e-mail from a gardener who lost two Osmanthus which he kept inside during the winter and wanted us to replace them. I looked at our block of 800 plants from which his plants were taken and which had been outside all winter with no protection, and not one of them was dead. They were all lush and thriving. What did he do wrong? There are four major reasons for a plant not surviving indoors, during the winter or summer. Any one of these will lead to a plant’s demise.

1. Too much water seems to be number one.  Unfortunately too much water and too little water give the same symptoms. Too much water and the roots die from lack of oxygen. They cannot then take up water. It doesn’t take much over-watering to kill the roots of a container plant. And yes, the pot needs plenty of holes in the bottom for good drainage, so that any excess water can immediately escape.
2. Too little water The plant just dries out and the leaves shrivel just like they do when they get too much water.
3. Too much fertilizer.  When growing an outdoor plant indoors during the winter it doesn’t need any fertilizer. The plant is not actively growing, so it doesn’t need to be fertilized. If you have fertilized your plant with a liquid or soluble fertilizer, you need to put it under a faucet in a sink or bathtub and allow the water to slowly trickle on the surface of the soil for 3 or 4 hours and drain out of the bottom of the pot. This will leach the excess fertilizer salts out. But fertilizer will also cause the same symptoms as numbers 1 and 2. It dehydrates the roots, they die and then the plant cannot take up water.
4. Lastly, too little light.  These plants are designed to grow in full sun not at the low light one experiences indoors especially during the winter. This light is probably one tenth or less of the light intensity outside. Leaves will drop. And during the winters further north, the days are much shorter than further south. It would be beneficial to have some form of artificial light directly on them. As soon as it is warm enough in the spring, the plant needs to be put outside with at least a half day of direct sun. All day full sun would be acceptable.

Potting medium: Some of the manufactured potting soils will compact after a few months and begin to smother a plant’s roots and they might even retain too much water. We use a potting soil that is 90% pine bark with a little sand added as well as lime, superphosphate and minor elements. We then top dress with a slow release fertilizer that will last up to 12 months. One can also use water soluble fertilizer such as Miracle Grow or Peters, but I have found that gardeners tend to over-do these. If a pine bark based potting medium is not available, one can amend one of the commercial mixes with plenty of perlite (white granular material) or any other locally available material which a nurseryman would recommend.

If one insists on trying to grow a plant outside of its natural hardiness range, one should site it where it will get as little cold winter winds as possible. The extremely cold temperatures can “freeze-dry” a plant. When a plant remains frozen, its stems cannot transport water upward. The leaves and stems simply dry out or freeze dry. This is how freeze dried coffee is made. Water can go from its solid frozen state to a gaseous state without passing through the liquid state. This freeze drying can damage or actually kill a plant. One help in such situations is to spray your plant with one of the readily available “anti-transpirants” such as Cloud Cover™ or Wilt Pruf™. There are many other brands on the market as well. These materials are like a liquid plastic which will coat the plant and prevent excessive moisture loss when frozen. We love to use these to spray a plant when it is being transplanted. They greatly facilitate transplanting larger plants or planting out of season like during warm summer months.