First, read the English article below.

The amount of oil in a plant depends on where it is in its life cycle, so it’s important to harvest each species of plant at the right time. You need to do some research to figure out when to harvest the plants you want to distill. For example, lavender should be harvested when about half the flowers on the stem have withered. Rosemary, on the other hand, should be harvested when the plants are in full bloom. Just as you need to research when to harvest plants for best essential oil production, you need to research how to harvest them. Careless handling, harvesting the wrong parts, even harvesting at the wrong time of day can reduce the quantity and quality of the essential oils. For example, you only want to use the flowering tops of the rosemary plant when making rosemary essential oil. Discard or use the rest of the plant some other way. Most essential oils are held in the plant's oil glands, veins and hairs, and these are very fragile. If you disturb or break them, you’ll get less oil from your plants. Handle the plants with care, and handle them as little as possible. If you're buying already-harvested plants, you will not have much control over the harvesting process. Look for plants that appear healthy and undamaged, and ask the seller when they were harvested. Generally, plants that are in whole form (not crushed or powdered) are best. While distillation removes many impurities, pesticides and herbicides can contaminate your oil. It’s best to use organically grown plants, whether you purchase them or grow your own. Drying reduces the amount of oil in each plant, but can greatly increase how much essential oil you make per batch. This is because you’ll be able to fit more material into each batch. Drying should be done slowly and away from direct sunlight. Commercially grown plants such as lavender and peppermint may be allowed to dry in the field after cutting for a day or so.  The ideal drying method varies from plant to plant, but in general, you shouldn’t overheat the plants. Drying in the shade or... 

Now, please write a short abstract for it in Vietnamese. Abstract:
Xác định thời điểm thu hoạch cây nguyên liệu. Thu hoạch đúng cách. Khi mua cây, hãy chọn kỹ. Làm khô cây nguyên liệu.